Battery - uu.diva-portal.orguu.diva-portal.org/smash/get/diva2:166685/FULLTEXT01.pdf · 10 ley, Kroto och Curl fick Nobelpriset i kemi år 1996 för upptäckten av kol-strukturer

Battery

List of papers

This thesis is based on the following publications, referred to in the text by their Roman numerals.

I The performance of vanadium oxide nanorolls as cathode material in a rechargeable lithium battery Sara Nordlinder, Kristina Edström and Torbjörn Gustafsson, Electrochemical and Solid State Letters, 4 (2001) A129.

II The structure and electrochemical performance of Na+-, K+-, and Ca2+-vanadium oxide nanotubes Sara Nordlinder, Jan Lindgren, Torbjörn Gustafssonand Kristina Edström, Journal of the Electrochemical Society, 150 (2003) E280.

III Redox behavior of vanadium oxide nanotubes as studied by X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy Sara Nordlinder, Andreas Augustsson, Thorsten Schmitt, Jinghua Guo, Laurent C. Duda, Joseph Nordgren, Torbjörn Gustafsson and Kristina Edström, Chemistry of Materials, 15 (2003) 3227.

IV The electronic structure and lithiation of electrodes based on vanadium oxide nanotubes Andreas Augustsson, Thorsten Schmitt, Laurent C. Duda, Joseph Nordgren, Sara Nordlinder, Kristina Edström, Torbjörn Gustafsson and Jinghua Guo, Journal of Applied Physics, 94 (2003) 5083.

V Lithium insertion into vanadium oxide nanotubes: electrochemical and structural aspects Sara Nordlinder, Leif Nyholm, Torbjörn Gustafsson and Kristina Edström, Submitted to Chemistry of Materials.

Reprints were made with permission from the publishers.

Papers and patents of relevance to this work not included in this thesis

Vanadium oxide nanotubes – electrochemistry and structure Sara Nordlinder, Kristina Edström, Torbjörn Gustafsson and Jun Lu, Electrochemical Society Proceedings, Volume 2000-21 (2000) 208.

Vanadium oxide electrode materials and structure Sara Nordlinder, Kristina Edström and Torbjörn Gustafsson, Patent No. US 6.653.022 (Telefonaktiebolaget LM Ericsson).

Nanosized productSara Nordlinder and Tom Eriksson, International patent application number: SE2005/000312 (St. Jude Medical Inc.).

Comments on my contribution In the papers where I am the first author (paper I-III and V), I have planned and performed most of the experimental work myself, and evaluated the results. The exceptions are: In paper II: The Raman measurements and evaluation of the spectra were done in collaboration with J. Lindgren. In paper III: The SXAS measurements and evaluation of the spectra were done in collaboration with A. Augustsson et al.In paper V: Evaluation of the electrochemical results was done in collabora-tion with L. Nyholm.

In paper IV: I prepared all the samples myself, and participated in planning and performing of the experiments as well as contributing to the evaluation of the results.

Contents

Svensk sammanfattning / Summary in Swedish .............................................9Nanomaterial ..............................................................................................9

Nanotuber ..............................................................................................9Litiumbatteriet ..........................................................................................10Vanadinoxid-nanotuber ............................................................................12

Resultat ................................................................................................13

1 Introduction................................................................................................141.1 Nanomaterials.....................................................................................14

1.1.1 Nanotubes ...................................................................................151.2 Vanadium oxide nanotubes ................................................................15

1.2.1 Structure......................................................................................181.3 Scope of this thesis .............................................................................19

2 The lithium battery.....................................................................................202.1 Electrode materials .............................................................................21

2.1.1 Vanadium oxides for battery applications ..................................232.2 Nanostructured electrode materials ....................................................24

2.2.1 Nanotubular electrode materials .................................................25

3 Methods .....................................................................................................273.1 Synthesis.............................................................................................273.2 Electrode and battery preparation.......................................................273.3 Characterization .................................................................................28

3.3.1 Electrochemical methods............................................................283.3.2 X-ray diffraction .........................................................................293.3.3 Spectroscopic methods ...............................................................293.3.4 Microscopy .................................................................................30

4 Results........................................................................................................314.1 Characterization of the synthesized products .....................................314.2 Electrochemical performance.............................................................33

4.2.1 Galvanostatic measurements ......................................................344.2.2 Potentiostatic measurements.......................................................36

4.3 Structural response to Li+ insertion ....................................................394.3.1 Electronic structure.....................................................................39

4.3.2 Atomic structure .........................................................................444.3.3 The oxidation state dilemma.......................................................49

5 Concluding remarks ...................................................................................50

Acknowledgements.......................................................................................53

Bibliography .................................................................................................55

Abbreviations

CCD Charge coupled device CV Cyclic voltammetry DMC Dimethyl carbonate EC Ethylene carbonate en Ethylene diamine FTIR Fourier transform infrared spectroscopy IR Infrared spectroscopy LiTFSI Lithium bis(trifluoromethylsulfonyl)imide NiCd Nickel cadmium NiMH Nickel metal hydride OCP Open circuit potential PDOS Projected density of states PES Photoelectron spectroscopy RSXES Resonant soft X-ray emission spectroscopy SEI Solid electrolyte interphase SEM Scanning electron microscope SHE Standard hydrogen electrode SXAS Soft X-ray absorption spectroscopy SXES Soft X-ray emission spectroscopy TEM Transmission electron microscope TEY Total electron yield XRD X-ray diffraction

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Svensk sammanfattning Summary in Swedish

Det finns saker, som man måste vara fackman för att inte förstå.

- Hjalmar Söderberg

NanomaterialNanovetenskap/nanoteknologi är ett forskningsområde som rör sig i gräns-landet mellan fysik och kemi. En nanometer (nm) är en miljarddels meter (10-9 m). För att få en förståelse för hur litet det är kan man som jämförelse tänka på att ett hårstrå är ca 50 000 nm tjockt och att en bakterie är runt 1 000 nm stor. Diametern hos en atom ligger mellan 0,1 och 0,6 nm. Man brukar definiera nanovetenskap som det område där material med dimensio-ner upp till 100 nm studeras. Vad är det då som är så intressant med nanoma-terial? Jo, när man använder nanometerstora byggklossar kan helt nya egen-skaper uppträda, jämfört med de konventionella motsvarigheterna, hos t.ex. metaller eller kristallina ämnen. Kemiska, mekaniska, elektriska, optiska och magnetiska egenskaper kan ändras vilket kan leda till nya spännande an-vändningsområden.

Det finns många tänkbara tillämpningar för den här typen av material. Några exempel är: nya hårda ytbeläggningar, ultralätta och starka material, effektivare katalysatorer, elektronikkomponenter 100-1000 gånger mindre än dagens, solceller med ökad kapacitet och selektiva läkemedelsbärare. Vissa egenskaper kan tas tillvara inom litiumbatteriområdet. Sprickbildning som uppkommer i vissa typer av elektrodmaterial kan till exempel undvikas om partiklarna är tillräckligt små. En ökad ytarea kan också bidra till högre kapacitet hos batteriet.

NanotuberNanotuber är en speciell form av nanomaterial. Som hörs på namnet är dessa uppbyggda av tuber eller rullar. Ett vanligt förekommande exempel är kol-nanotuber som är en speciell form av så kallade fullerener. Forskarna Smal-

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ley, Kroto och Curl fick Nobelpriset i kemi år 1996 för upptäckten av kol-strukturer som ser ut som fotbollar (fullerener)i och 1991 publicerades en artikel av en Japansk forskareii som upptäckt att kolatomer även kunde bilda tuber bestående av ett eller flera skikt. Inte långt efter började forskare runt om i världen att rapportera att även metallbaserade material kunde bilda den här typen av strukturer. Exempel på kända nanotubulära material är WS2,TiS2, TiO2, ZnO och ZrO2. Nanostavar och nanofibrer är vanligare före-kommande än tuber eftersom de är termodynamiskt mer stabila. De har dock inte något hålrum i mitten.

Som batteriforskare är man intresserad av att veta om det går att använda nanotuber som elektrodmaterial. Det finns några få rapporter om mer eller mindre lyckade försök där man använt kolnanotuber, TiS2-tuber eller koltu-ber belagda med CuO. Det första nanotubulära materialet som visade sig vara användbart i över 100 upp- och urladdningar är, så vitt vi vet, de vana-dinoxidnanotuber som diskuteras i den här avhandlingen.

Litiumbatteriet Ett batteri omvandlar kemisk energi till elektricitet. Det består av två elek-troder, en positiv och en negativ, samt en elektrolyt som kan vara fast, flytande eller i gelform. Den ena elektroden producerar elektroner medan den andra elektroden tar upp elektroner. Elektrolyten fungerar som länk mel-lan de två genom vilken joner kan vandra. Genom att koppla elektroderna till ett motstånd, till exempel en apparat, via en yttre krets, kan man få en elekt-risk ström att flyta. Det finns två huvudtyper av batterier: primära batterier som endast kan användas en gång och sekundära eller uppladdningsbara batterier där den kemiska reaktionen kan vändas och återupprepas många gånger.

I början av 1970-talet inleddes forskning på en ny typ av uppladdnings-bart batteri där litiumjoner användes som laddningsbärare. Litium är ett grundämne med hög energidensitet som tidigare framgångsrikt använts i primära batterier. Idag används litiumjonbatterier i stor utsträckning som kraftkälla i mobiltelefoner, videokameror och bärbara dataprodukter. An-vändningsområdena ökar dessutom i takt med att utvecklingen av batterierna går framåt.

De litiumjonbatterier som finns att köpa i affärerna har vanligtvis en kol-baserad negativ elektrod och en positiv elektrod som består av en metalloxid. Det finns en rad kriterier som ett potentiellt elektrodmaterial måste uppfylla:

i För mer information om Nobelpris se t.ex. Kungliga Vetenskapsakademiens hemsida: www.kva.se. ii S. Iijima, Helical Microtubules of Graphitic Carbon, Nature, Vol. 354 (1991) s. 56-58.

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Det måste kunna ta emot och släppa ifrån sig litiumjoner reversibelt, det vill säga, materialets atomära struktur får inte förändras för mycket. Materialet bör ha bra elektronisk ledningsförmåga och bra förmåga att leda litiumjoner. Det bör vara kemiskt stabilt mot elektrolyten och de andra batterikompo-nenterna, de får alltså inte reagera med varandra. Slutligen bör det vara billigt, lätt att producera, miljövänligt och ha låg vikt.

I dagens kommersiella batterier används vanligen LiCoO2 eller dopade varianter av LiNiO2 som positiv elektrod. Dessa material är dock dyra att tillverka så man vill gärna byta ut dem mot billigare alternativ som till ex-empel LiMn2O4 eller LiFePO4. På den negativa sidan hittar man normalt grafit, men även intermetalliska föreningar (t.ex. Cu6Sn5, InSb och Mg2Si)studeras flitigt idag. Vanadinoxider har en lång historia som påtänkta positi-va elektrodmaterial. Problemet med dessa är dock att de inte innehåller liti-um i sin orginalform. De måste därför kombineras med ett material som innehåller litium, vilket utesluter många populära negativa elektrodmaterial.

Figur 1. Så här fungerar ett litiumbatteri. När mobiltelefonen används vandrar liti-umjonerna från grafitelektroden till manganoxidelektroden. Under uppladdningen vänds processen åt andra hållet.

Elektrolyten är baserad på vattenfria lösningsmedel, till exempel en kom-bination av dimetylkarbonat och etylkarbonat, som blandas med ett litium-salt, till exempel LiPF6, LiBF6 eller LiN(SO2CF3)2. Vattenbaserade elektro-lyter kan inte användas eftersom de sönderfaller vid de höga potentialer som litiumbatteriet ger.

När batteriet laddas ur och upp vandrar litiumjonerna mellan elektroderna via elektrolyten och inkorporeras i elektrodmaterialen (se figur 1). Denna process skall kunna göras 1000-tals gånger utan att batteriet tappar allt för

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mycket kapacitet, vilket ställer stora krav på de material som används i elek-troder och elektrolyter.

Forskningen kring litiumjonbatterier är huvudsakligen inriktad på att för-stå hur och varför de material som används i elektroder och elektrolyt funge-rar, samt att hitta bättre, billigare och mer miljövänliga material. När man vet vilka parametrar som är viktiga för att batteriet ska fungera bra kan man lättare designa ett material som har optimala egenskaper för en specifik till-lämpning, till exempel, för ett bilbatteri eller ett batteri i en mobiltelefon.

Vanadinoxid-nanotuberJag har syntetiserat och studerat ett unikt, nanostrukturellt material som visat sig vara ett potentiellt elektrodmaterial till uppladdningsbara litiumjonbatte-rier. Materialet består av upp till 1000 nm (10-6 m) långa rullar av vanadin-oxidskikt med en inre och yttre diameter på ca 10 nm respektive 100 nm. Oxidskikten är separerade av aminer, alternativt metalljoner (t.ex., Na+, Ca2+,Mn2+ eller Cu2+) som verkar stabiliserande på strukturen. Om dessa joner tas bort kollapsar vanadinstrukturen. Man kan jämföra det med en rulltårta, där kakan är vanadinoxidskikten och sylten är de stabiliserande molekylerna, alternativt metalljonerna. Figur 2 visar en bild av nanotuberna sett vinkelrätt mot tubernas axel, så att man tittar in i tuberna när de står på högkant. I bil-den ser man tydligt att rullarna består av vanadinoxidskikt (mörka linjer) separerade av aminmolekyler (ljusa partier).

Figur 2. Bilden till vänster är tagen med ett transmissionselektronmikroskop (TEM) och visar nanotuberna i hög förstoring. Tuberna har samma utseende som en rulltårta (bilden till höger) och TEM-bilden är tagen så att man ser en skiva av tårtan.

Jag har studerat hur dessa nano-rullar uppför sig när de används som ka-todmaterial. Materialets kapacitet (Ah) och effekt (Wh) undersöks med elek-trokemiska mätmetoder, däribland galvanostatisk och potentiostatisk upp och urladdning samt cyklisk voltammetri. Morfologin karakteriseras med

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hjälp av svepelektronmikoskop (SEM) och transmissionselektronmikroskop (TEM) som kan producera bilder med mycket hög upplösning och den ato-mära strukturen studeras med röntgendiffraktion (XRD). Jag har även under-sökt den elektroniska strukturen i vanadinoxiden med fotoelektronspektro-skopi (PES) och mjukröntgenspektroskopi (SXAS/SXES).

ResultatSom nämndes tidigare visade det sig att vanadinoxidtuber fungerar bra som elektrodmaterial i litiumbatterier. Kapaciteten man kan få ut är jämförbar med den för andra vanligt förekommande elektrodmaterial. Reversibiliteten är bra och materialet kan användas under minst 100 ur- och uppladdningar utan att tappa allt för mycket kapacitet.

Materialets prestanda är knutet till vilket litiumsalt man använder i elek-trolyten och vilken jon som sitter mellan vanadinoxidskikten. Vi har sett att tuberna fungerar bäst när man använder saltet LiN(SO2CF3)2 i elektrolyten (artikel I). Det kan bero på att detta salt har en stor, relativt inert anjon. Stu-dier där vi använt Ca2+, K+ eller Na+ som stabiliserande joner visade att Ca2+

ger de stabilaste ur- och uppladdningsegenskaperna (artikel II).Spektroskopiska experiment visade att vanadinet, som i orginalmaterialet

har oxidationstalen V5+ eller V4+, reduceras under urladdningen. I slutet av urladdningen har man en kombination av V5+, V4+ och V3+ (artikel III, IV).Fördelningen av oxidationstal har en stor inverkan på hur vanadinoxidstruk-turen ser ut. Strukturen är i sin tur viktig för att förstå hur litiumjonerna rör sig in och ut ur strukturen och även inom skikten. Spektroskopiska mätning-ar ger dock inte en komplett bild av hur denna litiumdiffusion går till. För att få en fullständig bild gjordes röntgendiffaktionsstudier in situ, det vill säga direkt på ett fungerande batteri. De visade att litium går in mellan vanadin-oxidskikten när materialet laddas ur och att det orsakar en snabb minskning av skiktavståndet. En omstrukturering inom skikten sker sedan men i en långsammare takt (artikel V).

Sammanfattningsvis kan man konstatera att vanadinoxidnanotuber kan användas i litiumjonbatterier om de kombineras med ett material som kan fungera som litiumkälla.

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1 Introduction

Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment.

- Buddha

1.1 Nanomaterials Following the increasing resolution and precision of the instruments used for characterizations and the availability of new synthesis methods, nanomateri-als have developed into a large research area over the last decade. A nano-structure, typically layer-like, wire-like or plate-like, can be defined as a system in which at least one dimension is less than 100 nm and the family includes:

free clusters of atoms (quantum dots, nano dots, fullerenes, inorganic macromolecules) materials consisting of grains less than 100 nm (nanocrystalline, nano-phase and nanostructured materials) fibers less than 100 nm in diameter (nanowires, nanorods, nanotubes, quantum wires) layers less than 100 nm thick composite materials (nanoparticles or fibers embedded in a matrix, for example, a polymer)

Interestingly, materials with nanometric dimensions have significantly dif-ferent properties compared to bulk material, since such a large portion of the atoms reside at the surface or at grain boundaries. These include, for exam-ple, increased electrical conductivity or resistivity, changed magnetic proper-ties and supermagnetic behavior, increased hardness and strength and en-hanced ductility, toughness and formability. Lower melting points have also been reported, as well as higher catalytic activities.1 Some of these features can be exploited in a lithium battery context and this will be further discussed in chapter 2.3.

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Possible applications range from new hard surface coatings and ultralight materials, catalysts with tailored reactivity, sensors, scanning probes, super-plastic ceramics as well as fuel cells and rechargeable lithium batteries. The latter will be discussed in this thesis.

Nanomaterials can be synthesized following a number of different routes, for example, vapor-phase growth (including thermal evaporation, chemical vapor deposition, arc-discharge, laser ablation, etc.),2-4 spray pyrolysis,5 sol-gel methods,6-8 molecular self-assembly,9,10 and template-based methods.11-13

The choice of method depends, to some extent, on the geometrical morphology one wishes to produce.

1.1.1 Nanotubes Carbon nanotubes have become an important research area and their physical and chemical properties have been thoroughly investigated.14,15

Soon after the discovery of carbon nanotubes, by Iijima in 1991,16 reports on inorganic nanotubes and fullerene-like structures started to emerge.17-19 It is believed that all compounds possessing layered, graphite-analogue structures should be able to form nanotubes or fullerene-like structures.19,20 A large number of inorganic nanotubes have already been reported, including metal disulfides like WS2,21,22 MoS2,4,23-25 and TiS2,3 as well as transition metal oxides, for example, TiO2,7,8 ZnO,7 MoO3,26 ZrO2,27 Co3O4,28 In2O3 and Ga2O3

6. More common are nanorods and fibers. They have an advantage over tubes in that they are thermodynamically more stable, but lack an accessible inner volume.17

1.2 Vanadium oxide nanotubes Synthesis of redox-active vanadium oxide nanotubes by a ligand assisted templating approach was first presented by Spahr et al.29 The tubes were prepared by a sol-gel reaction of vanadium (V) alkoxide precursors together with primary amines, followed by hydrothermal treatment.29-31 The synthesis can also be performed using the less expensive precursors, V2O5 or VOCl3.32

Alkylamines have earlier been shown to interact with different types of lay-ered compounds,33 and several reports have shown the formation of vana-dium oxide composite materials with organoamine ligands, for example, tetramethylammonium (N(CH3)4).34-38

Vanadium oxide nanotubes consist of multiple layers of vanadium oxide separated by structure-directing molecules or ions, for example, alkylamines, alkaline, alkaline-earth or transition metal ions. The layers are commonly arranged in a scroll-like manner with open ends. Cylindrical tubes have been found, although in much lower concentration than the scroll-like morphol-ogy. The length and diameter of the tubes depend on the choice of template

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and can therefore be controlled to a certain extent.31,39 In general, the rolls consist of 2-30 vanadium oxide layers and are up to 15 m long. The tube openings range from 5 nm to 50 nm, and the outer diameters are 15-100 nm.

Embedded structure-directing agents can readily be exchanged (to 70-90 %) by other cations, for example diamines,40 aromatic amines41 or metal ions such as Na+, K+, Ca2+, Co2+ and Ni2+,39 without destroying the tubular structure. If the structure-directing agents are completely removed the structure collapses. Successful ion-exchange with Li-ions has, however, not been possible. It is possible that the Li-ions have too large hydration shells, which thereby hinder the diffusion of lithium into the structure. The ratio between guest charges (amines, cations) and vanadium atoms is always close to 0.27 mol guests/mol V which can be explained by a fixed number of binding sites for the guest moieties (paper II).39 The above stated ratio also agrees with the relationship between guest charges and vanadium in BaV7O16, which is the proposed structure of the vanadium oxide layers.

A synthesis using monoamines produces thin tube walls with few vana-dium oxide layers, while diamines give thick walls with a correspondingly larger number of layers.31,40 Krumeich et al. found that intercalated monoamines could be readily exchanged for diamines but that the reversed process was impossible.40 This points towards a thermodynamically more stable arrangement for the diamine, which could explain the larger number of layers appearing in those nanotubes.

Alternating interlayer distances have been found in tubes synthesized with addition of ammonia (NH4

+) prior to the hydrothermal step.42 Adjusting the pH-value to between 9 and 10, which is substantially higher than for normal synthesis conditions (pH = 5-7), led to tubes where the vanadium oxide lay-ers were separated by amine and ammonia alternatively. Addition of the NH4

+ molecules, not the pH-value, proved to be the important factor in the preparation of these nanotubes, since the use of other alkaline agents to raise the pH did not lead to formation of nanotubes. The larger outer diameters (>250 nm) and thinner walls found for these tubes are probably due to an increased stiffness induced by the short inter-layer distance created by the ammonia.42

Over the last few years, a number of articles discussing different areas of vanadium oxide nanotube research have emerged. The unique combination of redox-activity and tubular structure is interesting for both battery and catalysis applications as well as for the field of nonlinear optics. The follow-ing sections contain a short review of the scientific papers published about nanotubular vanadium oxide.

Several reports on synthesis of vanadium oxide nanotubes have been pub-lished, all based on the procedure first published by Spahr et al.43-47 Doping the tubes with transition metal ions have been performed in several varieties. Normally the metal ions are situated between the vanadium oxide layers, as seen for Cu2+,48 Fe2+,48 and Mn2+.49 However, there are also reports on

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growth of copper particles within the tubes,50 as well as Mo-doping of the vanadium oxide layers.46

Band structure calculations have been performed to elucidate the electronic properties of the nanotubes, using models based on V2O5

51,52 and Mo-doped V2O5

53. Results from calculations on cylidrical V2O5 tubes showed that they were wide-band-gap semiconductors. Scroll-like tubes had smaller bandgaps than cylindrical tubes. Also, the values of the bandgaps calculated using the scroll-like model were dependent on the inter-wall spacings. Larger diameters and larger interwall distances produced more stable tubes. No dramatic differences in the relative stability could be found between the Mo-doped and non-doped tubes.

Cao et al., examined the effect of sheet distance on the vibrational and electronic properties of vanadium oxide nanotubes and found no systematic dependence between the optical gap and the tube size.54 Substiution of various amines, producing different sheet distances, caused only small changes in the curvature due to the large tube diameter (<100 nm). The microscopic strain induced by the increased curvature was apparently not enough to affect the bandgap, however, it did produce a red-shift in the calculated vibrational spectra. Nonlinear transmittance of vanadium oxide nanotubes was studied by Xu et al., and the results were compared to multi-walled carbon nanotubes, which have shown to be efficient broad band optical limiters.55 Optical limiter can be used as filters when working with, for example, lasers, to limit the intensity at some maximum value. Vanadium oxide nanotubes were found to show limiting effects at 532 nm but not in the infrared range (1064 nm).

Raman and IR spectroscopy were used by Souza Filho et al., to study temperature effects on the structure of nanotubes with embedded dodecylamine and Cu.56 They found a nonreversible collaps of the tubular structure, leading to formation of V2O5, even at low laser power densities. The decompostion was found to occur through an intermediate compound, isostructural to V2O5 xerogel, which appeared at temperatures around 300 C.Structural rearrangements in the vanadium oxide layers, after laser irradiation, were also followed in a recent FTIR study by Chen et al.43 The authors claim that the organic template could be removed with preservation of the tubular structure.

Electrochemical testing in a lithium battery context has been reported by a few research groups. The first study showed that the specific capacity dropped considerably over 12 cycles.57 Possibly, the choice of cut-off poten-tials induced the poor performance. Successful cycling was reported for tubes doped with Mn2+, which could be charge and discharged reversibly giving a capacity of 140 mAh/g.49 Mo-doped tubes showed an initially high capacity, close to 200 mAh/g, but the capacity decreased to 80-100 mAh/g after 50 cycles.46 Sun et al., explored disordered vanadium oxide nanotubes and found that defect-rich tubes performed better than well-ordered tubes.58

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This effect was related to cracks in the tube walls and residual organic surfactant between the vanadium oxide layers favouring Li-diffusion, as well as on a higher V5+/V4+ ratio for the defect-rich tubes.

1.2.1 Structure Since the vanadium oxide sheets are curved to form scrolls or cylinders, the X-ray diffractograms consist of broad, asymmetric peaks of relatively low intensities. The tubular structure also contain defects wich can be clearly seen in TEM micrographs. As a consequence of the curvature, there is a lack of three-dimensional periodicity, which makes a proper structure determination difficult. To describe the structure, an approximate unit cell can be compared to a similar three-dimensional structure. This was done by Wörle et al., who used the related structure (ethylene diamine)V7O16 as a starting point.59 The choice of V7O16 was based on an earlier model which pointed out the similarities of the vanadium oxide sheets in the nanotubes to the compound BaV7O16 xH2O.60 In this tetragonal structure, V7O16 double layers are stacked along the <001> direction with Ba-ions and H2Omolecules residing between the layers.61 The vanadium atoms are coordinated in either a distorted octahedral or tetrahedral arrangement. The octahedra can also be considered as square pyramids since the distance to the 6th oxygen is very long (~ 2.3 Å). By comparing X-ray diffractograms of the nanotubes to the diffractogram of the related V7O16 phase, Wörle and co-workers concluded that the vanadium oxide layers consist of the same building blocks. The structural model is shown in Figure 3.

Interestingly, the scrolling always appear in the <110> direction, in other words the <110> direction is parallel to the tube axis.59 This is, according to Wörle et al.,59 a consequence of the way the layers are bent during the hydrothermal treatment. Partial reduction of V5+ to V4+ in an anisotropic manner, where an enrichment of V4+ ions in one layer would cause an increase in size of that specific layer, could be the driving force of the scrolling.

Petkov et al. used another non-traditional approach to solve the vanadium oxide structure. By applying an atomic pair distribution function technique (PDF), where both diffuse scattering as well as the Bragg component of the diffraction data is taken into account, they determined the intralayer structure to be that of V7O16,62 which concur with the conlusions stated by Wörle.

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Figure 3. Schematic picture of the scroll-like structure of the nanotubes and the structural model for V7O16 viewed along the b-axis. Atomic coordinates from Ref. 59 were used. Embedded ions are represented as gray spheres between the VOxlayers. Note that the positions of the embedded ions are arbitrary.

1.3 Scope of this thesis In this thesis, redox-active vanadium oxide nanotubes have been investigated in a lithium battery context, focusing on structural and electrochemical re-sponse in a “real battery” environment. The object was to find answers to the following questions:

Can this material compete with other known electrode materials? Which are the prerequisites for optimal performance? Do the nanometric dimensions provide interesting properties not seen for other vanadium oxides? What is the mechanism for lithium insertion?

In order to find the answers, the material was investigated using different characterization methods, including X-ray diffraction (XRD) and spectro-scopic methods together with electrochemical testing.

More specifically, the electrochemical performance of the tubes related to different lithium salts (LiBF4, LiPF6 and LiN(CF3SO2)2) in the electrolyte and with different cations (Na+, K+ and Ca2+) embedded in between the va-nadium oxide layers was studied (paper I and II). Spectroscopic methods were used to study the electronic structure as the material was discharged (paper III and IV). A combination of electrochemical techniques and in situX-ray diffraction was used to elucidate the lithium insertion mechanism (pa-per V).

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2 The lithium battery

Science never solves a problem without creating ten more.

- George Bernard Shaw

Today, the wide use of portable electronics, such as cellular phones and lap-top computers, has created a demand for small and efficient, rechargeable power sources. Lithium has high standard potential (-3.04 V vs. the standard hydrogen electrode, SHE), small radius (76 pm) and it is also the lightest metal (specific gravity 0.53 g/cm3). These two properties combined, allow high energy density, which makes lithium a perfect candidate for high-performance battery applications. Lithium technology can be found in both primary (non-rechargeable) and secondary (rechargeable) batteries. Watches, calculators and medical implants (such as pacemakers, defibrilators and neurostimulators) make use of primary lithium batteries, while the secondary batteries can be found mainly in portable electronics. The market for indus-trial batteries, that is, larger systems for electric vehicle and stationary appli-cations (e.g. emergency lighting, railway signalling and space applications), is still dominated by other battery technologies, for example, NiMH and NiCd, mainly due to cost issues.

Lithium-ion batteries are currently responsible for the highest sales value, 63% of worldwide sales,63 of small rechargeable cells. This is impressive considering that the first lithium battery was introduced by Sony as late as 1991. Today, almost all manufacturing of rechargeable lithium batteries is located in Asia (China, Japan and Korea). Only 3 % are manufactured in Europe.64

Charge and discharge of a lithium battery involves insertion and extrac-tion of lithium ions into a host matrix. As the host matrix is reduced or oxi-dized, ions are transported through the electrolyte while electrons flow through an external circuit.

The specific or volumetric capacity, usually expressed as mAh/g or mAh/cm3, is the amount of charge that can be obtained from the active mate-rial, and the energy that the battery can deliver (expressed as Wh/kg or Wh/dm3) is a function of the capacity and the cell potential. These character-istics are closely linked to the chemistry of the system. For a battery system to be called rechargeable it must be able to deliver 80 % of the original

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capacity over at least 300 cycles.65 Figure 4 shows a schematic picture of a lithium-ion battery.

When the battery is being used in, for example, a cell phone, Li-ions mi-grate from the anode through the electrolyte and are finally incorporated into the cathode host lattice, while the electrons travel in the outer circuit and feed the device with electrical energy. During charge, the reversed process is activated.

Figure 4. Schematic picture of a lithium ion battery. The Li-ions are transported between the negative and positive electrode through the electrolyte during charge and discharge. Reprinted with permission from Chemistry of Materials, 10, 2895 (1998) Copyright 1998 Am. Chem. Soc.

2.1 Electrode materials Choosing the right electrode materials is crucial for the battery’s overall performance. In order to be a successful electrode material, the compound should satisfy a number of criteria.66

The insertion compound should be able to host a large number of Li-ions to maximize the cell capacity. This depends on the number of available lithium sites in the host structure as well as the ability of the transition metal in the cathode material to access multiple oxidation states. To allow cyclability, the lithium insertion/extraction has to be reversible and accompanied by no, or small, changes in the host structure.

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The insertion compound should have good electronic and Li-ion conduc-tivity to minimize polarization losses and thereby support a high current and power density. This depends mainly on the crystal structure of the host.The compound must be chemically stable with respect to the electrolyte over the entire operating potential. To prevent unwanted oxidation or reduction of the electrolyte, the poten-tial of the cell must lie within the stability window of the solvents and salts used. The compound should also be inexpensive, easy to produce, environmen-tally benign and lightweight.

The third key to a successful battery (besides the anode and cathode materi-als) is to find a suitable electrolyte. This task is not trivial since the environ-ment in which it will operate is highly oxidizing (>4 V vs. Li/Li+) and reduc-ing (~0 V vs Li/Li+). Some non-aqueous solvents, polymers or gels can han-dle these extreme conditions without decomposing and are used together with lithium salts (for example LiPF6, LiBF4 or LiN(SO2CF3)2). More infor-mation on liquid and polymer electrolytes and their role in the battery can be found in for example references 67-71.

The most commonly used negative electrode material for lithium-ion bat-teries today is carbon, which is a low-cost material with good cycling per-formance.72,73 Other proposed anode materials are intermetallic alloys, based on Al, Si, Sn, In, or Bi, for example, Cu6Sn5,74 InSb,75,76 Cu2Sb,77 and Mg2Si78. Some transition metal oxides, among them LiMVO4 (M = Co, Zn, Cd, Ni)79 and the nanosized MO materials (M = Co, Fe, Ni),80 can also be used as negative electrode.63,81,82

Transition metal oxides are normally found on the positive side. Exam-ples are layered LiMO2 oxides (M = Mn,83 Co,84 Ni85). LiCoO2 and doped forms thereof, for example LiNi1-x-yCoxMyO2 (x+y<0.25, M = Mg or Al), is used in commercial batteries. However, since cobalt is expensive the battery manufacturers are looking for other materials, which can help reduce the price of the final product without decreased performance.

One alternative is LiMn2O4,86 which is attractive due to its low price and low toxicity. However, the material experience severe problems in storage and cycle life when stored at high temperatures. Another possible cathode material is LiFePO4,87,88 which unfortunately suffers from poor electronic conductivity. It has been shown that the effects from this can be minimized by, for example, coating the LiFePO4-particles with carbon or by tailoring the synthesis procedure as to reduce the particle size.89,90 Recently, another iron based compound, Li2FeSiO4, proved to be an promising electrode mate-rial.91 An overview of the different electrode materials, displaying their indi-vidual potentials and capacities, is shown in Figure 5.

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Figure 5. Voltage versus capacity for positive and negative electrode materials cur-rently used, or under serious consideration, for the next generation of rechargeable Li-based cells. (based on Fig. 5, in Tarascon, et al. Nature, 414, 359 (2001))

2.1.1 Vanadium oxides for battery applications One way of achieving higher capacities is to use an electrode material where the metal ion can change oxidation state by two or more units. Vanadium based oxides is one group where this is possible. Examples of materials which have been investigated for 3 V battery cathodes are, V2O5, V6O13 and Li1+xV3O8.

V2O5 can be easily synthesized by sol-gel methods and is built up of lay-ers of edge sharing and corner sharing VO5 square pyramids. The Li-ions are inserted between the vanadium oxide layers during discharge. Up to three Li can be inserted while going through five different structural phases, some of which are not reversible.92 Promising results has also been reported for V2O5xerogels,93-96 and aerogels96-98 which are amorphous, low-crystalline forms of V2O5. There is also an aerogel-like form, which has slightly larger surface area than the aerogel.99,100 The large surface area and low density, found in these materials, give rise to high specific capacities. However, the capacity depends on the rate of insertion/extraction and is significantly decreased when higher rates are used.101

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V6O13 was earlier considered as an electrode material but is not so ac-tively studied nowadays. The structure can accept up to eight Li-ions. How-ever, only six lithium can be used reversibly.102-105 Large channels in the structure, formed between the single and double layers, built up by distorted VO6 octahedra, provide pathways for lithium diffusion. So far, five different LixV6O13 phases have been identified where x = 2/3, 1, 2, 3 and 6.102,106-110

Lix+1V3O8 is a pseudo-layered material which can host lithium ions re-versibly.111-114 Also, the isostructural Na1+xV3O8 has been studied in a battery context.115-117 In this group of vanadates, the alkaline or alkaline-earth metal atoms function as structure stabilizing pillars between the vanadium oxide units. This enhances the lithium diffusion rate, and increases the amount of lithium that can be inserted, leading to good electrode performance. The specific capacities for the different vanadium oxide electrode materials have been summarized in Table 1.

Vanadium oxide nanotubes will be discussed in detail later in this thesis and will therefore not be mentioned further in this section. Worth pointing out is, however, that the nanotubular material shows capacities comparable to the other vanadium oxide electrode materials. Important to note is that all of the above mentioned materials must be coupled to a lithium source, for example, lithium metal to work as cathodes.

Table 1. Specific capacities for different vanadium oxide electrode materials

Material Typical Specific Ca-pacity (mAh/g)

Theoretical Capacity i

(mAh/g) References

Crystalline V2O5 148-310 148 (1) 118, 119 V2O5 xerogel 200-560 560 (4) 93, 94, 101 V2O5 aerogel 300-780 97, 101 V2O5 aerogel-like 400-560 560 (4) 100, 101 V6O13 260-314 314 (6) 102, 105 Li1+xV3O8 150-279 279 (3) 111, 112 Na1+xV3O8 150-265 265 (3) 115-117 VOx nanotubes 150-330 ~370ii 49, paper I, IIi) The calculations are based on the number of Li+ shown in parenthesis ii) If all vanadium in M2+V7O16 (M = guest ions) is reduced to V3+.

2.2 Nanostructured electrode materials Decreasing the particle size can improve the performance for some electrode materials considerably. But there are also disadvantages associated with using nanosized electrode components.120 In this section, the advantages and disadvantages of using nanomaterials in lithium battery electrodes will briefly be discussed, together with examples of nanostructured anode and cathode materials.

25

One advantage of using a material with a large surface area is that higher charge/discharge rates can be facilitated. Particle dimensions in the nano-range also leads to shorter lithium diffusion lenghts, which can provide a higher power output. Particle cracking, leading to pulverization and corresponding capacity loss, can be prevented if the particles are small enough, which has been shown, for example, in Sn-alloys used as anode material.121-123

Other examples of nanostructured anode materials are the recently reported nanoparticles of transition metal oxides, MO (where M is Co, Ni, Cu or Fe).80,124,125 These materials demonstrated high capacitites (up to 700 mAh/g) and fast recharging rates. Another approach is to support small particles of active material in a less active (or inactive) matrix. One example is the nanocomposites of active Sn2Fe and inactive SnFe3C studied by Mao et al.126-128

On the cathode side, nanocomposites of LiMn2O4 and nanometer sized particles of Na0.7MnO2 show good cyclability.129 A positive effect of particle size reduction can also be seen for LiFePO4.89,90 High rate performance can be seen for nanocrystalline Li4Ti5O12.130 Monodispersed nanorods (diameters = 70 nm) of V2O5 where the ends are attatched to a current collector has been presented by Sides et al.131 This brush-like electrode proved to perform twice as good at low temperatures than rods with larger diameters, due to the increased solid state lithium diffusion. The above described type of architecture can also be used to assemble three-dimensional batteries, in which the parallel-plate design is exchanged for interdigitated electrodes.132

The enhanced surface electrochemical reactivity is generally expected to improve the performance of lithium-ion batteries. However, a large surface area can also increase the solvent decomposition occurring at both anode and cathode during charge and discharge, resulting in large irreversible capacities. Also, poor packing of the nano particles can lead to low volumetric energy density. It is therefore important to optimize particle size and morphology to maximize the performance of the electrode material.101

Tubular or wire-like materials may have an advantage over nanoparticles since the inherent shape favors good connectivity.

2.2.1 Nanotubular electrode materials There have been several attempts to use nanotubes as electrode material in lithium ion batteries. Carbon nanotubes were the first to attract interest, since graphite is a commonly used anode material. There have been several reports on lithium intercalation into both single-walled and multi-walled carbon nanotubes.133-141 However, the material normally show high irreversible ca-pacities during the first cycle. Carbon nanotubes have also been used in

26

nanocomposites, together with, for example, vanadium oxide aerogels,142,143

CuO,144 or Sn and SnSb0.5145.

Inorganic tubes have not been investigated as thoroughly in a battery con-text as their carbon analogues. There are, however, a few examples. On the anode side, MoS2-xIy (x 0 and y 1/3) has been studied.146 However, this material suffers from high irreversible capacity. Microtubular TiS2,synthesized by a template-based method was reported as a possible cathode material by Che et al.3 The same method was used to prepare nanotubes of LiMn2O4, which showed higher capacities than a normal composite elec-trode.12 Anatase TiO2 nanotubes have been shown to cycle reversibly and exhibited good rate capability.147 To the best of our knowledge, the first nanotubular electrode material, which cycled reversibly for more than 100 cycles was vanadium oxide nanotubes (paper I and II).

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3 Methods

Experience is simply the name we give our mistakes.

- Oscar Wilde

This chapter will briefly describe the experimental methods used in this the-sis. The different methods will not be described in detail. Instead, references will be given to where further information can be found. Specific details on the experimental setups can also be found in paper I-V.

3.1 Synthesis Vanadium oxide nanotubes were prepared as described in the literature using vanadium (V) triisopropoxide (paper I-IV) or crystalline V2O5 (paper V) as precursors and dodecylamine (C12H25NH2) as structure-directing agent.29,31,32

Dodecylamine was chosen because it empirically proved to be the best start-ing material for ion-exchange.31 Syntheses were also performed using hexa-decylamine (C16H33NH2),148 but this material will not be discussed in this summary, since the material proved to be difficult to ion-exchange. The alkoxide and the amine were dissolved in ethanol and stirred under inert atmosphere for 2 h. The liquid was then hydrolyzed and the resulting gel was left to age for 24 h. After aging the gel was transferred to an autoclave and heated at 180 C for 7 days. The resulting black powder was carefully washed and dried.

Ion-exchange with Na+, K+ or Ca2+ was then performed by stirring a mix-ture of nanotubes and a metal salt in an ethanol/water mixture for 2-24 h, after which the powders were washed and dried as described above. The ion-exchanged material was later used for electrode preparation.

3.2 Electrode and battery preparation Electrodes were made by spreading an 80:10:10 wt. % mixture containing the active material, carbon black and a binder, ethylene propylenediene ter-polymer (EPDM), dissolved in hexane, onto an aluminum foil. The slurry

28

was dried over night to remove all the solvent from the binder, leaving a thin layer of the electrode material on the foil. Circular electrodes were then cut and dried under vacuum at 120 C inside a glove-box with argon atmosphere.

Generally, two-electrode cells were used for galvanostatic measurements and three-electrode cells for the potentiostatic experiments. In all case, lith-ium foil was been used as counter electrode and as reference electrode (for the three-electrode cells). The electrolyte consisted of lithium salt dissolved in a mixture of ethylene carbonate (EC) and dimethylcarbonate (DMC), 2:1 by volume. An imide salt, LiN(SO2CF3)2 (LiTFSI), was used for most of the electrochemical measurements. In paper I and II, LiBF4 and LiPF6 were also tested. All three salts are commonly used in electrolytes for recharge-able lithium batteries. Assembly of the battery was performed inside the glove-box, and the cell components were sealed into a polymer-coated alu-minum pouch to avoid exposure to air.

3.3 Characterization Several techniques were used to study the structure and the electrochemical performance of the vanadium oxide nanotubes. Structural characterization was made on pure powder, electrodes or on the final battery (in situ). All experiments were performed at room temperature.

3.3.1 Electrochemical methods Galvanostatic methods, in which the potential is monitored under controlled current conditions, are important tools to evaluate a compound’s appropri-ateness as electrode material. The results give information about, for exam-ple, the long term cycling behavior and the rate capability. Also, the shape of the potential versus time curve can hold information on phase transitions. A gently sloping curve points to a solid solution of lithium in the host mate-rial, while discrete plateaus can identify the existence of stoichiometric phases. In this thesis, galvanostatic measurements have been used mainly to evaluate the long term capacity and rate capability under different circum-stances (paper I and II). The rate of charge and discharge is specified as C/x, where x is the time (h) for a complete discharge. For example, C/4 means one discharge in 4 h.

Potentiostatic methods involve altering the potential and measuring the current response. Cyclic voltammetry (CV), where the current is followed as the potential is scanned between two cut-off values, belongs to this family. In paper V, CV was used to investigate the potential stability window of the material and to determine the rate controlling process. Chronoamperometric studies, in which potential steps are applied and the current-time response followed, were also applied in paper V to further elucidate the lithium inser-

29

tion mechanism. Two-electrode cells were used for all galvanostatic meas-urements and for the in situ measurements. Three-electrode cells were used for all potentiostatic measurements (except for the in situ measurements). More information on electrochemical methods can be found in, for example, references 149 and 150.

3.3.2 X-ray diffraction The diffraction experiments in this thesis were generally performed using Cu K radiation ( = 1.5418 Å). However, X-rays produced by a synchrotron source ( = 1.0886 Å or 0.9547 Å) were used in paper V. The advantage of performing diffraction experiments at a synchrotron facility is the very high intensity of the X-ray beam. The experimental setup was equipped with a CCD (charge coupled device) camera, which allowed collection of the total diffraction pattern instantaneously. High intensity together with the area detector considerably shortened the measurement time needed to achieve one diffraction pattern, thus improving in situ measurements on the vanadium oxide nanotubes. The battery was mounted in transmission geometry using a purpose built sample holder that allowed electrochemical control. General information about diffraction can be found, for example, in references 151 and 152.

3.3.3 Spectroscopic methods Information about the orbital energies can be obtained by using

photoelectron spectroscopy (PES) where the ionization energies of atoms are measured when electrons are ejected from different orbitals. Since both core level and valence levels can be accessed, PES gives elemental information as well as information on the chemical environment. Element specificity allows investigation on one selected constituent of the cathode or anode. Hence, interference from other electrode components, which can cause problems in, for example, XRD studies are eliminated. The escape depth of electrons is only 5-50 Å, which means that PES only probes the outermost surface.153,154

Soft X-ray emission and absorption spectroscopy (SXES/SXAS) also give information on the electronic structure of solids. Since photons are used as probes, these methods are not limited to electronically conducting materials. Also, charging effects, common in PES measurements, can be avoided. The energy of the photons used are in the range 100-1000 eV, which can be compared to hard X-rays that have energies above 1000 eV. Al K radiation (h = 1486.6 eV) was used for the PES measurements and the soft X-ray experiments were performed at energies around 500 eV.

Depending on the detection method used, SXAS and SXES can probe the surface or the bulk. Total electron yield (TEY), used for the SXAS meas-

30

urements in paper III and IV, is a surface sensitive detection method. If in-stead photons are detected, as in the SXES measurements described in paper IV, bulk properties can be explored.25,155 By using resonant emission spectroscopy (RSXES), where the spectra are recorded at different excitation energies, specific electronic states can be selected giving information on local electronic structures. Resonant soft X-ray emission spectroscopy was used in paper IV.

In paper II, Raman spectroscopy was used as a complement to the XRD measurements to characterize the structural properties of the as-synthesized products. Information on Raman spectroscopy can be found in, for example, reference 156. Low laser power (1 % laser power = 0.027 mW, correspond-ing to 6 mW at the sample position) had to be used for the measurements, since high temperatures destroy the nanotubes. If the sample was exposed to a high intensity laser beam it was immediately burnt and decomposed into orange colored V2O5. All samples were visually inspected in the microscope before and after the measurement to make sure that the samples were unal-tered.

3.3.4 Microscopy Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the morphology of the nanotubes. TEM offers high resolution, facilitating exploration of the individual vanadium oxide layers. It was also possible to study the nanotubes perpendicular to the tube axis using TEM. For these measurements a special preparation tech-nique was used in which the tubes were embedded in a glue matrix, which was then cut and polished.

31

4 Results

The most exciting phrase to hear in science, the one that heralds the most discoveries, is not “Eureka”, but “That’s funny…”.

- Isaak Asimov

4.1 Characterization of the synthesized products XRD and Raman spectroscopy were used to verify the success of the

synthesis. In the diffraction pattern for the vanadium oxide nanotubes, 00l-reflections resulting from the regular interlayer distances created by the vanadium oxide walls can be seen at 2 <20 (Figure 6). The d-value associated with the highest intensity peak (001) corresponds to the distance between the vanadium oxide layers. Substituting larger structure-directing molecules, for example, hexadecylamine (C16H33NH2) or dodecylamine (C12H25NH2), for smaller metal cations, for example, Na+, results in a pronounced decrease in interlayer distance and accordingly a shift in the 001-peak position towards a higher 2 value.

5 10 15 20 25 30 35 40 45

(320

)

(210

)(2

00)

(100

)

(003

)(002

)(0

01)

Na-VOx

K-VOx

Ca-VOx

C12-VOx

Inte

nsity

(arb

. uni

ts)

2 (deg.)

Figure 6. X-ray powder diffractograms (CuK radiation, = 1.5418 Å) for vana-dium oxide nanotubes with four different layer-separating agents: Na+, K+, Ca2+ and dodecylamine (C12). For the ion-exchanges samples, the highest intensity peak corresponds to the 001 reflection.

32

The peaks at 2 > 20 are mainly hk0-reflections originating from the intralayer vanadium oxide structure. These peaks remain at the same 2 -positions for the as-synthesized and ion-exchanged nanotubes, suggesting that the structure within the vanadium oxide layers is independent of the type of molecules embedded. All diffractograms can be indexed on the basis of a pseudo tetragonal (used in paper II) or triclinic cell with the c-axis being the distance between the vanadium oxide layers.59,62

Raman spectra of the nanotubes, presented in paper II, showed no major differences between the samples (as-synthesized and ion-exchanged), which agree with the XRD results. The Raman spectra of the nanotubes consist of one broad, low-intensity peak at ~ 955 cm-1, belonging to the vanadyl V=O stretching mode, and two broad bands at 800-600 cm-1 and 500-200 cm-1,associated with bridging V–O bonds (Figure 7). Hardcastle and Walchs showed that the vibration frequencies of stretching modes can be related to the bond length by an empirically derived exponential function.157 Applied to the band at 955 cm-1, this corresponds to a bond length of 1.62 Å, which is reasonably close to the bond length for the two V=O bonds in BaV7O16(1.600 Å and 1.611 Å for the V(1)-O(2) and V(2)-O(4), respectively61) as well as to the mean vanadyl bond length in (en)V7O16 (1.611 Å).

1400 1200 1000 800 600 400 200

955

e)

d)

c)

b)

a)

Inte

nsity

(arb

. uni

ts)

Wavenumbers (cm-1)

Figure 7. Raman spectra of vanadium oxide nanotubes with embedded (a) dode-cylamine (785 nm laser), (b) K-ions, (c) Ca-ions, (d) Na-ions. The bottom spectrum is for crystalline V2O5 (e).

Figure 8 shows TEM micrographs of vanadium oxide nanotubes. Most of the tubes adopt a scroll-like morphology, and this can be seen in the micro-graph taken perpendicular to the tube axis. The tubular structure is preserved after ion-exchange as seen in the micrograph with lower magnification. It is evident that the scrolls are not perfect but contain defects. Some tubes con-tain S-shaped layers (not shown here), where one vanadium oxide layer on

33

the outside of a tube starts to bend in the opposite direction compared to the curvature of the original tube.

Figure 8. TEM micrographs showing the tubular structure of the vanadium oxide nanotubes. The left micrograph shows tubes with templating hexadecylamine seen perpendicular to the tube axis. The micrograph to the right shows Na+ containing nanotubes.

As can be seen in Figure 9, which shows an SEM image of an electrode, the tubular shape is preserved during the electrode preparation. The image clearly shows the porous structure of the electrodes, in this case made from Ca2+ containing nanotubes. Some tubes have aligned and formed larger agglomerates, but generally the tubes are well separated.

Figure 9. SEM image of an electrode made from Ca2+ containing nanotubes.

4.2 Electrochemical performance Galvanostatic and potentiostatic measurements were performed to investi-gate if vanadium oxide nanotubes could compete with other cathode materi-als and what the prerequisites for optimal performance were.

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4.2.1 Galvanostatic measurements As a first approach, vanadium oxide nanotubes with embedded amines or Na-ions were tested. The ion-exchanged material proved to have superior performance and was therefore used in the subsequent studies. The as-synthesized material, containing dodecylamine, had a maximum capacity of <80 mAh/g, (using LiBF4 or LiTFSI salt) compared to ~150 mAh/g for the Na+-exchange tubes. If the capacity was calculated with respect to the weight of vanadium instead of the total weight, the as-synthesized material was still not as good as the ion-exchanged. The reason can be that the bulkier amine molecules impede lithium diffusion and thereby limit the overall cell per-formance.

The theoretical capacity of vanadium oxide nanotubes can be estimated to approximately 370 mAh/g if all vanadium in the proposed structure, M2+V7O16 (M = guest ion) is reduced to V3+. A total of nine Li-ions must be inserted to reach this capacity. There are, however, some question marks considering the experimentally obtainable capacity.

0 20 40 60 80 1000

50

100

150

200

250

0 100 200

2.0

2.5

3.0

3.5

Pote

ntia

l (V)

Capacity (mAh/g)

Dis

char

ge c

apac

ity (m

Ah/

g)

Cycle no.

Figure 10. Discharge capacities for cells cycled with three different salts in the elec-trolytes: LiTFSI ( ), LiBF4 ( ), LiPF6 ( ). The inset displays the first discharge/ charge cycle with LiTFSI-electrolyte. The potential is given vs. Li/Li+. The cells were discharged/charged at 50 mA/g, which corresponds to a C/4 rate (> cycle 20).

In order to find a salt which provided good electrochemical performance, Na+-exchanged tubes were cycled with three different electrolytes. Figure 10shows the discharge capacities for cells with Na+-exchanged nanotubes as working electrode, cycled with three different salts in the electrolyte: LiBF4,LiPF6 and LiTFSI.

Interestingly, for cells cycled with LiBF4 or LiPF6 containing electrolytes the capacity increases and reaches a maximum of ~150 mAh/g after 20 cy-cles (paper I). Electrodes cycled with LiTFSI electrolyte showed a different behavior, with an initially high capacity (~250 mAh/g) followed by a slow

35

decrease. The reason for this difference was assumed to be partly connected to the anion and partly to the electrode preparation technique. The electrode slurries for these experiments were made from very small quantities of active material, which created a problem to mix the electrode components in a satisfactory way. Larger aglommerates of nanotubes may have remained intact due to the absence of grinding in the mixing procedure. The increasing capacity could therefore be due to the existence of initially inactive areas of the electrodes, which was gradually accessed by electrolyte. TFSI¯ is a bulky anion where the negative charge is delocalized over the nitrogen and oxy-gens. Possibly, the TFSI¯ anion provided a better environment for lithium transportation to and from the nanotubes.

The next step was to investigate the electrochemical dependence on the embedded metal ion. Nanotubes with embedded Na+, K+ or Ca2+ ions were cycled with LiBF4 or LiTFSI based electrolytes (paper II).

0 20 40 60 80 1000

40

80

120

160

200

0 20 40 60 80 1000

40

80

120

160

200LiTFSI

Dis

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ge C

apac

ity (m

Ah/

g)

Cycle No.

Na-VOx K-VOx Ca-VOx

LiBF4

Cycle No.

Na-VOx K-VOx Ca-VOx

Figure 11. Discharge capacities for the ion-exchanged material cycled 100 times with (a) LiBF4 and (b) LiTFSI in the electrolyte. The cells were discharged/charged at 25 mAh/g, which corresponds to a C/5 rate.

Using LiTFSI in the electrolyte generally provided a more stable cycling behavior over the first 10-20 cycles resulting in less capacity increase (Figure 11). Na-exchanged nanotubes, however, experienced a pronounced increase in capacity when cycled with LiTFSI electrolyte, contradictory to the earlier results. Electrodes from the same batch, used for rate experiments, did not show the same behavior (paper II). This can, again, be due to the technique used to prepare the electrode films. Since the batches were very small, the electrode film deposited on the aluminum current collector could have been inhomogenous in terms of presence of agglommerates. An electrode with a more compact film (less porous) or an electrode film containing larger agglomerates could behave as seen in the electrolyte study (see above), due to the existence of grains inaccessible to the electrolyte.

All cells experienced an initial increase in capacity independent of the nature of the embedded ion or the electrolyte composition. This can be

36

connected to the structure and composition of the material. Vanadium oxide has been shown to reversibly intercalate both monovalent and polyvalent cations. One example is the insertion of Mg2+, Al3+ or Zn2+ into V2O5aerogels.158

Keeping this in mind, one possible explanation for the capacity increase is that the cations between the vanadium oxide layers are electrochemically active and thus participate in the insertion/extraction process. Lithium ions may replace the embedded cations gradually creating a more effective environment for lithium diffusion. Another explanation can be a continuous modification of the tubular structure facilitating more effective lithium insertion. Continuous charge/discharge cycles may induce more defects in the structure thereby incresing the capacity, similary to the improved performance seen for defect rich tubes presented by Sun et al.58 Perhaps the most reasonable explanation is, however, that parts of the tubes are initially inaccessible for lithium ions due to lack of sufficient separation of the individual tubes, as was discussed above.

Comparing the performance of the three different ion-exchanged materials, K+ substituted nanotubes generally show the lowest specific capacities. Considering both the capacity and the stability, tubes containing Ca2+ are the best choice for a future battery material. Since the performance seem to depend on not only the material but also on the composition and morphology of the electrode, the capacity values should be considered as approximate and not absolute.

Nanomaterials can have superior rate performance due to the high contact area between electrode and electrolyte. Consequently, the responses of Na+,K+ and Ca2+ nanotubes to the applied current was investigated (paper II).The cells were cycled at three different rates: C/30, C/5 and C/2. Increasing the rate from C/30 to C/2 resulted in a 20-30 % decrease in capacity. The capacities at the C/2 rate were between 100 and 140 mAh/g as compared to the original capacities, at C/30 rate, of 150-180 mAh/g. All materials re-gained most of, or all of, their initial capacity when returned to the C/30 rate. It is difficult to compare the rate performances of different materials since the rate is also closely linked to the electrode preparation. Generally, the displayed rate performance for vanadium oxide nanotubes is comparable to other cathode materials, but it can not compete with the high rate perform-ance seen, for example, in nanocrystalline Li4Ti5O12

130 or carbon black coated LiCoO2.159

4.2.2 Potentiostatic measurements A more comprehensive electrochemical investigation, to further assess the response of the material in a lithium battery, was performed on vanadium oxide nanotubes containing Ca-ions (paper V). This specific material was chosen due to its stable galvanostatic cycling behavior. Cyclic voltammetry

37

was used to obtain information on whether the lithium insertion was a sur-face process, governed by kinetic control, or if it was diffusion controlled. Also, the available potential window was studied.

The cyclic voltammograms are shown in Figure 12. Irreversible peaks, at 3.06-2.90 V, appeared on the first cathodic scan (Figure 12 a). The exact origin of the peaks is not yet fully understood. As seen in Figure 12 a, three broad peaks are also clearly visible in the cathodic region (discharge) and two peaks appear on the anodic sweep (charge).

1.8 2.1 2.4 2.7 3.0 3.3-0.10

-0.05

0.00

0.05

0.10

1.6 2.0 2.4 2.8 3.2 3.6 1.5 2.0 2.5 3.0 3.5 4.0

pre-cycle

a

Cur

rent

(mA)

Potential vs. Li/Li+ (V)

1.8-3.2 V 1.6-3.4 V

b

Potential vs. Li/Li+ (V)

1.4-3.6 V 1.2-3.8 V

c

Potential vs. Li/Li+ (V)

Figure 12. Cyclic voltammograms for the first cycle (a) and for four different poten-tial windows (b, c) The sweep rate was 0.05 mV/s.

The anodic peak was used to study the relationship between peak current (Ip)and the sweep rate, which can give information about the diffusion behavior of lithium ions in the electrolyte and in the electrode material. For a finite diffusion distance, infinite rate of diffusion, the peak current will be directly proportional to the sweep rate.160 The latter relationship is expected for the thin laminate cell used in these investigations, since the working electrode and counter electrode are situated very close to each other (~500 m) and low scan rates were used.

To establish that the lithium insertion into the vanadium oxide nanotubes behaved as a surface process and not a diffusion controlled reaction, a sweep rate study was performed using a three-electrode cell, which was scanned between 1.8 V and 3.5 V. In Figure 13, the anodic peak current (Ip) and peak potential are plotted against the sweep rate. As expected, a linear relationship was found between Ip and the sweep rate, confirming that the lithium inser-tion took place under thin layer electrochemical conditions.

38

0.0 0.2 0.4 0.6 0.8

0.0

0.4

0.8

1.2

Sweep Rate (mV/s)

I p (m

A)

2.6

2.8

3.0

3.2

3.4

Ep v

s. L

i/Li+ (V

)

Figure 13. Anodic peak current ( ) and peak potentials ( ) as a function of sweep rate. The sweep rates used were: 0.05 mV/s, 0.1 mV/s, 0.2 mV/s, 0.4 mV/s and 0.8 mV/s.

The anodic peak potential (Ep) will normally shift towards more positive potentials (more negative potentials for a cathodic peak on the cathodic sweep) with increasing sweep rate if the process is controlled by the rate of charge transfer and/or ohmic drop effects.160 In other words, the peak poten-tial separation, Ep, increases. Constant Ep is only seen in a diffusion con-trolled system with fast electron transfer. In the presence of significant ohmic drop, Ep should increase linearly as implied by Ohm’s law. As seen in Figure 13, Ep shifts towards more positive potentials. The relationship be-tween Ep and the sweep rate is non-linear while Ip depends linearly on the scan rate. This implies that the lithium insertion process is mainly kinetically controlled. However, the effect of the ohmic drop could dominate at low sweep rates, as seen by the linear relationship for the first three points.

To investigate the available potential window, electrodes were submitted to scans between different potential limits. The global limits were chosen to be 1.2 V and 3.8 V, starting with a scan between 3.2 V and 1.8 V. For every cycle the window was increased by 0.1 V in each direction until the global limits had been reached. Figure 12 b, c show voltammograms for the poten-tial windows 3.2-1.8 V, 3.4-1.6 V, 3.6-1.4 V and 3.8-1.2 V respectively. The shapes of the peaks differed on the cathodic and anodic scans. The anodic peak centered at 2.5 V, was larger than the two cathodic peaks observed. This asymmetry can be due to different mechanisms for lithium insertion and extraction, and was also found in the chronoamperometric results (paper V).

With the widest potential window (1.2-3.8 V) the shape of the peak on the anodic sweep differed substantially from those seen on the scans with the other potential windows. The anodic peaks were broadened and shifted to more positive potentials. This shift in the anodic peak potential can be due to a resistance of the vanadium oxide layers to readopt their original structure.

39

The use of the low cathodic potential limit could generate considerable amounts of V3+, initiating side reactions and/or phase transitions detrimental to the battery performance. According to a PES study, V3+ is formed already at potentials below 2.0 V (this will be discussed further in chapter 4.3.1). At 1.8 V, the three vanadium oxidation states were found to coexist with the distribution: 23 % V5+, 51 % V4+ and 26 % V3+. The amount of V3+ should naturally be even higher at a potential of 1.2 V. The differently shaped voltammogram seen when using cathodic potential limit below 1.2 V could be a result of a distortion of the vanadium oxide structure. Charging and discharging the battery galvanostatically over these potential limits caused the capacity to degrade over time, which is consistent with the presence of irreversible reactions. In summary, the battery can be discharged to about 1.5 V, but no further than 1.3 V, and charged to 3.5 V to allow reversible operation of the battery.

4.3 Structural response to Li+ insertion In an attempt to understand the lithium insertion mechanism, the change in the electronic and atomic structure was followed as the material was charged and discharged. For the spectroscopic studies of the electronic structureex situ methods were used, since the experimental set-ups did not allow in situ experiments. However, the XRD measurements, used to evaluate the atomic structural changes, were made in situ.

4.3.1 Electronic structure This section will be divided into two parts: surface studies, performed by PES and SXAS, and bulk studies, using RSXES. All the spectroscopic stud-ies discussed in this section were performed on vanadium oxide nanotubes with embedded Na-ions. At the time of this study, Na+ containing tubes de-livered the highest capacities and was therefore chosen for the investigation. To follow the changes in the electronic structure as Li-ions were inserted, five samples with different Li-ion content were prepared. The electrodes were submitted to one cycle between 1.8-3.5 V to avoid the influence of first cycle effects, and then discharged to four different potentials: 3.0 V, 2.5 V, 2.0 V, and 1.8 V. The pre-cycled electrode had an OCP of ~3.3 V.

Absorption spectra of the electrodes are shown in Figure 14, together with reference spectra of V2O5 (3d0), VO2 (3d1), V2O3 (3d2) and V6O13(3d2/3). All spectra contain two large peaks, which contain information about the electronic state of vanadium. Transitions from V 2p core states to unoc-cupied 3d states give rise to the spin-orbit splitting in the SXAS spectra pro-ducing the peaks: L3 (2p3/2 3d) at 515-520 eV and L2 (2p1/2 3d) at 522-527 eV. Reduction of vanadium from V5+ to V4+ and V3+ increases the

40

occupancy of electrons in the 3d states from d0 to d2, which will in turn en-hance the screening of the core hole and thereby lead to lower absorption energy. This effect could be clearly seen in the absorption spectra of vana-dium oxide nanotubes at different potentials.

510 515 520 525 530 510 515 520 525 530

a

1.8 V

2.0 V

2.5 V

3.0 V

1 cycle

Inte

nsity

(arb

. uni

ts)

Energy (eV)

b

V2O3

VO2

V6O13

V2O5

Energy (eV)

Figure 14. (a) V 2p SXAS spectra of Na+ vanadium oxide nanotubes. The top spec-trum represents an electrode cycled once (E ~3.3 V). (b) V 2p spectra of reference materials.

The overall peak area was shifted towards lower energies when Li-ions were inserted between the vanadium oxide layers. A shoulder on the low-energy side of the L2 peak (523 eV) increased in intensity as vanadium was reduced, and the L3 peak shifted towards lower energy. The electrodes discharged to 2.0 V and 1.8 V showed a small peak at 520.7 eV, which appears to be the same characteristic peak found in the valley between the L3 and L2 for V2O3.There were also pre-edge structures in the energy region of 513.5-516 eV observed in both electrodes and the V2O3 reference sample. These experi-mental findings suggest that the chemical surroundings of vanadium in the 2.0 V and 1.8 V electrodes are similar to that in V2O3, thus indicating the presence of V3+ in the material. The fact that the electrode only subjected to one cycle does not show these features indicates a reversible reduction proc-ess. Analysis of the O 1s absorption spectra can be found in paper III.

It was difficult to draw any conclusions on the specific reduction behavior from absorption spectra alone. Therefore, PES measurements were per-formed to estimate the distribution of oxidation states for the different elec-trodes.

Figure 15 presents the V 2p PES results for the pristine electrode and the discharged electrodes. A typical two-peak structure can be seen in the spec-tra, originating from the spin-orbit splitting of the V 2p3/2 at 514-518 eV and V 2p1/2 at 522-526 eV. When the electrodes were discharged, the shape of the

41

V 2p peaks changed significantly. The peak weight was shifted towards lower binding energies, consistent with the absorption spectra.

To interpret the changing of peak shapes, the experimental spectra of the electrodes were deconvoluted with two or three Voight functions, represent-ing V5+, V4+ and V3+. The relationship between the areas of the fitted peaks can be seen as a measure of the different oxidation states of vanadium. Bind-ing energies for the V 2p3/2 peak and relative areas of the fitted peaks are displayed in Table 2.

Table 2. PES binding energies (in eV) for the fitted peaks.i

V 2p3/2

V5+ V4+ V3+ O 1s

pristine 517.3 (0.60) 516.0 (0.40) 530.0 (1.01) 14.0 3.0 V 517.5 (0.41) 516.2 (0.59) 530.1 (0.83) 13.9 2.5 V 517.5 (0.34) 516.2 (0.66) 530.3 (0.83) 14.1 2.0 V 517.9 (0.18) 516.5 (0.59) 515.3 (0.23) 530.5 (0.69) 14.0 1.8 V 517.9 (0.23) 516.5 (0.51) 515.3 (0.26) 530.5 (0.64) 14.0 i) Numbers in parentheses give the area of the individual peak divided by the total V 2p3/2area. The energy difference between V4+ 2p3/2 and O 1s (in eV) is displayed as .

According to the PES results, pristine material contains approximately 40 % V4+, equivalent to an oxidation state of +4.60. The oxidation states of vana-dium estimated from PES spectra and those derived from electrochemical measurements agree well (paper III).

Oxygen atoms bonded to vanadium atoms give rise to a peak close to 530 eV in the O 1s spectra (Figure 15). Binding energies for this peak are displayed in Table 2, together with the area relative to the total V 2p3/2 peakarea. The pristine electrode had a second peak at 531 eV appearing as a shoulder, which originated from the binder and/or functional groups, such as hydroxyl groups or oxygen. As the electrode was discharged, a peak at 531-535 eV increased in intensity from relatively small at 3.0 V to dominant at 1.8 V. A peak at this binding energy is commonly found in electrodes after electrochemical cycling and can be attributed to electrolyte decomposi-tion products present in a solid electrolyte interface (SEI).146,161-163 This peak was fitted with one Voight function but more likely it consists of several contributions.

42

525 522 519 516 513

e

Binding energy (eV)

V 2p1/2 V 2p3/2

+3

+4

+5

d

Inte

nsity

(arb

. uni

ts)

c

b

+4+5a

536 534 532 530 528

e

Binding Energy (eV)

d

Inte

nsity

(arb

. uni

ts)

c

b

O 1sa

Figure 15. PES spectra for the pristine electrode (a) and electrodes discharged to 3.0 V (b), 2.5 V (c), 2.0 V (d) and 1.8 V (e). Left: V 2p spectra. The fitted Voight functions for V5+, V4+ and V3+ are marked in the spectra. Right: PES O 1s spectra. The solid lines represent the individual peaks and the total fit. The experimental data are represented by dots. Excitation source, Al K (h = 1486.6 eV).

The relatively small size of this peak in the electrode submitted to one cycle and discharged to 3.0 V suggests that the process is reversible and also po-tential dependent. A possible dissolution of the SEI layer depends on the nature of the electrolyte and the chosen potential range, as well as on the electrode material itself. Reversible formation of a polymeric-type coating around nanoparticles of CuO164 and CoO165, and other metal oxides,166 has been found to occur at potentials below 1 V. It is possible that a similar dis-solution process occurs at higher potentials. What the exact constituents of this potentially reversible SEI layer are, and how it is connected to the poten-tial is not known.

43

The presence of V3+ at low potentials is not only a surface effect, as could be seen from the RSXES results presented in paper IV. These data concurred with the PES and SXAS result in that V3+ is present at potentials below 2.0 V. By tuning the excitation energies to different features in the absorp-tion spectra (Figure 14), V L emission from a specific vanadium site, that is V5+, V4+ or V3+, can be enhanced. Emission spectra excited at 520.5 eV, in the valley between L3 and L2 in the absorption spectra (seen in Figure 14) for the electrode submitted to one pre-cycle and the four electrodes cycled to different potentials and the are shown in Figure 16. This specific choice of excitation energy facilitates exploration of the V3+ states. The sharp feature at 520.5 eV is the elastic peak, which is at the same energy as the excitation energy. Two features, centered at 509 and 515 eV, dominate the V L emis-sion. They originate from V 3d states strongly hybridized with O 2p (O 2p-h,in Figure 16) and states of pure V 3d character projected density of states (PDOS).

Inte

nsity

(a.

u.)

530525520515510505500495

Energy (eV)

Eexc= 520.5 eV

Cell potential

1.8 V

2.0 V

2.5 V

3.0 V

3.3 Vdd-excV3dO 2p-h

Figure 16. Resonant V L emissions of lithiated vanadium oxide nanotubes excited at 520.5 eV. The electrode submitted to one cycle had an OCP of 3.3 V (bottom spec-trum).

As the material was discharged, that is, as vanadium was reduced, the ratio of these two features changed. With increasing lithiation the spectral weight of the V 3d PDOS increased and the O 2p-h PDOS feature became broader. The broadening comes from an additional intensity around 507 eV originat-ing from states which are strongly hybridized with the V 3d component (O 2p-V 3d). Hence, the degree of lithiation has a pronounced effect on the V-O hybridization. The ratio of these two components is sensitive to the choice of excitation energy due to the simultaneous existence of several va-nadium oxidation states.

As discussed in paper IV, the ratio between the V 3d and O 2p-h peaks can be used to indicate the presence of V3+ in the material. If the ratios are

44

compared to those of reference compounds it becomes clear that in vana-dium oxides containing V3+ the V 3d feature is dominating. This is also the case for the electrodes discharged to potentials below 2.0 V.

In summary, the spectroscopic measurements showed that vanadium is reduced when the nanotubes are discharged, as could be expected. Below 2.0 V, V3+ started to form, and this may have a large impact on the vanadium oxide structure. The spectroscopic measurements alone can, however, not give any answers on the specific lithium insertion mechanism. The electronic structure must be related to the atomic structure.

4.3.2 Atomic structure In situ XRD measurements of the 001 reflection performed on Na+ contain-ing vanadium oxide nanotubes, presented in paper I, revealed small changes in the peak position during discharge and charge. Unfortunately, the inter-pretation was difficult due to very broad peaks, and the low intensity radia-tion used for the experiment. Ex situ XRD measurements on a pristine cell and on a cell submitted to 100 cycles were also presented in paper I. These data showed a clear shift in the 001 reflection to higher 2 for the cycled electrode.

In situ synchrotron XRD, presented in paper V, proved to be a more ef-fective tool to study the vanadium oxide nanotubes, allowing exploration of the interlayer distance as well as of the intralayer structure. Time-resolved measurements were made with a resolution of approximately 15 s (time be-tween two consecutive measurements).

The powder diffraction pattern for the Ca2+-containing vanadium oxide nanotubes compared to the pattern for an electrode enclosed in a polymer coated aluminum pouch are shown in Figure 17. The latter reveals peaks from the binder and from aluminum as well as several minor peaks originat-ing from aluminum oxide and pouch materials. There is a slight mismatch at low angles between the powder pattern and the pattern from the electrode. The 001 reflection is shifted to higher 2 for the electrode compared to the powder sample, indicating a decrease in interlayer distance. Probably this contraction of the vanadium oxide layers is induced by removal of embedded water, remnant from the ion-exchange procedure, and/or rearrangement of the Ca2+ ions caused by the heating step during electrode preparation (see chapters 3.1 and 3.2). The intensity ratios between the 001 reflection and the hk0 reflections are clearly different for the electrode compared to for the pure powder (Figure 17). Preferred orientation of the nanotubes in the elec-trode film may have caused this altered intensity distribution.

The interlayer distance decreased further to 8.79(5) Å as the electrode was incorporated into a battery as can bee seen for the pattern marked P in Figure 18 a. Interactions with solvent molecules and/or Li-ions from the electrolyte could therefore have caused the additional compression of the

45

vanadium oxide layers. Li-ions have a smaller ionic radius than Ca-ions (76 pm and 100 pm, respectively), which means that an exchange of Ca-ions for Li-ions would result in a decreased interlayer distance. The amines can not be spontaneously exchanged for Li-ions during the commonly used ion-exchange process.7 However, it is possible that the environment in the cell, where aprotic solvents are coordinated to the Li-ions, favors such a substitu-tion. It is important to note is that all pristine batteries had the same d-value for the 001 reflection

10 20 30 40 50 60(220)

(420)

(330)

(400)

(320)

(210)

(200)

(100) (310)

(001)

*

*****

Inte

nsity

2 (degrees)

Figure 17. Powder X-ray diffractograms ( = 0.9457 Å) of a Ca2+-VOx powder (bot-tom) and a Ca2+-VOx electrode enclosed in the polymer laminated aluminum pouch (top). Diffraction peaks from Al are marked with ( ) and from the binder material with ( ). The reflections of interest are marked with their hkl-assignments in the powder pattern.

Diffractograms recorded during a potential step from OCP (~3.4 V) to 1.71 V are shown in Figure 18, together with the corresponding current tran-sient. Two regions of the diffractogram have been chosen to highlight the reflections of interest: (a) the 001 and 100 peaks at 2 ~7 and ~10 respec-tively and (b) the 310 peak at 2 ~33 . The other hk0 reflections showed similar behaviors, but were difficult to analyze properly and are therefore not included. The bottom patterns, marked P in Figure 18 a, b, were obtained with the pristine battery, before the potential step was made. Pattern A was recorded 2 min after the potential step, and the corresponding position in the current transient is marked in Figure 18 c. The uppermost diffractogram, marked F in Figure 18 a, b, was recorded after 2 h.

When a potential was applied, the 001 reflection exhibited a shift from 7.1º to 7.4º corresponding to a decrease in interlayer distance from 8.79 Å to 8.43 Å. If the diffraction patterns recorded before 2 min are considered it is clear that the shift of the 001 peak occurred gradually and not in a single step

46

(paper V). A shift of the 310 reflection, belonging to the intralayer vanadium oxide structure, occurred during a much longer time period than the shift of the 001 reflection (Figure 18 b).

0 1000 2000 3000 4000 5000-7-6-5-4-3-2-101

31 32 33 34

6 8 10 12

cEX

DCB

A

XX

X

X

Cur

rent

(mA)

Time (s)

Al (310)

F

AP

b

Inte

nsity

(arb

. uni

ts)

2 (degrees)

(100)

(001)

F

AP

a

Inte

nsity

(arb

. uni

ts)

2 (degrees)

Figure 18. In situ XRD measurements ( = 1.0886 Å) during the potential step from OCP to 1.71 V: (a) the 001 and 100 reflections, (b) the 310 reflection, (c) the corre-sponding current transient (X mark the positions of the diffractograms). The diffrac-tograms were recorded after (from bottom to top): P) pristine battery, A) 2 min (120 s), B) 7 min (420 s), C) 14 min 30 s (900 s), D) 25 min (1500 s), E) 1 h (3600 s) and F) 2 h (not shown in 18 c).

After seven minutes, almost all of the charged had passed, as can be seen in the current transient (Figure 18 c), but, the 310 reflection continued to shift. This indicates that the vanadium oxide structure relaxes as the Li-ions diffuse into more energetically favorable sites in the interlayer spacing, fol-lowing the initial flow of Li-ions in between the vanadium oxide layers. The 100 reflection did not move at all during the potential step to 1.71 V and the

47

same trends could be seen for the other cathodic potential steps. For the an-odic steps, the 310 reflection returned to its original position, but the 001 peak did not shift back to lower 2 . Possibly, the decrease of the inter-layer distance is a first-cycle effect.

Figure 19 a, b show diffraction patterns for cathodic steps from OCP, recorded when the current had dropped below 4 A. The latter occurred after 1-2 h depending on the size of the potential step ( Estep), implying a system close to equilibrium. The 001 reflection remained at the same position inde-pendent on Estep. For the 310 reflection, the dependence of the potential was evident since an increasing shift could be detected as Estep increased. As is seen in Figure 19 b, the 310 reflection finally merged into the aluminum peak.

6 8 10 12 31 32 33 34

6 8 10 12 31 32 33 34

(100)(001)

a

Inte

nsity

(arb

. uni

ts)

2 (degrees)

Cha

rge

Dis

char

ge

(310)Alb

Inte

nsity

(arb

. uni

ts)

2 (degrees)(100)

(001)

c

Inte

nsity

(arb

. uni

ts)

2 (degrees)

(310)Ald

Inte

nsity

(arb

. uni

ts)

2 (degrees)

Figure 19. In situ X-ray diffractograms ( = 1.0886 Å) for (a, b) the cathodic poten-tial steps from OCP to 2.62 V, 2.47 V, 2.15 V, 1.71 V and 1.52 V (from bottom to top), respectively, and (c, d) anodic (charge) potential steps from ~1.5 V to: 1.87 V, 2.04 V and 2.78 V (from bottom to top), respectively. The frames were recorded when I(t) <4 A.

Diffractograms obtained for the three anodic steps are shown in Figure 19 c, d. During charge, the 310 peak emerged again, and after the largest anodic step (to 2.78 V) it had returned to 32.9 (Figure 19 d). Only a very small shift of the 001 reflection towards lower 2 could be detected for the largest anodic step.

Summarizing the results from the in situ synchrotron XRD study, it be-comes clear that there are at least two processes occurring as the Li-ions

48

enter into the vanadium oxide nanotubes. Initially, Li-ions are inserted be-tween the vanadium oxide layers. This is a fast process, which occurs in a fairly disordered way and causes the interlayer distance to decrease. Secondly, the Li-ions diffuse to find energetically favorable sites in the inter-layer spacing. This process affects the intralayer vanadium oxide structure.

The reduction of vanadium is accompanied by an increase in its coordina-tion number. Different possible coordination polyhedra are shown in Figure20. Tetrahedral vanadium is always in the oxidation state +5, while vana-dium in oxidation state +3 are always octahedrally coordinated. Vanadium in oxidation state +5 and +4 can adopt three types of five-coordinated arrange-ments: trigonal bipyramides, square pyramids and distorted octahedra. Ini-tially, the vanadium oxide sheets in the nanotubes are composed of double layers of square pyramids separated by tetrahetra (Figure 3).

Figure 20. Different vanadium-oxygen coordination polyhedra. The figure is based on Fig. 1 in Zavalij et al., Acta Cryst. B55, 627 (1999).

When lithium is inserted between the sheets, vanadium is reduced from V5+

to V4+ and from V4+ to V3+, as could be seen by the spectroscopic data dis-cussed in chapter 4.3.1. The formation of regular octahedra around vanadium in oxidation state +3 will cause buckling of the double layers. This will lead to changes in the cell parameters, thereby producing a shift in the reflections, as seen in the diffraction patters. Discharging the material to 1.2 V resulted in a decreased electrochemical performance, as was discussed in chapter 4.2.2. If a large amount of the vanadium in tetrahedral arrangements is re-duced, and thereby increase their coordination number, the structure will be significantly altered. Evidently the reorganization of the layers does not allow lithium ions to be inserted to the same extent as in the original struc-

49

ture. However, since only a limited number of reflections could be used, it is difficult to elaborate further on the precise lithium insertion process.

4.3.3 The oxidation state dilemma As was stated in chapter 4.3.1, the experimentally measured vanadium oxi-dation state for Na+ containing vanadium oxide nanotubes was +4.60 (40 % V4+). Magnetic measurements on tubes synthesized with ten different templating primary amines performed by Krumeich et al., showed that the tubes contained 33-66 % V4+.31

The proposed structure is similar to that of BaV7O16 H2O. In this structure the vanadium must have the average oxidation state +4.28 to balance the charges from oxygen and Ba2+. Two guest charges per seven vanadium atoms (0.28 mol charges per mol vanadium) are also what have been ex-perimentally found. The proposed structure model predicts a V4+ content of around 72 %. If the experimentally obtained oxidation states are compared solely to the vanadium oxide, not taking in to account the positive charge of the guest ion, the fit is better. V7O16 has a mean oxidation state of +4.57, containing 43 % V4+. However, there is a problem with the embedded posi-tively charged ions.

This mismatch in oxidation state between experiments and theory can have several possible origins. Perhaps the vanadium oxide structure holds a larger oxygen content than predicted due to the defect-rich structure. An-other possibility is that the positively charged ions are balanced from another source than the vanadium oxide structure. PES is a surface sensitive method, and the measured oxidation states could be a surface effect.

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5 Concluding remarks

Reality is merely an illusion, albeit a very persistent one.

- Albert Einstein

This thesis has discussed the performance of vanadium oxide nanotubes in a lithium battery context. The four questions asked in the end of chapter 1 have been completely or partly answered, and here follows a brief summary of the main findings.

Can vanadium oxide nanotubes compete with other known electrode ma-terials?

Yes, the material can be compared to other cathode materials. Lithium inser-tion into the tubes occurs reversibly, and can be repeated for more than 100 cycles with capacities around 150 mAh/g. However, for a potential commercial use, there are other more obvious choices from the vanadium oxide family, for example the V2O5 aerogels and xerogels. These materials can display high capacities (over 500 mAh/g) and are fairly easy to synthe-size.

Which are the prerequisites for optimal performance?

Ion-exchange is crucial to the materials performance as electrode material. Tubes with embedded amine molecules do not cycle as well as those con-taining metal ions (Na+, K+ or Ca2+). The most stable cycling behavior is seen for Ca-exchanged tubes cycled with an electrolyte based on LiTFSI salt. Also, the performance is sensitive to the preparation of the electrode film. Porous electrodes and/or small aggregates of nanotubes seem to per-form better than dense films and/or large aggregates. Initially, the electrode mixtures were stirred with a magnetic stirrer due to the small quantities of available material. Later, the electrode films were prepared by using ball-milling, which produced better films without destroying the tubular struc-ture.

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Do the nanometric dimensions provide interesting properties not seen for other vanadium oxides?

Reports on nanotubular electrode materials are not commonly found in the literature. Vanadium oxide nanotubes are therefore interesting to study due to their unique shape and size. However, in this case, the nanometric dimen-sions are probably not the most important factor. More likely the vanadium oxide sheet structure holds the key to successful lithium insertion. The rate properties are not good enough to provide a niche for this nanosized vana-dium oxide as high rate material.

What is the mechanism for lithium insertion?

Characterizing and understanding the lithium insertion mechanism into the vanadium oxide nanotubes has not been totally straightforward. It is not a well-ordered crystalline material in the normal sense. The vanadium oxide layers are curved, some in a scroll-like manner and some forming closed cylinders. The scrolls also normally contain defects, adding to the non-periodicity.

Lithium ions will reside between the layers, and not inside the vanadium oxide structure. This means that there must inevitably be some interaction between Li-ions and the originally embedded guests. These guests can also participate in the electrochemical process. Any clear evidence for this par-ticipation has, however, not been found.

In situ X-ray diffraction experiments showed that the interlayer distance decreases as the material is discharged, followed by a slower relaxation of the structure within the vanadium oxide layers. Lithium insertion induces a rearrangement of the structure due to the reduction of vanadium, which leads to increased coordination numbers. Spectroscopic measurements showed that vanadium is reduced to V3+ at potentials below 2.0 V, and that the three oxidation states (V5+, V4+ and V3+) co-exist below that potential limit. Dis-charging the material too far induces irreversible phase transitions as a result of the increasing V3+ content. The precise lithium insertion mechanism could not be established due to the small number of usable reflections in the diffractograms.

Many questions still remain concerning this material. For example: What happens to the structure at low potentials? How and why are the properties of the material dependent on the embedded guest? Can single tubes be aligned? Why does the total vanadium oxidation state differ between the experimentally obtained values and the theoretical values? There are certainly much more to be discovered about vanadium oxide nanotubes, and scientific reports on this fascinating material will most likely continue to appear. This thesis, however, has come to an end. Hopefully it has provided

52

both answers and new questions concerning the use of this material in lith-ium ion batteries.

53

Acknowledgements

Actors who are rewarded with an Oscar always have a long list of people to thank. And so do I.

My first thank you goes to my two advisors, Kristina Edström and Torbjörn Gustafsson, for skillfully guiding me through the research jungle. Thank you also prof. Josh Thomas for nice scientific discussions and for initially including me in the battery group.

I have worked with many great scientists during these five years. Thank you all co-authors: Andreas Augustsson, Thorsten Schmitt, Laurent Duda, prof. Joseph Nordgren (Physics Department, Uppsala University) and Jinghua Guo (Advanced Light Source, LBNL Berkeley, USA) for introduc-ing me to the world of soft X-ray physics. Jun Lu (Department of Engineer-ing Sciences, Uppsala University) and prof. Eva Olsson (Chalmers Univer-sity of Technology, Göteborg) for magnificent TEM micrographs. Prof. Leif Nyholm and prof. Jan Lindgren (Department of Materials Chemistry, Upp-sala University) for valuable inputs concerning electrochemistry and Raman spectroscopy.

I could not have managed without the help of the lovely administrative staff. A big hug to: Ulrika Bergvall, Eva Larsson, Gunilla Lind and Katarina ”Tatti” Israelsson. When computer problems appeared, instruments refused to co-operate, or chemicals were missing I called on Nisse Ersson, Peter Lundström, Anders Lund or Hilding Karlsson. They have always found a way to solve any possible, or impossible, problem with a smile on their faces. Thank you!

Thank you prof. Elton J. Cairns, prof. Jeffrey A. Reimer, Young-Joo Lee, Mike Tucker and the rest of the guys in the NMR basement, who took such good care of me during my stay at University of California at Berkeley.

I would also like to acknowledge: Ångpanneföreningens Forskningsstif-telse, Stiftelsen Blanceflor, Stiftelsen Bengt Lundqvists minne, Vetenskaps-rådet, Svenska Kemistsamfundet, Liljewalchs stiftelse and Rektors Wallen-bergmedel. Their financial aid made it possible for me to present my results in various places of the world.

Thank you everyone at the department of materials chemistry, both the oldies and the new gang, for keeping me company at the coffee table. Spe-cial thanks to Mina, Therese and Sabina for being great friends, Jonas my sci-fi companion and structure guru, J.P. for luring me into the PhD board, my room mate Daniel I will never forget the West Ham tune… Marie,

54

thanks for the brilliant comments on this thesis. To all past and present members of Ångström Advanced Battery Centre (you know who you are), may the force be with you!

Slutligen har jag några tack att framföra på svenska. Frida-konstprinsessan, du har alltid varit en klippa. Anders - globetrottern, vart går nästa resa? Alla vänner och bekanta bosatta i Norrlands Aten och på resten av jordklotet. Tack familjen Eriksson (och de håriga monstren) för er fantas-tiska gästfrihet. Hela Nordlinders/Edbergs-klanen: mamma, pappa, bror, farmor, moster, morbror och kusinerna ni är bäst! Mormor, morfar och farfar ni kommer alltid att finnas med mig.

Tom och Märta, ni gör mitt liv komplett. Jag älskar er för evigt och evigt och evigt...

Uppsala, den 31 maj 2005

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