Research Article Enhanced Structural Integrity and …downloads.hindawi.com/journals/isrn/2014/359019.pdf · 2017. 12. 4. · Research Article Enhanced Structural Integrity and Electrochemical

Research ArticleEnhanced Structural Integrity and ElectrochemicalPerformance of AlPO4-Coated MoO2 Anode Material forLithium-Ion Batteries

Joseacute I Loacutepez-Peacuterez123 Edwin O Ortiz-Quiles14 Khaled Habiba12

Mariel Jimeacutenez-Rodriacuteguez123 Brad R Weiner134 and Gerardo Morell123

1 Institute of Functional Nanomaterials University of Puerto Rico Rio Piedras Campus San Juan PR 00931-3334 USA2Department of Physics University of Puerto Rico-Rio Piedras Campus San Juan PR 00936-8377 USA3 Center for Advanced Nanoscale Materials University Research Center University of Puerto Rico-Rio Piedras Campus San JuanPR 00931-3346 USA

4Department of Chemistry University of Puerto Rico Rio Piedras Campus San Juan PR 00931-3346 USA

Correspondence should be addressed to Jose I Lopez-Perez lopezjoseismaelgmailcom

Received 24 December 2013 Accepted 15 January 2014 Published 4 March 2014

Academic Editors S-M Lee and E Valles

Copyright copy 2014 Jose I Lopez-Perez et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

AlPO4nanoparticleswere synthesized via chemical depositionmethod andused for the surfacemodification ofMoO

2to improve its

structural stability and electrochemical performance Structure and surfacemorphology of pristine andAlPO4-coatedMoO

2anode

material were characterized by electron microscopy imaging (SEM and TEM) and X-ray diffraction (XRD) AlPO4nanoparticles

were observed covering the surface of MoO2 Surface analyses show that the synthesized AlPO

4is amorphous and the surface

modification with AlPO4does not result in a distortion of the lattice structure of MoO

2 The electrochemical properties of pristine

and AlPO4-coated MoO

2were characterized in the voltage range of 001ndash25 V versus LiLi+ Cyclic voltammetry studies indicate

that the improvement in electrochemical performance of the AlPO4-coated anodematerial was attributed to the stabilization of the

lattice structure during lithiation Galvanostatic chargedischarge and electrochemical impedance spectroscopy (EIS) studies revealthat the AlPO

4nanoparticle coating improves the rate capability and cycle stability and contributes toward decreasing surface layer

and charge-transfer resistancesThese results suggest that surfacemodificationwithAlPO4nanoparticles suppresses the elimination

of oxygen vacancies in the lattice structure during cycling leading to a better rate performance and cycle life

1 Introduction

Lithium ion batteries are extensively used in a variety ofportable electronic devices due to their high power densityand long cycle life [1] As reported they are criticallyimportant for electrichybrid vehicles as the power storage ofthe future [2] Therefore lithium ion batteries have attractedmuch interest in the field of fundamental study and appliedresearch Most commercialized lithium ion batteries usegraphite as an anode material due to its accessibility andlow cost but its theoretical capacity is only 372mAhsdotgminus1calculated by forming the compound of LiC

6and cannot

meet the ever-increasing demands for high capacity lithium

ion technology [3] By replacing graphite with transitionmetal oxides as anode materials the capacity is enhancedThis is due to their close packed oxygen array providing aframework structure and specific site for topotactic insertionand removal of lithium ions during chargedischarge processA number of transition metal oxides have been studied andreported so far including Mn

3O4 Co3O4 MnO TiO

2 NiO

MoO2

and SnO2 because of their possibility of various

oxidation states and the search of new materials for energystorage [3 4]

In order to improve structural stability and electrochem-ical behavior many groups have demonstrated that the addi-tion of a thin coating of metal phosphates fluorides oxides

Hindawi Publishing CorporationISRN ElectrochemistryVolume 2014 Article ID 359019 12 pageshttpdxdoiorg1011552014359019

2 ISRN Electrochemistry

or other analogous materials onto the cathode particle resultsin reduced irreversible capacity improved rate capabilityand cycle life [5] Surface modification of the electrodematerial by substitution is an effective method to improvethe electrochemical properties [6] Such substitutions areusually done for electrochemically active elements causinglower capacity and Li+ diffusion because the substitutionsare usually electrochemically inactive ingredients A coatingapproach is beneficial with respect to delivery of the initialcapacity because there is no reduction of the amount ofelectrochemically active element in the electrode materialTherefore a small amount of coating on the surface of elec-trode materials can improve the electrochemical properties[7ndash9]The improvements in performance of these lithium ioncathodes by surface modification via the addition of coatingshave been attributed to a diverse series of mechanismssuch as the coating promoting the retention of oxide ionvacancies in the crystal lattice after the first charge [10]suppression of the decomposition of the electrolyte [11] andthe maintenance of low microstrain for better structuralintegrity and crystallinity during cycling [12]

Aluminum phosphate (AlPO4) an environmentally fri-

endly lower cost and thermally stable material is of greatinterest in both environmental and technological fields [13]With regard to the application of AlPO

4for lithium ion

batteries other groups reported improvement concerningthe safety and the electrochemical properties of the cathodematerials by applying a direct coating of AlPO

4nanoparticles

from an aqueous solution [14ndash16] Jiao et al [17] successfullyprepared AlPO

4-coated LiV

3O8powders by mixing active

material LiV3O8

with AlPO4

nanoparticle suspensionfollowed by a low temperature heat treatment The AlPO

4-

coated material was found to reduce the capacity fadingsignificantly Manthiram and Wu [18] studied the effectsof surface modification of Li

2MnO3and LiMO

2(where M

= Mn Ni and Co) solid solutions modified with 3wtAl2O3 CeO

2 ZrO

2 SiO

2 ZnO AlPO

4and 005 atom

Fminus per formula unit and were characterized by XRD andchargedischarge measurements in lithium cells Among allcoating materials results showed that the AlPO

4modified

sample had the largest reduction in irreversible capacitycompared to the rest of the samples modified with differentcoatings Cho [19] reported that LiCoO

2cathodes coatedwith

AlPO4have improved their electrochemical performance

due to the formation of homogeneous surface layers incontrast with other coating materials (Al

2O3and ZrO

2)

Recently MoO2 with a theoretical reversible capacity

of sim838mAhsdotgminus1 has received much attention and hasbeen considered as a promising anode material in lithiumion batteries because of its low electrical resistivity highelectrochemical activity and high chemical stability [20]One of the intrinsic drawbacks of MoO

2for lithium ion

battery applications is its volume expansion during Li+insertionextraction process The irreversible volume changecauses MoO

2particles to pulverize and crack causing

the detachment of the active material from the currentcollector and consequently leading to a substantial loss incapacity [21] In this context we hereby present a study of

the effects of AlPO4nanoparticle coating on the structural

and electrochemical properties of MoO2anode material

2 Experimental

Commercially available high purity chemicals were directlyused without further purification Pristine MoO

2powder

(Molybdenum (IV) oxide Sigma Aldrich) was sintered at350∘C for 2 hours and ground thoroughly with an agatemortar and pestle until a fine and homogeneous powder wasobtained To prepare AlPO

4-coated MoO

2 stoichiometric

amounts of aluminum nitrate nonahydrate (Al(NO3)39H2O-

98 Alfa Aesar) and ammonium hydrogen phosphate((NH4)2HPO4 Alfa Aesar) were dissolved separately in

nanopure water Ammonium hydrogen phosphate solutionwas slowly added to the aluminum nitrate nonahydratesolution until a white AlPO

4nanoparticle suspension was

observed MoO2powder with an average particle size of

sim5 120583m was added to the coating solution and stirred thor-oughly for 2 hours The amount of AlPO

4in the solution was

sim3wt of the MoO2powder The solution was then filtered

dried at room temperature in air and sintered at 400∘C for 4hours in flowing argon

21 Electrode Preparation Electrodes were prepared by spraycoating Cu foil substrates with slurries of 90wt anodepowder 5 wt carbon black (100 compressed 995metalbasis Alfa Aesar) and 5wt PVDF binder (poly-vinylidenefluoride Alfa Aesar) in 1-Methyl-2-pyrrolidinone (anhy-drous 995 SigmaAldrich)The pristine andAlPO

4-coated

MoO2electrode materials were used as working electrodes

Coin cells were assembled inside an argon-filled glove box(M Braun USA) using stainless steel CR2032 coin cellhardware Li metal foil was used as the counter and thereference electrode (075mm thick times 19mm wide 999metal basis Alfa Aesar) Electrodes inside the coin cell wereseparated using a Celgard 2400 membrane Lithium hexaflu-orophosphate (LiPF

6) dissolved in a 1 1 molar ratio solution

of dimethyl carbonate (DMC) and ethylene carbonate (EC)was used as the electrolyteMultiple coin cells were assembledin order to validate the reproducibility of the surface analysisand electrochemical experiments

22 Imaging and Surface Analysis Characterization PowderX-ray diffraction (XRD) measurements were carried outusing a Rigaku Ultima III X-ray diffractometer (Cu K120572radiation Rigaku Japan) at an accelerating potential of40 kV and a tube current of 20mA to identify the crystallinephase of the synthesized pristine powders and AlPO

4-coated

powders before and after lithiation XRD data were collectedat 3∘minminus1 in the 2-theta range of 20ndash80∘ Field emissionscanning electron microscopy (FE-SEM JSM-7500F JEOLJapan) was employed at working voltage of 15 kV to studythe surface morphology of the prepared powders and cycledelectrodes Transmission electron microscopy (TEM CarlZeiss-LEO 922 Germany) at a working voltage of 200 kV andequippedwithX-rays energy dispersive spectroscopy (XEDS)was used to determine the morphology and composition of

ISRN Electrochemistry 3

5120583m

(a)

Fractures

5120583m

(b)

5120583m

(c)

5120583m

(d)

Figure 1 Scanning electron microscopy of pristine MoO2electrodes (a) before and (b) after cycling and AlPO

4-coated MoO

2electrodes (c)

before and (d) after cycling

the pristine and AlPO4-coated samples The samples were

placed in a copper grid

23 Electrochemical Characterization Cyclic voltammetry(CV) tests were carried out at room temperature on a SeriesG-750 PotentiostatGalvanostatZRA Gamry workstation inthe potential window of 001ndash25 V versus LiLi+ at a scan rateof 02mV sminus1 Galvanostatic charge and discharge capacitycycles were also carried out in this workstation at currentdensities of 50 100 and 200mAsdotgminus1 between 001ndash25 V ver-sus LiLi+ at room temperature Electrochemical impedancespectroscopy (EIS) measurements were performed on aPARSTAT 2273 PotentiostatGalvanostat (Advanced Mea-surement Tech Inc) with an applied AC signal amplitudeof 5mV peak-to-peak over a frequency range of 1MHz to10mHz

3 Results and Discussion

31 Imaging and Surface Analysis Characterization

311 Scanning Electron Microscopy (SEM) The morphologyof the pristine and AlPO

4-coated MoO

2electrodes before

and after cycling is shown in Figure 1 in the scanning electron

microscopy (SEM) images Before cycling the two powderswere generally indistinguishable from one anotherThey havean average size of sim5 to 10 120583m indicating that the AlPO

4

coating did not lead to clumping or any other observablechange in the microstructure of the anode particles Incomparison cracks and crumbles are observed in the pristinematerial after cycling (Figure 1(c)) as a result of the largevolume expansion during lithium insertionextraction Thiscracking and crumbling during cycling keeps generating newactive surfaces that were previously passivated by the stablesurface films [22] Such cracks and crumbles are not observed(Figure 1(d)) in the AlPO

4-coated MoO

2after cycling It is

quite likely that the AlPO4nanoparticle coating significantly

reduces the formation of surface cracks induced by thevolume expansion of the electrode material and thereforediminishes the repetitive formation of electrodeelectrolyteinterfaces affecting the capacity fading [22]

312 Transmission Electron Microscopy (TEM) and X-RayEnergy Dispersive Spectroscopy (XEDS) TEM images of pris-tine and AlPO

4-coated MoO

2anode material were collected

in order to determine the nature of the AlPO4coating

nanoparticles Figure 2(b) shows the coreMoO2anodemate-

rial uniformly covered by the AlPO4nanoparticles Study

4 ISRN Electrochemistry

MoO2

1120583m

(a)

500nm

MoO2

AlPO4

coating

(b)

200nm

MoO2

AlPO4

coating

(c)

Figure 2 Transmission electron microscopy (TEM) images of (a) pristine MoO2 (b) AlPO

4-coated MoO

2 and (c) AlPO

4nanoparticle

coating

at higher magnification (Figure 2(c)) further reveals that theAlPO

4nanoparticle coating consists of uniformparticleswith

an average diameter of sim80 nm The distribution of Al andP was examined by X-ray energy dispersive spectroscopy(XEDS) characterization technique and the results are dis-played in Figure 3 EDS data confirm the presence of Al andP in the coating layer and the absence of Al or P componentsin the pristine sampleThe presence of the Cu signal is due tothe copper grid used in TEM analysis

313 X-Ray Diffraction Analysis The XRD patterns of pris-tine MoO

2and AlPO

4-coated MoO

2powders are shown in

Figure 4 Figures 4(a) and 4(b) show the XRD patterns ofthe pristine andAlPO

4-coatedMoO

2powders before cycling

respectively Both powders were confirmed to bewell-defined

monoclinic structure with the space group of P21119899

withno additional diffraction patterns related to AlPO

4coating

layer Pristine and AlPO4-coated powders showed the same

lattice parameter values of 119886 = 5606 A 119887 = 4859 Aand 119888 = 5537 A (JCPDS card 32-0671) revealing thatthe AlPO

4coating was not incorporated into the anode

material as no changes were perceived in the structure [23]Furthermore the two diffraction patterns overlap nearlyidentically indicating that the sintering treatment or otherprocedures involved with the AlPO

4coating did not result

in distortion of the crystal lattice [5] This result showsthat the AlPO

4is just coated on the surface of the MoO

2

powders [24] Peaks between sim40ndash45∘ are characteristic ofgraphite [25] while the peaks at sim50∘ and sim74∘ correspondto the Cu-foil substrate (JCPDS card number 04-0836) [26]As we want to evaluate if there are significant changes in

ISRN Electrochemistry 5

Cou

nts (

k)56

49

42

35

28

21

14

OCMo

Mo

Mo

310 610 910 1210 1510 1810 2110 2410

Cu

Energy (keV)

(a)

Cou

nts (

k)

P

C

Mo

Mo

Energy (keV)

Cu

CuAl

63

56

49

42

35

28

21

14

7

2 4 6 8 10 12 14 16 18 20

(b)

Figure 3 X-ray Electron Dispersion Spectroscopy (XEDS) data of (a) pristine MoO2and (b) AlPO

4-coated MoO

2anode materials

the lattice structure after cycling lithium cells were openedinside and argon-filled glove box to recover the electrodesThese electrodes were rinsed in EC dried under vacuumand studied exposed by XRD Figures 4(c) and 4(d) show theXRD data of the pristine and AlPO

4-coated MoO

2samples

after 50 cycles of galvanostatic charge and discharge In thepristine sample (Figure 4(c)) a careful inspection revealsthat diffraction peaks evolved in the 25∘ndash35∘ 2theta rangeThis peak evolution corresponding to Li

2O formation during

lithiation process [27] may indicate a partial interchange ofoccupancy of Li+ and transition metal ions giving rise todisordering in the lattice structure due to an irreversible lossof oxygen during cycling [28]This interchange of occupancyis known to deteriorate the electrochemical performance ofthe layered material [29 30] Such peaks are not observedin the AlPO

4-coated sample (Figure 4(d)) This probably

suggests that the evenly dispersed AlPO4coating suppresses

microstructural defects and structural degradation acting asa protective coating layer and therefore enhancing structuralstability of MoO

2electrode material

32 Electrochemical Characterization

321 Cyclic Voltammetry (CV) Studies Cyclic voltammetry(CV) of pristine and AlPO

4-coated MoO

2between 001ndash

25 V at a scan rate of 02mV sminus1 was performed at roomtemperature to understand the effect of AlPO

4coating on the

Li+ insertionextraction behavior of MoO2 Figure 5 shows

two pairs of redox peaks at sim123157V versus LiLi+ andsim150180V versus LiLi+ corresponding to the reversiblephase transition of Li

119909MoO2and MoO

2caused by the

insertion and extraction of lithium ions [3 31] According toprevious research [32 33] the two reactions correspondingto the two redox processes observed in the cyclic voltammo-grams in Figure 5 are as follows

MoO2+ 4Li+ + 4eminus 997888rarr Mo + 2Li

2O (1)

Mo + 119909Li+ + 119909eminus larrrarr Li119909MoO2

(2)

During discharge the lithium bonds to the oxygenin MoO

2 forming Mo metal and Li

2O Then the Mo

8000

7000

6000

5000

4000

3000

2000

1000

0

20 25 30 35 40 45 50 55 60 65 70 75 80

2120579 (deg)

Inte

nsity

(au

)

(a)

(b)

(c)

(d)

lowastlowast

lowastlowast

(111

)

(211

)

(222

)

(031

)

(402

)(204

)(411

)(413

)

(132

)

Figure 4 X-ray diffraction (XRD) patterns of (a) pristineMoO2and

(b) AlPO4-coated MoO

2before cycling and (c) pristine MoO

2and

(d) AlPO4-coatedMoO

2 Note the additional peaks of Li

2O (marked

by asterisk) after 50 cycles of galvanostatic charge and discharge

partially alloysdealloys up to the theoretical limit ofLi119909MoO2(sim838mAhsdotgminus1) For pristine MoO

2(Figure 5(a))

oxidation peaks slightly shift to higher potentials while thereduction peaks slightly shift to lower potentials (indicatedwith arrows) In addition as cycling proceeds oxidationand reduction peak intensities decrease rapidly This elec-trochemical behavior indicates the structural degradationof MoO

2anode material and an increase in the internal

resistance during cycling leading to the fast capacity lossof the pristine MoO

2anode material [24 34] Electrodes

suffer from capacity loss and poor rate capability becausethere are incomplete reversible phase transition and localstructural damages during lithiation On the other handit is observed that the AlPO

4-coated MoO

2(Figure 5(b))

shows better cycling stability compared to pristine MoO2

During cycling almost no oxidation and reduction peakshifts are observed suggesting a more stable lattice structureFurthermore the peak intensity declines much slower thanthat of the pristine MoO

2 indicating that capacity retention

is noticeably enhanced after the AlPO4nanoparticle coating

6 ISRN Electrochemistry

minus0002

minus0001

0000

0001

0002

0003

0004

149V148V

123V122V

182V

180V

184V

156V

159V

157V

124V 150V

Curr

ent (

A)

00 05 10 15 20 25

1st cycle5th cycle

10th cycle

Potential (V) (versus LiLi+)

(a)

minus0002

minus0001

0000

0001

0002

0003

0004

149V

123V122V

180V180V

181V155V

155V

157V

126V150V150V

Curr

ent (

A)

00 05 10 15 20 25

1st cycle5th cycle

10th cycle

Potential (V) (versus LiLi+)

(b)

Figure 5 Cyclic voltammetry (CV) of (a) pristine MoO2and (b) AlPO

4-coated MoO

2in the potential window of 001ndash25 V versus LiLi+ at

a scan rate of 02mV sminus1 with 1 1 molar solution of LiPF6as electrolyte

0 200 400 600 800 1000

00

05

10

15

20

25

Discharge

Charge

Capacity (mAhmiddotgminus1)

Pote

ntia

l (V

) (ve

rsus

LiL

i+)

(a)

0 200 400 600 800 1000

00

05

10

15

20

25

Discharge

Charge

Capacity (mAhmiddotgminus1)

Pote

ntia

l (V

) (ve

rsus

LiL

i+)

(b)

Figure 6 Initial chargedischarge curves of (a) pristine MoO2and (b) AlPO

4-coated MoO

2anode material at a current density of 50mAsdotgminus1

in the voltage range of 001ndash25 V versus LiLi+

322 Galvanostatic Charge and Discharge Capacity StudiesTo study the electrochemical performance of pristine andAlPO

4-coated MoO

2 charge and discharge capacities were

measured at a potential window of 001ndash25 V at currentdensities of 50 100 and 200mAsdotgminus1 at room temperatureThe first charge and discharge cycles for pristine and AlPO

4-

coated MoO2electrodes at a constant current density of

50mAsdotgminus1 are represented in Figure 6 The first cycle chargecapacity has been observed to be higher in the case of

the AlPO4-coated anode material (sim1008mAhsdotgminus1) com-

pared to the pristine anode material (sim625mAhsdotgminus1) Onthe other hand a higher first cycle discharge capacity isobserved in the case of AlPO

4-coatedMoO

2(sim1015mAhsdotgminus1)

compared to the pristine MoO2(sim650mAhsdotgminus1) These

enhanced first cycle charge and discharge capacities can beattributed to the effective removal of lithium and oxygenfrom the host structure [35] In both samples there are twoconstant potential plateaus at sim140 and 170V on the first

ISRN Electrochemistry 7

0 200 400 600 800 1000

00

05

10

15

20

25

middotgminus1)

Pote

ntia

l (V

) (ve

rsus

LiL

i+)

200mAmiddotgminus1 100mAmiddotgminus1 50mAmiddotgminus1

(a)

0 200 400 600 800 1000

00

05

10

15

20

25

Capacity (mAhmiddotgminus1)

200mAmiddotgminus1 100mAmiddotgminus1

50mAmiddotgminus1Pote

ntia

l (V

) (ve

rsus

LiL

i+)

(b)

Figure 7 Initial charge and discharge curves of (a) pristineMoO2and (b) AlPO

4-coatedMoO

2at current densities of 50 100 and 200mAsdotgminus1

between 001ndash25 V versus LiLi+ at room temperature

charge cycles as well as two potential plateaus at sim157 and13 V on the first discharge cycles These results are consistentwith those reported by Liang et al [33] since the inflectionpoints between these potential plateaus represent a transitionbetween monoclinic phase and orthogonal phase in thepartially Li

119909MoO2 It is clearly observed that surface modi-

fication with AlPO4nanoparticles can significantly improve

the electrochemical performance of MoO2anode material

PristineMoO2electrode shows an irreversible capacity (IRC)

of 25mAhsdotgminus1 during the first cycle while the AlPO4-coated

MoO2electrode shows an irreversible capacity of 7mAhsdotgminus1

during the first cycle The observed IRC and initial dischargecapacity values confirm that oxide ion vacancies are partiallyretained in the lattice during the initial charge In otherwords we can imply that surface modification suppresses theelimination of oxide ion vacanciesThis could be attributed tothe mechanism proposed by Armstrong et al [36] suggest-ing that surface modification suppresses the elimination ofoxygen vacancies during the initial charge and consequentlyallows a reversible insertionextraction of higher amountsof lithium in the subsequent discharge cycles [36] Figure 7shows the initial charge and discharge profiles of the pristineandAlPO

4-coatedMoO

2anodematerials at current densities

of 50 100 and 200mAsdotgminus1 As shown in Figure 7(a) theinitial discharge capacity of the pristineMoO

2is 434mAhsdotgminus1

at a current density of 100mAsdotgminus1 When the current densityis increased to 200mAsdotgminus1 pristineMoO

2only undergoes an

initial discharge capacity of 219mAhsdotgminus1 The pristine MoO2

exhibits a relatively poor rate capability Comparatively theAlPO

4-coated MoO

2exhibits an enhanced rate capability

as illustrated in Figure 7(b) The discharge capacities ofthe AlPO

4-coated MoO

2at current densities of 100 and

200mAsdotgminus1 are 647 and 341mAhsdotgminus1 respectively indicatingthat the AlPO

4nanoparticle coating significantly improves

rate capability The electrochemical data collected from thepristine and AlPO

4-coated MoO

2electrodes are denoted in

Table 1Now let us compare the cycle performance of pristine and

AlPO4-coated MoO

2electrodes considering the discharge

capacity as a function of cycle number for the first 50 cyclesas presented in Figure 8 At a current density of 50mAsdotgminus1pristine MoO

2exhibits an initial discharge capacity of

650mAhsdotgminus1 as discussed above It declines to 297mAhsdotgminus1after 50 cycles with a capacity loss of 54 By contrast theAlPO

4-coated MoO

2electrode delivers an initial discharge

capacity of 1015mAhsdotgminus1 It declines to 787mAhsdotgminus1 after50 cycles with a capacity loss of 22 Rate capabilitycycling stability and discharge capacities of the AlPO

4-

coated samples are improved after 50 cycles compared to thepristine samplesHowever with ongoing cycling lithium ionscan eventually penetrate the coating protective layer thusbecoming incorporated into the lattice of MoO

2 This can be

ascribed to the gradual elimination of oxygen vacancies inthe anode material which can be part of the reason for thecapacity fading during cycling Generally this improvementin the discharge capacity rate capability and cycling stabilitycan be explained due to the obstruction of the transitionmetal ions by theAlPO

4nanoparticle coating tomigrate from

the surface to the bulk in the vacant sites for the lithiuminsertion thereforemaintaining the high concentration of theavailable sites for lithium insertion [10] The AlPO

4coating

is an electronic insulator as reported by Kim et al [22]indicating that most of the oxidation and reduction reactionswith lithium ions and electrons occur mainly at the interfacebetween the anode material and AlPO

4coating and not at

the interface of AlPO4coating and electrolyte From these

results we conclude that AlPO4-coated anode material holds

better cycling performance compared to the pristine anodematerial

8 ISRN Electrochemistry

0 5 10 15 20 25 30 35 40 45 50100

200

300

400

500

600

700

800

900

1000

1100

Cycle number

Disc

harg

e cap

acity

(mA

hmiddotgminus

1)

AlPO4-coated MoO2

Pristine MoO2

50mAmiddotgminus1

(a)

0 5 10 15 20 25 30 35 40 45 50100

200

300

400

500

600

700

Cycle number

Disc

harg

e cap

acity

(mA

hmiddotgminus

1)

AlPO4-coated MoO2

Pristine MoO2

100mAmiddotgminus1

(b)

0 5 10 15 20 25 30 35 40 45 50

100

200

300

400

Cycle number

Disc

harg

e cap

acity

(mA

hmiddotgminus

1)

AlPO4-coated MoO2

Pristine MoO2

200mAmiddotgminus1

(c)

Figure 8 Discharge capacity as a function of cycle number of pristine MoO2and AlPO

4-coated MoO

2

Table 1 Electrochemical data of galvanostatic charge and discharge cycles for pristine and AlPO4-coated MoO2

Currentdensity(mA gminus1)

Pristine MoO2 AlPO4-coated MoO2

Initialdischargecapacity(mAh gminus1)

Initialcharge capacity

(mAh gminus1)

IRC(mAh gminus1)

Capacity lossafter 50 cycles

Initialdischargecapacity(mAh gminus1)

Initialcharge capacity

(mAh gminus1)

IRC(mAh gminus1)

Capacity lossafter 50 cycles

50 650 625 25 54 1015 1008 7 22100 434 413 21 mdash 677 673 4 mdash200 201 201 18 56 341 338 3 24

ISRN Electrochemistry 9

0 100 200 300 400 500 6000

100

200

300

400

500

600

OCVCharged

Discharged

Zi

(Ohm

)

Zr (Ohm)

(a)

0 100 200 300 400 500 600 7000

100

200

300

400

500

600

700

OCVCharged

DischargedZi

(Ohm

)

Zr (Ohm)

(b)

Figure 9 Electrochemical impedance spectroscopy (EIS) data of (a) pristine MoO2and (b) AlPO

4-coated MoO

2with an applied AC signal

amplitude of 5mV peak-to-peak over a frequency range of 1MHz to 10mHz EIS data were obtained after 3 cycles of galvanostatic charge anddischarge at room temperature

323 Electrochemical Impedance Spectroscopy (EIS) To bet-ter understand the reason for the enhanced electrochemi-cal properties of the AlPO

4nanoparticle coating electro-

chemical impedance spectroscopy (EIS) was carried out forthe pristine and AlPO

4-coated MoO

2anode materials The

electrochemical impedance data were obtained after 3 cyclesof galvanostatic charge and discharge at room temperaturesince the solid electrolyte interface (SEI) film is formed dur-ing the first few cycles and changes very little during ongoingcycling [37] EIS is an effective nondestructive technique tounderstand the various phenomena occurring at the interfacebetween the electrode and electrolyte It is used to determineelectrochemical cell impedance in response to a small ACsignal at constant DC voltage over a broad frequency rangefromMHz to mHz [38] Impedance spectroscopy is a crucialparameter to determine the electrochemical performance oflithium ion batteries With this characterization techniquedifferent electrochemical processes occurring inside lithiumion batteries such as charge transfer double layer capaci-tance and diffusion of ions in the electrode can be studiedby calculating the real and imaginary parts of the impedanceEIS measurements have been carried out on the lithium ionbatteries to examine the electrochemical systems involvinginterfacial processes and kinetics of electrode reactions forthe pristine MoO

2and the AlPO

4-coated MoO

2 The results

are shown in Figures 9(a) and 9(b) respectively in the formofNyquist plots Determining the possible equivalent circuit inorder to interpret the data is crucial in this electrochemicalcharacterization technique [39] The equivalent circuit usedfor fitting the impedance data is shown in Figure 10 From

Re

RctZw

Rsl

CPECPE

Figure 10 Equivalent circuit model for the EIS where CPE arethe constant phase elements119877emdashelectrolyte resistance119877slmdashsurfacelayer resistance 119877ctmdashcharge transfer resistance and 119885wmdashWarburgimpedance

the Nyquist plots it can be perceived that they are composedof two parts The first one is a suppressed semicircle inthe high-middle frequency region related to charge-transferprocess and the second one is an oblique straight linein the low frequency region representing typical Warburgimpedance

The suppression of the semicircle in the Nyquist plots isdue to the overlap of two different semicircles The appear-ance of two suppressed semicircles indicates the contributionof two different resistive elements to the total impedanceof the electrochemical cell This is observed generally inthe impedance plot due to the combination of a capacitorelement and a resistor element in parallel The semicircle inthe high frequency region corresponds to the resistance (119877sl)due to the surface layer or solid electrolyte interface (SEI)formation [40] Capacity fading of the anode material duringcycling is associated with the thickness of such layer on theanode particles During cycling the SEI layer grows thick due

10 ISRN Electrochemistry

Table 2 Electrochemical impedance spectroscopy (EIS) data parameters obtained after fitting based on the model shown in Figure 10

Fitted parameters Pristine MoO2 AlPO4-coated MoO2

OCV Charged Discharged OCV Charged Discharged119877e (Ohm) 159 749 132 774 724 1281119877ct (Ohm) 3136 1679 2887 244 1236 2717119877sl (Ohm) 3807 2181 3462 2457 1578 2837

to the electrodeelectrolyte reaction thus deteriorating theelectrochemical performance of the cell Middle frequencysemicircle corresponds to the charge transfer resistance (119877ct)across the interface and the low frequency oblique straightline arises due to the lithium ion diffusion in the bulk ofthe anode material [41] The intercept value on the 119909-axisin the high frequency region corresponds to the resistance(119877e) due to the lithium ion conduction in the electrolyte[41] Depression in the semicircle has been calculated byplacing constant phase elements (CPEs) instead of purecapacitance as shown in the equivalent circuit Impedanceparameters obtained after fitting the EIS experimental dataare summarized in Table 2

By analyzing the datawe observed that themain influenceto the impedance is from the charge transfer resistance(119877ct) and surface layer resistance (119877sl) 119877e behavior has beenobserved to be similar in both samples In the charged stateit is observed that the 119877ct value for the AlPO4-coated MoO

2

is lower compared to that of the pristine MoO2 and an

increase in 119877sl is observed respectively This increase in thevalue of 119877sl is expected due to the growth of the SEI layer atthe electrodeelectrolyte interface In the case of the AlPO

4-

coated sample the decrease in the 119877ct value can be explaineddue to the fact that during cycling irreversible extractionof the oxygen and lithium occurs creating vacancies inthe crystal structure of the anode material and thereforeleading to the decrease in the charge transfer resistance [42]The decrease in 119877ct is helpful for improving the electronkinetics of the anode material and hence enhancing theelectrochemical performance of MoO

2as anode material

for lithium ion batteries [43] On the other hand in thedischarged state we observed that both 119877ct and 119877sl fromthe AlPO

4-coated sample are relatively low compared to the

pristine sample Charge transfer process is considered to bea rate determining process and the rate performance of theanode material particularly depends on the 119877ct [40] AlPO4nanoparticle coating can support reducing the increase incharge transfer resistance and therefore implying a betterrate performance compared to the pristine sample Theseresults are consistent with previous studies indicating thatcharge transfer resistance decreases significantly with theincorporation of coatings [41 44]

4 Conclusions

MoO2anode material has been successfully coated by AlPO

4

nanoparticles and the AlPO4-coated electrode displays an

enhancement in cycle-life performance The AlPO4coating

significantly reduces the formation of surface cracks induced

by the volume expansion of MoO2anode material diminish-

ing the repetitive formation of electrodeelectrolyte interfacesthat affects the capacity fading Electrochemical performanceof pristine and AlPO

4-coated MoO

2has been studied by

galvanostatic charge and discharge cyclic voltammetry (CV)and electrochemical impedance spectroscopy (EIS) in thevoltage range of 001ndash25 V indicating that the AlPO

4-coated

MoO2exhibits enhanced rate capability and excellent cycle

stability Galvanostatic charge and discharge measurementsat a current density of 50mAsdotgminus1 reveal that pristine MoO

2

exhibits an initial discharge capacity of 650mAhsdotgminus1 and 54capacity loss in 50 cycles while the AlPO

4-coated MoO

2

exhibits an initial discharge capacity of 1015mAhsdotgminus1 andonly 22 capacity loss at 50 cycles Cyclic voltammetrystudies indicate that the improvement in cycling performanceof the AlPO

4-coated MoO

2that is attributed to the stabi-

lization of the lattice structure due to the suppression of theelimination of oxygen vacancies from the anode materialElectrochemical impedance spectroscopy (EIS) shows thatthe AlPO

4nanoparticle coating reduces the surface layer and

charge transfer resistance Surface modification with AlPO4

nanoparticles is an effective way to improve the structuralstability and electrochemical performance of MoO

2as anode

material for lithium ion batteries

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This research project was carried out under the auspicesof the Institute for Functional Nanomaterials (NSF Grantno 1002410) This research was also supported in part byNSF GK-12 (NSF Grant no 0841338) PR NASA EPSCoR(NNX13AB22A) PR NASA Space Grant (NNX10AM80H)and NASA Center for Advanced Nanoscale Materials(NNX08BA48A) The authors gratefully acknowledge theinstrumentation and technical support of the NanoscopyFacility (Dr M Guinel) the XRD and Glovebox Facilities(Dr R S Katiyar) and helpful discussions with Dr VladimirMakarov

References

[1] B Scrosati ldquoRecent advances in lithium ion battery materialsrdquoElectrochimica Acta vol 45 no 15-16 pp 2461ndash2466 2000

ISRN Electrochemistry 11

[2] B Kang and G Ceder ldquoBattery materials for ultrafast chargingand dischargingrdquo Nature vol 458 no 7235 pp 190ndash193 2009

[3] Q Tang Z Shan L Wang and X Qin ldquoMoO2-graphene

nanocomposite as anode material for lithium-ion batteriesrdquoElectrochimica Acta vol 79 pp 148ndash153 2012

[4] V Pralong ldquoLithium intercalation into transition metal oxidesa route to generate new ordered rock salt type structurerdquoProgress in Solid State Chemistry vol 37 no 4 pp 262ndash2772009

[5] W C West J Soler M C Smart et al ldquoElectrochemicalbehavior of layered solid solution Li

2MnO

3-LiMO

2(MNi Mn

Co) li-ion cathodes with andwithout alumina coatingsrdquo Journalof the Electrochemical Society vol 158 no 8 pp A883ndashA8892011

[6] J Sun X Ma C Wang and X Han ldquoEffect of AlPO4coating

on the electrochemical properties of LiNi08Co02O2cathode

materialrdquo Journal of Alloys and Compounds vol 453 no 1-2 pp352ndash355 2008

[7] S T Myung and K Izumi ldquoRole of alumina coating onLiminusNiminusCominusMnminusO particles as positive electrode material forlithium-ion batteriesrdquo Chemistry of Materials vol 17 pp 3695ndash3704 2005

[8] A M Kannan L Rabenberg and A Manthiram ldquoHigh capa-city surface-modified LiCoO

2cathodes for lithium-ion batter-

iesrdquoElectrochemical and Solid-State Letters vol 6 no 1 ppA16ndashA18 2003

[9] H Cao B J Xia Y Zhang and N X Xu ldquoLiAlO2-coated

LiCoO2as cathodematerial for lithium ion batteriesrdquo Solid State

Ionics vol 176 no 9-10 pp 911ndash914 2005[10] Y Wu and A Manthiram ldquoEffect of surface modifications on

the layered solid solution cathodes (1-z) Li[Li13Mn23]O2minus (z)

Li[Mn05minus119910

Ni05minus119910

Co2119910]O2rdquo Solid State Ion vol 180 pp 50ndash56

2009[11] J Ying C Wan and C Jiang ldquoSurface treatment of LiNi

08

Co02O2cathodematerial for lithium secondary batteriesrdquo Jour-

nal of Power Sources vol 102 no 1-2 pp 162ndash166 2001[12] A M Kannan and A Manthiram ldquoSurfacechemically modi-

fied LiMn2O4cathodes for lithium-ion batteriesrdquo Electrochem-

ical and Solid-State Letters vol 5 no 7 pp A167ndashA169 2002[13] B Hu X Wang Y Wang et al ldquoEffects of amorphous AlPO

4

coating on the electrochemical performance of BiF3cathode

materials for lithium-ion batteriesrdquo Power Sources vol 218 pp204ndash211 2012

[14] J Cho Y-W Kim B Kim J-G Lee and B Park ldquoA break-through in the safety of lithium secondary batteries by coatingthe cathode material with AIPO4 nanoparticlesrdquo AngewandteChemie (International Edition) vol 42 no 14 pp 1618ndash16212003

[15] K S Tan M V Reddy G V S Rao and B V R Cho-wardi ldquoEffect of AlPO

4-coating on cathodic behaviour of

Li(Ni08Co02)O2rdquo Journal of Power Sources vol 141 pp 129ndash142

2005[16] J Y Shi C-W Yi and K Kim ldquoImproved electrochemical

performance of AlPO4-coated LiMn

15Ni05O4electrode for

lithium-ion batteriesrdquo Journal of Power Sources vol 195 no 19pp 6860ndash6866 2010

[17] L F Jiao L Liu J L Sun et al ldquoEffect of AlPO4nanowire

coating on the electrochemical properties of LiV3O8cathode

materialrdquo Journal of Physical Chemistry C vol 112 no 46 pp18249ndash18254 2008

[18] A Manthiram and Y Wu ldquoEffect of surface modifications onthe layered solid solution cathodes (1-z) Li[Li

13Mn23]O2-(z)

Li[Mn05minus119910

Ni05minus119910

Co2119910]O2rdquo Solid State Ion vol 180 pp 50ndash56

2009[19] J Cho ldquoCorrelation between AlPO

4nanoparticle coating thick-

ness on LiCoO2cathode and thermal stabilityrdquo Electrochimica

Acta vol 48 no 19 pp 2807ndash2811 2003[20] Y M Sun X L Hu W Luo and Y H Huang ldquoSelf-assembled

hierarchicalMoO2graphene nanoarchitectures and their appli-

cation as a high-performance anode material for lithium-ionbatteriesrdquo ACS Nano vol 5 no 9 pp 7100ndash7107 2011

[21] P Poizot S Laruelle S Grugeon L Dupont and J-M Taras-con ldquoNano-sized transition-metal oxides as negative-electrodematerials for lithium-ion batteriesrdquo Nature vol 407 no 6803pp 496ndash499 2000

[22] T-J Kim D Son J Cho B Park and H Yang ldquoEnhancedelectrochemical properties of SnO

2anode by AlPO

4coatingrdquo

Electrochimica Acta vol 49 no 25 pp 4405ndash4410 2004[23] Y-K Sun S-W Cho S-W Lee C S Yoon and K Amine

ldquoAlF3-coating to improve high voltage cycling performanceof Li[Ni

13Co13Mn13]O2cathode materials for lithium sec-

ondary batteriesrdquo Journal of the Electrochemical Society vol 154no 3 pp A168ndashA172 2007

[24] D Liu Z He and X Liu ldquoIncreased cycling stability of AlPO4-

coated LiMn2O4for lithium ion batteriesrdquoMaterials Letters vol

61 no 25 pp 4703ndash4706 2007[25] H Shi J Barker M Y Saıdi and R Koksbang ldquoStructure

and lithium intercalation properties of synthetic and naturalgraphiterdquo Journal of the Electrochemical Society vol 143 no 11pp 3466ndash3472 1996

[26] T Theivasanthi and M Alagar ldquoX-ray diffraction studies ofcopper nanopowderrdquoArchives of Physics Research vol 1 pp 112ndash117 2010

[27] C-H Doh H-M Shin D-H Kim et al ldquoImproved anodeperformance of thermally treated SiOC composite with anorganic solution mixturerdquo Electrochemistry Communicationsvol 10 no 2 pp 233ndash237 2008

[28] Z H Lu and J R Dahn ldquoUnderstanding the anomalouscapacity of Li Li [ Ni

119909Li(1 3 minus 2119909 3)

Mn(2 3 minus 119909 3)

] O2cells using

in situ x-ray diffraction and electrochemical studiesrdquo Journal ofthe Electrochemical Society vol 149 pp A815ndashA822 2002

[29] C P Grey W-S Yoon J Reed and G Ceder ldquoElectrochemi-cal activity of Li in the transition-metal sites of O

3

Li[Li(1minus2119909)3

Mn(2minus119909)3

Ni119909]O2rdquo Electrochemical and Solid-State

Letters vol 7 no 9 pp A290ndashA293 2004[30] J R Mueller-Neuhaus R A Dunlap and J R Dahn ldquoUnder-

standing irreversible capacity in Li119909Ni1minus120574

Fe1minus120574

O2cathodemate-

rialsrdquo Journal of the Electrochemical Society vol 147 no 10 pp3598ndash3605 2000

[31] W Luo X Hu Y Sun and Y Huang ldquoElectrospinningof carbon-coated MoO

2nanofibers with enhanced lithium-

storage propertiesrdquo Physical Chemistry Chemical Physics vol 13pp 16735ndash16740 2011

[32] J R Dahn and W R McKinnon ldquoStructure and electrochem-istry of LixMoO

2rdquo Solid State Ionics vol 23 no 1-2 pp 1ndash7 1987

[33] Y Liang J Sun S Yang Z Yi and Y Zhou ldquoPreparation char-acterization and lithium-intercalation performance of differentmorphological molybdenum dioxiderdquoMaterials Chemistry andPhysics vol 93 pp 395ndash398 2005

[34] B-C Park H-B Kim S-T Myung et al ldquoImprovementof structural and electrochemical properties of AlF

3-coated

12 ISRN Electrochemistry

Li[Ni13Co13Mn13]O2

cathode materials on high voltageregionrdquo Journal of Power Sources vol 178 no 2 pp 826ndash8312008

[35] G Singh R Thomas A Kumar R S Katiyar and A Mani-vannan ldquoElectrochemical and structural investigations onZnO treated 05 Li

2MnO

3-05LiMn

05Ni05O2layered composite

cathode material for lithium ion batteryrdquo Journal of the Electro-chemical Society vol 159 no 4 pp A470ndashA478 2012

[36] A R Armstrong M Holzapfel P Novak M Thackerayand P Bruce ldquoDemonstrating oxygen loss and associatedstructural reorganization in the lithium battery cathodeLi[Ni

02Li02Mn06]O6rdquo Journal of the American Chemical Soci-

ety vol 128 pp 8694ndash88698 2006[37] G Li Z Yang and W Yang ldquoEffect of FePO

4coating on

electrochemical and safety performance of LiCoCO2as cathode

material for Li-ion batteriesrdquo Journal of Power Sources vol 183no 2 pp 741ndash748 2008

[38] B V Ratnakumar M C Smart and S Surampudi ldquoElec-trochemical impedance spectroscopy and its applications tolithium ion cellsrdquo ChemInform vol 33 p 229 2009

[39] M D Levi D Aurbach G Salitra et al ldquoSolid-state elec-trochemical kinetics of Li-ion intercalation into Li

1minus119909CoO2

simultaneous application of electroanalytical techniques SSCVPITT and EISrdquo Journal of the Electrochemical Society vol 146no 4 pp 1279ndash1289 1999

[40] G Ning B Haran and B N Popov ldquoCapacity fade study oflithium-ion batteries cycled at high discharge ratesrdquo Journal ofPower Sources vol 117 no 1-2 pp 160ndash169 2003

[41] J Liu and A Manthiram ldquoUnderstanding the improvementin the electrochemical properties of surface modified 5 VLiMn

142Ni042

Co016

O4spinel cathodes in lithium-ion cellsrdquo

Chemistry of Materials vol 21 pp 1695ndash1707 2009[42] S Sivaprakash and S B Majumder ldquoSpectroscopic analy-

ses of 05Li[Ni08Co015

Zr005

]O2-05Li[Li

13Mn23]O2compos-

ite cathodes for lithium rechargeable batteriesrdquo Solid StateIonics vol 181 no 15-16 pp 730ndash739 2010

[43] A Chen C Li R Tang L Yin and Y Qi ldquoMoO2-ordered

mesoporous carbon hybrids as anode materials with highlyimproved rate capability and reversible capacity for lithium-ionbatteryrdquo Physical Chemistry Chemical Physics vol 15 pp 13601ndash13610 2013

[44] M C Smart B L Lucht and B V Ratnakumar ldquoElec-trochemical characteristics of MCMB and LiNix Co

1minus119909O2

electrodes in electrolytes with stabilizing additivesrdquo Journal ofthe Electrochemical Society vol 155 no 8 pp A557ndashA568 2008

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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