B.SCI.Dissertation ... Projects Repository... · B.SCI.Dissertation CuOnanostructuresanditsphysicalandelectrochemical properties Author BaiJieA0105679W Supervisor MVVenkatashamyReddy

B. SCI. Dissertation

CuO nanostructures and its physical and electrochemical

properties

Author

Bai Jie �A0105679W�

Supervisor

MVVenkatashamy Reddy

Department of Physics

Faculty of Science

National University of Singapore

April 2016

Content1. Abstract......................................................................................................................................1

2. Introduction............................................................................................................................... 1

3. Synthesis of Copper Oxide and Cell Fabrication................................................................... 5

3.1 Preparation of macron-size CuO via molten salt method................................................5

3.2 Preparation of macron-size CuO via precipitation method............................................. 5

3.3 Preparation of macron-size CuO via thermal annealing method.................................... 6

3.4 Fabrication of batteries.......................................................................................................7

4. Characterization techniques..................................................................................................... 8

4.1 X-ray Diffraction analysis �XRD� ....................................................................................8

4.2 Scanning Electron Microscopy �SEM�............................................................................10

4.3 X-ray Photoelectron Spectroscopy �XPS� .....................................................................11

4.4 Battery performance characterization..............................................................................11

5. Result and discussions............................................................................................................12

5.1 Structure and morphology .............................................................................................. 12

5.2Chemical composition study............................................................................................ 16

6. Electrochemical studies on Li-ion Batteries......................................................................... 18

6.1 Galvanostatic Cycling �GC�............................................................................................. 18

7. Conclusion...............................................................................................................................23

8. Acknowledgements.................................................................................................................24

9. Reference.................................................................................................................................24

10. Appendix..................................................................................................................................26

1

1. AbstractConversion reaction says that normally, the stable lithium oxide, Li2O, is

electrochemically inactive and cannot be decomposed to the metal and oxygen.

However, in the presence of nanosize transition metal particles, which may or may

not be electrochemically generated, it can be decomposed, as exemplified by the now

well-known reaction:

nano-CuO + 2Li ↔ nano-Cu + Li2O.

Thus, at suitable potentials, depending on the nature of the metal, Li cycling can occur,

giving rise to large and reversible capacities, stable over a large number of discharge−

charge cycles. [1] Therefore, the hypothesis of this study is that CuO nanosucture

exhibits higher discharge capacity and charge capacity as compared to CuO bulk

structure lithium battery.

Therefore, in this report, the main objective is to study electrochemical properties and

nanostructures of Copper Oxide �CuO�. CuO nanoparticles is one of the anode

materials that gains considerable attentions due to its good electrical and chemical

properties. It shows excellent performance comparing the bulk or macron-sized

counterpart, with theoretical reversible capacity of 647 mAh g-1. Besides, it is also

very cheap and environmentally friendly due to its low toxicity. We report the

synthesis of CuO nanostructure material by both precipitation method and thermal

annealing method. Copper chloride was chosen as the precursors in precipitation

method and copper thin film was thermally annealed in static air in thermal annealing

method. The structural properties of the prepared CuO materials were then analyzed

by scanning eletron microscopy �SEM� and X-ray diffractrometer �XRD�. The

electrochemical properties were characterized galvanostatic cycling �GC� studies. GC

studies show that for the bulk material, continuous capacity fading was significantly

less than that of nanostructure copper oxide.

2. IntroductionLithium ion batteries�LIBs� are one of the most promising energy source for portable

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electronic devices such as cell phones, laptops, cameras and etc due to their higher

energy density, light weight and durability. Studies of lithium-ion batteries can be

traced back to early 1910s, however, the first non-rechargeable lithium batteries only

became commercially available until late 1970s. In 1991, the Sony Corporation

developed the first modern commercial lithium-ion battery, where graphite �C� was

used as the anode �negative electrode� and LiCoO2 was used as cathode �positive

electrode�. Graphite material have two major issues as the anode material. Firstly,

when the charge current rate is high, it suffers from a safety problem. The main reason

is that the operating potential of the lithiated-graphite electrode is close to that of

metallic lithium, which causes growth of Lithium-dendrite and electrical shorting.

Secondly, in the current world, there is an upward shift in demand for portable devices

such as cellphones and laptops in many highly populated countries. Most of portable

devices are equipped with faster processors that requires batteries with high

volumetric capacity and low capacity fading. According to Moore’s law, the

processing capabilities of hardware doubles every 18 months, however, current

batteries are not able to keep pace with these developments. In fact, the performance

of lithium ion batteries strongly relies on properties of electrode material, therefore, a

suitable choice of electrodes based on their electrochemical properties are essential to

meet the demanding requirements of these applications. Therefore, since then it began

to gain considerable attention and research groups from all over the world tried to

improve its overall performance by finding a replacement for graphite as the anode

material. �see e.g. [1]�. The anode material should satisfy the several requirements so

as to be used as an electrode in lithium ion batteries. Firstly, the ideal anode must have

a low redox potential �graphite: 0.1-0.2V vs Li� and must be stable and not be soluble

in electrolyte. Secondly, the host lattice of anode material must be able to

accommodate large numbers of lithium particles per unit volume to yield a high

specific capacity and should be an intercalation host where Li can be inserted and

deinserted without disrupting the host structure. Thirdly, the gravimetric and

volumetric energy density of the anode material must be high so that the mass and

3

volume of the material can be reduced in lithium ion battery. Lastly, the anode

material must be cost-effective and easy to synthesize for large scale manufacture and

must be environmentally friendly.

In 2000, the conversion reaction mechanism of simple binary oxide �Co3O4� was

found by the Tarascon group [2]. Such simple binary oxides have high theoretical

capacity and high reversibility and meets the requirements of an ideal anode.[3]

Extensive research has been conducted to investigate other relevant properties.

Generally, the stable lithium oxide, Li2O is electrochemically stable thus cannot be

decomposed to metal and oxygen.

However, decomposition can happen in the presence of nanosize transition of metal

particle, which may or may not be electrochemically generated. [4] The reactions

involved in the entire synthesis can be summarized as the following:

MO + 2Li+ + 2e-⇌ M + Li2O

�M = Mn, Fe, Co, Ni, Cu�

In our case, M = Cu. During discharge reaction, Li metal crystal structure is

destructed �amorphization of lattice�, which is followed by the formation of

nanoparticles of metal embedded into the Li2O matrix. During charge reaction, the

re-formation of CuO is deemed as a consequence of decomposition of Li2O [5]

Nowadays, the lithium-ion batteries have high energy densities, meaning they have

greater power for longer time in a smaller package. They can provide higher voltage

which allows it to power complex mechanical devices. The shelf life of lithium-ion

batteries is long, with less than 5% self discharge loss per month. The production is

pollution free. There is no mercury, cadmium and etc involved. Lithium-ion batteries

now is a billion dollar industry and have been applied to every possible electronic

devices ranging from mobile phones to laptops and electric vehicles.

In this paper, we evaluate the electrochemical performance of cupper oxide materials

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in the so-called ‘half-cell’ configuration, where the oxide practically serves as the

“cathode” and Lithium metal serves as the anode. Lithium metal acts as the reference

electrode whose voltage is arbitrarily taken to be zero. Therefore, in our configuration,

the process of Li being inserted into the oxide or composite is deemed as the

discharge reaction, whereas the process of Li being extracted from the oxide or

composite is deemed as the charge reaction. However, in commercial Lithium-ion

battery, which is the ‘full cell’ configuration, Li containing oxide, such as LiCoO2,

forms the cathode and an oxide material �or graphite� form the anode, hence, the

processes are named in the reverse way. Therefore, in full cell configuration, the

process of Li being extracted from the oxide �or graphite� anode, and the

corresponding Li of being inserted into the cathode, such as Li1−xCoO2, is deemed as

“discharge reaction”. Reversely, the process where Li being inserted in the oxide or

graphite, and the corresponding Li being inserted into the cathode, such as Li1−xCoO2,

is deemed as “charge reaction”. The reactions involved in the entire charge and

discharge process of can be summarized as the following:

Charging process:

Cathode: LiCoO2 Li1-xCoO2 + x Li+ + xe-

Anode: C + x Li+ + x e- LixC

Discharging process:

Cathode: Li1-xCoO2 + x Li+ + x e- LiCoO2

Anode: LixC x e- + x Li+ + C

The overall reaction is

LiCoO2 + C Li1-xCoO2 + LixC ; x=0.5

5

3. Synthesis of Copper Oxide and Cell FabricationIn this research project we first synthesized macron size CuO via the typical

experimental method -- molten salt method. In the electrochemical property study, we

used the macron size CuO sample as the control group to compare with its battery

performance with nano size CuO sample group. Subsequently, we synthesized nano

size CuO via precipitation method and thermal annealing method. We expected

different morphology of sample synthesized via these two method. The purpose to

prepared CuO sample via two different method is to study whether morphology of

CuO would have effect on its electrochemical property.

3.1 Preparation of macron-size CuO via molten salt method

In the first part of the experiment, we prepared the bulk CuO powder by Solid state

method where Cu�NO3�2·3H2O was heated in alumina crucible at 350 oC in air for 3h .

Next, we mixed the CuO sample with Super P Carbon dark �ENSACO, MMM Super

p�, which served as binder with a weight ratio of 70:15:15 that can improve the

conductivity and Polyvinylidene fluoride. Next ,we dispersed these mixtures were in

N-methyl pyrrolidone �NMP, Alfa Aesar� solvent to form viscous slurry, which was

then coated on a copper sheet by using Doctor Blade technique. Next, the coated

copper sheet sample was placed in the oven and dried for 12 hours at the temperature

of 70oC. Next, we pressed the copper sheet in between twin rollers and cut them into

circular disks with a diameter of 16mm, which were later used as electrodes to

fabricate batteries. The fabrication of cell is demonstrated in [3.2].

3.2 Preparation of CuO nanoparticles via precipitation method

We synthesized the CuO in nanostructure via precipitation method using copper

chloride �CuCl2�. We follow the typical experimental procedure, where we first

6

weighed 2g of PVP and 2g of copper chloride CuCl2 poured it into separate

volumetric flasks. Next, we added a total 100 mL distilled water to bring the solution

volumes to 100 mL. The solutions were added to a round-bottomed evaporating dish

and stirred using magnetic stirrer and heated for one hour. The resulting mixture color

was bright green. We added NaOH to the solution to adjust the pH, and kept the

solutions at 60 oC for 1 h. A large amount of black precipitates were obtained. After

being cooled to room temperature, the particles were repeatedly washed with distilled

water in order to remove impurities, and dried in an oven at 80 oC for 16 h. The

reactions involved in the entire synthesis can be summarized as the following

CuCl2+ 2NaOH →Cu�OH�2+2NaCl2

Cu�OH�2 →CuO +H2O

3.3 Preparation of CuO nanoparticle via thermal annealing method

In second part �B� of the experiment, copper oxide nanoparticles have been

synthesized by thermal annealing of copper thin films on aluminum bowl at constant

temperature of 650 oC. Thermal annealing of copper thin film in static air can produce

large-area, uniform, but not well vertically aligned CuO nanoparticles along the thin

film surface. When copper is oxidized in air, the major Cu is converted into Cu2O, and

CuO is then formed slowly through a second step of oxidation. In this case, Cu2O

served as a precursor to CuO. The reactions involved in the entire synthesis can be

summarized as the following:

4Cu + O2 = 2Cu2O

2Cu2O + O2= 4CuO

Therefore, we expected that sample prepared via this method could be a mixture of

CuO and Cu2O.

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3.4 Fabrication of batteries

The bulk and nanostructure CuO were prepared as described above respectively. And

we fabricated the cell in an argon filled glove box �MBraun, Germany� which

maintains <1ppm of H2O and O2 so that metallic Lithium can remain chemically

stable. We first placed the CuO was on a commercial stainless steel cup, which serves

as the positive terminal and on top of it, we covered it with a polymeric separator. The

polymeric separator is electronically non-conducting, however, Lithium ions are

permeable to penetrate freely. Thus the separator can prevent direct contact of the two

electrodes with opposite polarity. Next, we placed a few drop of electrolyte �1 M

LiPF6 in mixture of ethylene carbonate �EC�: dimethyl carbonate �DMC��1: 1 by

volume�� on the polymeric separator. Next we placed a piece of circular lithium metal

on the top of the polymeric separator. The metallic lithium has diameter of 13mm and

thickness of 0.6mm. Next we again dropped a few electrolyte on top of the metallic

lithium. Next, we placed a lid fitted with a plastic ring, which serves as the negative

terminal, on top of the metallic lithium. Lastly, the coin cell was sealed by using a

coin cell crimper to press the whole cell.

The assembled cells were have a diameter of 16mm and height of 2.0mm after proper

sealing. then transferred out of the glove box and placed in the for more than 10 hours

until it was entirely chemically stable before we conducted any subsequent

experiment.

Figure 1 shows the full component and sequence of assembling a lithium-ion coin

cell.

8

Figure 1. Component of Lithium-ion coin cells.

4. Characterization techniques

4.1 X-ray Diffraction analysis

XRD technique, which relies on the theory of Bragg’s diffraction law, is a common

characterization technique that is used to determines information of crystal structure

of unknown materials. XRD diffraction patterns �constructive interference� will be

observed when the monochromatic X-ray λ beam is incident at angles θ with respect

to the sample. The relationship is given by

nλ = 2dsin θ

where d is the distance between two adjacent atomic planes and n is the order of

diffraction.

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Figure 2. Simple illustration of Bragg’s law

In the this study, XRD patterns were obtained by using Siemens D5005 diffractometer

with Cu-Kα X-rays with a wavelength λ = 1.54 Å. With this technique, the

information about the crystallographic structure of the unknown samples can be

identified by matching the pattern with the literature value.

The original crystal structure of CuO was first determined by Tunnel in 1933 and was

then refined by single-crystal X-ray methods in 1970 [6]. The CuO crystal has

monoclinic structure with the Cu2+ ions are at centers of inversion symmetry in a

single fourfold site 4c �1/4,1/4,0�, and the oxygen ions occupy site 4e �0,y,1/4� with y

= 0.416�2� as illustrated figure 2. The structural parameters of CuO as summarized

above by Meyer et al.[7] are presented in Table 1.

Space group C 2/c �No.15�Unit cell a �� = 4.6837

b �Å� = 3.4226c �Å� = 5.1288beta �o� = 99.54oα, γ = 90o

Cell volume 81.08 ÅCell content 4 [CuO]Distances Cu–O 1.96 Å

O–O 2.62 ÅCu–Cu 2.90

Table 1. Structural parameters of CuO

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Figure 2: representation of a monoclinic CuO unit cell. The blue spheres represent Cuatoms and red spheres represent O atoms.

However, particle size and lattice parameters of CuO via different method would

normally have variation. One factor is the annealing temperature. Particle size and

lattice parameters of CuO would increase accordingly with increase in annealing

temperature as illustrated in Table 2. Vidyasagar et al. [8]

Table 2 Variation in crystallite size and lattice parameters with annealing temperature

Temperature �oC� a �� b �� c �� Crystallite size �nm�

400 4.688 3.427 5.132 20

500 4.711 3.429 5.133 21

600 4.715 2.430 5.135 25

700 4.719 3.432 5.136 27

In our report, we refined XRD using the TOPAS software. XRD Rietveld refined

patterns of bulk CuO sample prepared via MSM method at 350oC are shown in figure

3, where blue curve is experimental diffraction curve and red curve is theoretical

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refined diffraction curve. The continuous line is fitted with theoretical refined

diffraction curve and differences pattern are shown.

4.2 Scanning Electron Microscopy

SEM is a common technique that is used in the study of surface topography of

morphology of solid samples where images of near surface structure of solids can be

provided by scanning it with a focused beam of emitted electrons, which would

interact with those within the surface of the sample, during which X-rays and

secondary electrons would be ejected and scattered. Detectors would collect these

secondary electrons and transfer them into a signal on a screen, by which the images

of near surface structure of sample and information such as particle size, shape and

structure, are formed.

4.3 X-ray Photoelectron Spectroscopy

XPS is one extensively-used surface-sensitive quantitative spectroscopic technique to

measure the elemental composition, empirical formula and chemical state of the

elements existing within a sample material.

When a beam of X-rays are irradiated to a material, the kinetic energy and number of

electrons that escaped are simultaneously being measured. As the energy of an X-ray

with particular wavelength is known, and the kinetic energies of emitted electrons are

measured, the electron binding energy of each of the emitted electrons can be

determined by applying the conservation of energy equation:

Ebinding = Ephoton - � Ekinetic+ κ �

Where Ebinding is the binding energy of the electron, Ephoton is the energy of the

irradiating X-ray photons, Ekinetic is the kinetic energy of the emitted electron

measured by the instrument and κ, which is an adjustable instrumental correction

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factor that rarely needs to be adjusted in practice, is the work function dependent on

both the spectrometer and the material.

4.4 Battery performance characterization

The electrochemical properties of CuO are characterized Galvanostatic Cycling �GC�

tests. All GC tests carried out at the range of voltage between 0.005V and 3.0V vs

Lithium and at current rate of 60mAhg-1 and 240mAhg-1, respectively. Cyclic

voltammetry is an commonly used technique in electrochemistry studies to obtain

quantitative and qualitative information regarding to redox potentials, cell reversibility,

phase transitions and etc. Unfortunately, the experimental instrument for Cyclic

Voltammetry �CV� in Advance Batteries Lab 2, Physics Department, National

University of Singapore, is not working. Thus we cannot conduct CV studied during

the research project.

5. Result and discussions

5.1 Structure and morphology

We characterized the structures of the CuO synthesized via the molten salt method

and thermal annealing method by the powder X-ray Diffraction �XRD� technique to

identify and quantify the crystalline phases formed.

First we can observe that the experimental Rietveld refined XRD patterns for macron

size CuO prepared via MSM matches well with the reported value in literature, which

indicates the purity of CuO sample was very high. The Rietveld refined XRD patterns

show the lines with characteristic of monoclinic structure. The fitted lattice

parameters are a=4.692Å, b=3.431Å, c=5.137Å, which are very close to data obtained

by Meyer et al �a=4.684 Å, b=3.423 Å, c= 5.129 Å�[5].The Cell Volume is 81.556 Å3.

The structural parameters of the macron size CuO as summarized above are presented

in Table 3. The corresponding Retveld refined XRD patterns are shown in Figure 3.

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Space group C 2/c �No.15�Unit cell a �� = 4.692

b �Å� = 3.431c �Å� = 5.137beta �o� = 99.54oα, γ = 90o

Cell volume 81.18487ÅCell content 4 [CuO]

Table 3. Structural parameters of experimental Rietveld refined XRD patterns for

macron size CuO prepared via MSM matches

Figure 3. XRD Rietveld refined patterns of bulk CuO samples prepared via MSM

method at 350oC

The XRD patterns for CuO samples prepared via Thermal annealing method is shown

below in Figure 4. We did not conduct Rietveld analysis on nano size CuO because

with smaller particle size, less signals are observed so that it is not easy to refine them

accurately. For nano sized CuO, very broad peaks are observed and some small peaks

are merged into a single peak. Besides, we can observe that the sample is a mixture of

(2,2,0)(1,1,0)

(1,-1,-1) (1,1,1)

(2,0,-2) (2,0,2)(0,2,0)

(1,-1,-3)(0,2,2)bulk

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CuO and Cu2O as we can observe that the XRD characteristic peak for both CuO and

Cu2O coexist in the XRD graph. This is due to the growth mechanism: When copper

is oxidized in air, the major Cu is converted into Cu2O, and CuO is then formed

slowly through a second step of oxidation. In this case, Cu2O served as a precursor to

CuO. The reactions involved in the entire synthesis can be summarized as the

following:

4Cu + O2 = 2Cu2O

2Cu2O + O2= 4CuO

Figure 4. XRD patterns of nano size CuO samples prepared via thermal annealing

method at constant temperature of 650 oC

Due to the low concentration of CuO of the sample prepared via precipitation method,

we will not investigate the XRD pattern here, however, it is expected that the nano

size CuO prepared via different method should have very similar XRD pattern and the

intensity of XRD peaks of macron size CuO should be sharper than that of nano size

CuO as the particle size is bigger.

15

Next, We further investigate the morphology of the CuO prepared via all method by

scanning electron microscopy �SEM�. Before conducting SEM, in order to get clear

image, we heated the sample to 80°C to remove moisture from the sample as SEM is

carried out in vacuum conditions and uses electrons to form an image. The resulting

images are shown in Figure 5. The SEM images evidently exhibit different

morphologies and properties of architectures of CuO particles synthesized via

different method. However, we do not fully understand the exact chemical mechanism

behind the formation of these nanostructures. We can observe the macron size of

Nickel mesh in Figure 5�a� clearly. In figure 5�b�, we can observe that three

dimensional submicro-spherical structures a diameter in the range between 2 to 8 µm

are formed on the surface of Nickel mesh. Unfortunately, the image in figure 4 �c� to

�e� have low resolution and contrast so that we cannot see any clear nanostructure of

CuO. SEM image of CuO prepared via MSM method are shown in figure 5�e�. As

shown in figures, particle sizes of prepared samples are in micron size range and

showed irregular cauliflower-like shape.

a bc

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Figure 5: SEM Graph of CuO of samples prepared from �a� CuO precipitation method;bar scale 200 µm �b� CuO precipitation method; bar scale 2 µm �c� CuO precipitationmethod; bar scale 300nm �d� CuO thermal annealing method; bar scale 1µm �e� CuOthermal annealing method; bar scale 300nm �f� CuO MSM method; bar scale 1µm

5.2 Chemical composition study

We confirmed the chemical compositions of the metal oxides via X-ray photoelectron

spectroscopy �XPS�. The XPS spectra of CuO samples are shown in Figure 5. The

peak deconvolution and the fitting are conducted by using CASA XPS software.

Figures 6 �a� to �d� show the XPS spectra of Cu 2p, O 1s of sample CuO prepared via

thermal annealing method and precipitation method. We can clearly observe two

peaks at 933.69eV and 953.84eV of Cu 2p3/2 and Cu 2p½ spin orbit coupling which

are corresponding to Cu+ and Cu2+, respectively. [9]. Besides, we could also observe

two small peaks. They are due to background satellite signals which can be ignored in

f

c d

e

17

this study. Furthermore, XPS studies also provide the information on binding energy

and oxidation state of O 1s, where we can observe two peaks. The first peak at 529.4

eV which is corresponding to oxidation sate of O 1s. [10] The second peak is

corresponding to the surface oxygen of the sample, which could be due to surface

moisture. We can conclude that CuO have been successfully synthesized. Furthermore,

CuO and Cu2O are confirmed to coexist in our sample prepare via thermal annealing

method as expected. The binding energy of 933.69 ev corresponds to Cu2+ and 529.4

eV corresponds to O 1s in CuO.

a b

c d

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Figure 6: XPS Graph of CuO samples prepared via CuO thermal annealing method �a�Cu2p �b� O1s, XPS Graph of CuO of samples prepared via thermal annealing

method �c� Cu2p �d� O1s,

6. Electrochemical studies on Li-ion Batteries

6.1 Galvanostatic Cycling (GC)

We further study the electrochemical property of CuO samples prepared via all

method by investigating the performance of the cell which are characterized by

Galvanostatic Cycling study. Galvanostatic cycling requires the cell to be charged and

discharged at a constant current rate and the corresponding cell voltage, which varies

as a function of the discharge or charge state, is plotted as a function of step time.

Therefore, fundamentally, galvanostatic cycling explores the relationship between

time and voltage of an electrochemical cell. We can determine the suitability of

nanostructure CuO as the anode for the lithium ion battery by analyzing and

comparing its galvanostatic response in terms of achievable capacities and cyclability

with the theoretical values. The galvanostatic cycling was carried out in the voltage

range of 0.005V to 3V at current rate of 60 mAg-1 and 240 mAg-1, respectively.

The data collected was first plotted as a function of voltage vs the specific capacity.

To calculate the specific theoretical capacity, we can use the following formula:

e

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Specific capacity = �Step Time x Current� / �Active Material Mass × 3600�

The voltage vs specific capacity of prepared via all methods in 1st, 2nd, 10th, 20th and

40th cycles plotted with current rate of 60 mAg-1 and 240 mAg-1 are shown in Figure 6

and specific capacity vs cycle number plots are shown in Figure 7. We can observe

that during the 1st discharge cycle, there was a sharp decrease in voltage of all the

CuO samples from 3.0V to somewhere between 1.25-1.5V. By comparing the 1st

charge-discharge curve with the 2nd charge-discharge curve, we can calculate the

irreversible capacity loss �ICL� of the cell, which is shown in table three.

As showed in Figure. 7, the specific capacity for macron size CuO samples prepared

via solid state method is significantly less than that of the nano size CuO samples.

We next study and compare the coin cell performance with CuO nanostructure and

CuO bulk structures as cathode for low current rate �60mAg-1�. We can observe

significantly different features in charge and discharge process for nanostructure

samples prepared by both precipitation and thermal annealing method and bulk CuO

prepared via MSM method. The former has a significantly higher overall capacity.

The irreversible capacity loss of CuO nanostructure samples was around 30%, which

is significantly less that of the CuO bulk structure samples. From 2nd cycle onwards,

we found out that the capacity fading of nanostructure CuO is much lower than that of

bulk CuO. CuO nanostructure samples exhibit very low reversible capacity loss

�between 10-15%� up to 40th cycle, comparing that of CuO bulk structures, which

was 66.9%. Therefore, we may conclude that, compared with nanostructures CuO,

discharge capacity of bulk CuO at the same current rate were lower and less stable.

To study effect of current rate on battery performance, we raised the current rate to

240mAg-1 and compare the coin cell performance with CuO nanostructure for low

current rate �60mAg-1� and high current rate �240mAg-1�. We can observe significantly

different features in charge and discharge process for nanostructure samples for low

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current rate �60mAg-1� and high current rate �240mAg-1�. The former has a

significantly higher overall capacity. The irreversible capacity loss of CuO

nanostructure samples was around 30%, which is similar with that of the CuO bulk

structure samples. From 2nd cycle onwards, we observed that the reversible capacity

fading of CuO nanostructure for low current rate �60mAg-1� was significantly less

than that for high current rate �240mAg-1�. Therefore, we may conclude that,

compared with low current rate, discharge capacity of CuO at higher current rate were

lower and less stable.

We can observe a universal sharp decrease of capacity between the 1st and 2nd cycle of

all samples. This is due the formation of solid-electrolyte interphase �SEI� at the

electrode surface, which, in general, results from irreversible electrochemical

decomposition of the electrolyte or side reactions between lithium and electrolyte. In

our case of Li-ion batteries, the SEI is formed at the negative electrode, which is

lithium metal. It is easily imaged that if SEI formation were continued throughout

each charge and discharge cycle, Li-ion batteries would be unusable due to the

continual loss of lithium. However, the fact that batteries can operate is due to that

fact that SEI is not electron conductable, and is nearly impenetrable to any electrolyte

molecules. Therefore, once the initial SEI layer has formed, it would disable

electrolyte molecules to penetrate to further react with the lithium on the surface.

Thus the battery can actually experience many charge discharge cycles with minimum

additional formation of SEI layer. [11].

For clarity, we listed all information of discharge and charge capacity for all samples

in Table 3.

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Sample Dischargecapacity at1st cycle�mAh g-1�

Chargecapacity at 1st

cycle �mAhg-1�

Dischargecapacity at2nd cycle�mAh g-1�

Dischargecapacity at10th cycle�mAh g-1�

Dischargecapacity at40th cycle�mAh g-1�

Reversiblecapacity Fading

from 2-40cycle �%�

Irreversiblecapacityloss�%�

CuO- 60 mAg-1MSM method

1180 395 390 325 280 28.2 66.9

CuO- 60 mAg-1Thin film 650oC

1100 710 695 650 620 10.8 35.5

CuO- 240mAg-1Thin film 650oC

980 680 660 570 480 27.2 32.7

CuO- 60 mAg-1precipitationmethod

920 620 595 570 505 15.1 35.3

CuO- 240mAg-1precipitationmethod

970 600 580 520 450 23.4 40.2

Table 3: 1stdischarge, 1st charge, 2nd charge, 10th charge and 40th charge capacity,irreversible capacity loss, and reversible capacity fading of samples prepared by allmethods

22

Fig. 6: Voltage �0.005-3.0V� vs specific Capacity of sample CuO prepared by �a� CuO60 mAg-1 MSM method 350 oC �b� CuO 60 mAg-1 Thin film 650oC �c� CuO 240mAg-1 Thin film 650oC �d� CuO 60 mAg-1 precipitation method �e� CuO 240 mAg-1precipitation met

23

Figure. 7: Specific capacity vs Cycle number of CuO prepared by �a� CuO 60 mAg-1MSM method 350 oC �b� CuO 60 mAg-1Thin film 650oC �c� CuO 240 mAg-1Thin

film 650oC

7. ConclusionsWe have successfully verified the hypothesis that CuO nanosucture exhibits higher

discharge capacity and charge capacity as compared to CuO bulk structure lithium

battery. Furthermore, we also proved that the morphology of CuO would influence on

its electrochemical properties. First, we synthesized bulk and nanostructure of CuO

via various methods including molten salt method, precipitation method and thermal

annealing method. Nanostructure and physical properties of CuO samples were

characterized and analyzed through X-Ray Diffraction �XRD�, Scanning Electron

Microscopy �SEM� and X-ray Photoelectron Spectroscopy �XPS�. Electrochemical

properties of CuO were examined and characterized via galvanostatic cycling �GC�

studies on Lithium-ion coin cell where CuO samples are used as cathode.

Galvanostatic cycling of CuO nanostructures prepared by precipitation and thermal

annealing methods shows a high and stable reversible capacity at current rate of

60mA/g within the voltage range of 0.005-3V compared with CuO macron-sized

structure prepared via MSM method. Besides, Galvanostatic cycling of CuO

nanostructures prepared by precipitation and thermal annealing methods shows a high

and stable reversible capacity at current rate of 60mA/g within the voltage range of

0.005-3V compared with current rate of 240mA/g within the voltage range of

0.005-3V.

24

8. AcknowledgmentI would like to express my most sincere gratitude to my mentor Dr M.V. Reddy,

Advanced Batteries Lab, Department of Physics, National University of Singapore,

for his utmost patience and guidance throughout my final year project journey.

Furthermore, I would like to thank technical staff of Advanced Batteries Lab,

Department of Physics, National University of Singapore. Lastly, I would like to

thank Ministry of Education �MOE� and National University of Singapore for

providing me with the opportunity to be involved in the honor year project.

9. Reference1. Guyomard, D., et al., New amorphous oxides as high capacity negative

electrodes for lithium batteries: the LixMVO4 (M = Ni, Co, Cd, Zn; 1 &lt; x ≤

8) series. Journal of Power Sources, 1997. 68�2�: p. 692-697.

2. Poizot, P., et al., Nano-sized transition-metal oxides as negative-electrode

materials for lithium-ion batteries. Nature, 2000. 407�6803�: p. 496-499.

3. Sharma, Y., et al., Nanophase ZnCo2O4 as a High Performance Anode

Material for Li‐Ion Batteries. Advanced Functional Materials, 2007.

17(15):p.2855-2861.

4. Reddy, M.V., G.V. Subba Rao, and B.V.R. Chowdari, Metal oxides and

oxysalts as anode materials for Li ion batteries. Chemical Reviews, 2013.

113�7�: p. 5364-5457.

5. Laurent S, Forge D, Port M, Roch A, Robic C, Elst L, et al. Magnetic iron

oxide nanoparticles: synthesis, stabilization, vectorization, physicochemical

characterizations, and biological applications. Chem Rev 2008;108:2064–110.

6. Amin G. ZnO and CuO nanostructures: low temperature growth,

characterization, their optoelectronic and sensing applications. 1st ed.

LiU-Tryck: Norrköping �Sweden�; 2012 [SE-601 74]

25

7. Meyer B, Polity A, Reppin D, Becker M, Hering P, Klar P, et al. Binary copper

oxide semiconductors: from materials towards devices. Phys Status Solidi �b�

2012;249:1487–509.

8. Vidyasagar C, Arthoba Naik Y, Venkatesha T, Viswanatha R. Solid-state

synthesis and effect of temperature on optical properties of CuO nanoparticles.

Nano-Micro Lett 2012;4:73–7.

9. Anisimov VI, Zaanen J, Andersen OK. Band theory and Mott insulators:

Hubbard U instead of Stoner I. Phys Rev B 1991;44:943–54.

10. Anisimov V, Aryasetiawan F, Lichtenstein A. First-principles calculations of

the electronic structure and spectra of strongly correlated systems: the LDA+

U method. J Phys: Condens Matter 1997;9:767–808.

11. 21. P. Verma, P. Maire and P. Novák, Electrochimica Acta, 55, 6332 �2010�.

26

10. Appendix

Macron size CuO Rietveld refinement pattern

2Theta (Degrees)8070605040302010

Inte

nsity (

Co

un

ts) 80,000

60,000

40,000

20,000

0

-20,000

Tenorite 100.00 %

Analysis Report

Global R-Values

Rexp : 1.32 Rwp : 6.35 Rp : 4.08 GOF : 4.81

Rexp`: 3.79 Rwp`: 18.25 Rp` : 21.29 DW : 0.18

Range Number : 1

R-Values

Rexp : 1.32 Rwp : 6.35 Rp : 4.08 GOF : 4.81

Rexp`: 3.79 Rwp`: 18.25 Rp` : 21.29 DW : 0.18

Quantitative Analysis - RietveldPhase 1 : Tenorite 100.000 %

BackgroundChebychev polynomial, Coefficient 0 4867.283

1 -653.3883

2 511.1321

3 -310.869

27

4 173.5312

InstrumentPrimary radius (mm) 217.5

Secondary radius (mm) 217.5

CorrectionsCylindrical sample 2Th correction uR 0.8662125

LP Factor 24

Structure 1Phase name Tenorite

R-Bragg 3.040

Spacegroup C12/c1

Scale 0.0425493182

Cell Mass 318.183

Cell Volume (Å^3) 81.55658

Wt% - Rietveld 100.000

Crystallite Size

Cry Size Lorentzian (nm) 83.0

Crystal Linear Absorption Coeff. (1/cm) 283.338

Crystal Density (g/cm^3) 6.478

Lattice parameters

a (Å) 4.6915405

b (Å) 3.4311787

c (Å) 5.1374587

beta (°) 99.54

Site Np x y z Atom Occ Beq

Cu1 4 0.25000 0.25000 0.00000 Cu+2 1 1

O1 4 0.00000 0.41840 0.25000 O-2 1 1