Content 1. Abstract Introduction Synthesis of Copper Oxide and Cell Fabrication Preparation of macron-size CuO via molten salt

Transcription

1 B. SCI. Dissertation CuO nanostructures and its physical and electrochemical properties Author Bai Jie A W Supervisor M V Venkatashamy Reddy Department of Physics Faculty of Science National University of Singapore April 2016

2 Content 1. Abstract Introduction Synthesis of Copper Oxide and Cell Fabrication Preparation of macron-size CuO via molten salt method Preparation of macron-size CuO via precipitation method Preparation of macron-size CuO via thermal annealing method Fabrication of batteries Characterization techniques X-ray Diffraction analysis XRD Scanning Electron Microscopy SEM X-ray Photoelectron Spectroscopy XPS Battery performance characterization Result and discussions Structure and morphology Chemical composition study Electrochemical studies on Li-ion Batteries Galvanostatic Cycling GC Conclusion Acknowledgements Reference Appendix...26

3 1. Abstract Conversion 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. Introduction Lithium ion batteries LIBs are one of the most promising energy source for portable 1

4 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: V 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 2

5 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 3

6 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: Anode: LiCoO2 Li1-xCoO2 + x Li+ + xe- C + x Li+ + x e- LixC Discharging process: Cathode: Anode: Li1-xCoO2 + x Li+ + x e- LiCoO2 LixC x e- + x Li+ + C The overall reaction is LiCoO2 + C Li1-xCoO2 + LixC ; x=0.5 4

7 3. Synthesis of Copper Oxide and Cell Fabrication In 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 o C 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 70 o C. 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 5

8 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 o C 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 o C 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 o C. 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. 6

9 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. 7

10 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. 8

11 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 Cu 2+ 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 = 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 Å = b Å = c Å = beta o = o Cell volume Cell content Distances α, γ = 90 o Å 4 [CuO] Cu O 1.96 Å O O 2.62 Å Cu Cu 2.90 Table 1. Structural parameters of CuO 9

12 Figure 2: representation of a monoclinic CuO unit cell. The blue spheres represent Cu atoms 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 o C a Å b Å c Å Crystallite size nm In our report, we refined XRD using the TOPAS software. XRD Rietveld refined patterns of bulk CuO sample prepared via MSM method at 350 o C are shown in figure 3, where blue curve is experimental diffraction curve and red curve is theoretical 10

13 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 11

14 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= Å [5].The Cell Volume is Å 3. 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. 12

15 Space group C 2/c No.15 Unit cell a Å = b Å = c Å = beta o = o α, γ = 90 o Cell volume Å Cell content 4 [CuO] Table 3. Structural parameters of experimental Rietveld refined XRD patterns for macron size CuO prepared via MSM matches (1,-1,-1) (1,1,1) (1,1,0) (2,0,-2) (2,0,2) (0,2,0) (1,-1,-3) (0,2,2) bulk (2,2,0) Figure 3. XRD Rietveld refined patterns of bulk CuO samples prepared via MSM method at 350 o C 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 13

16 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 o C 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. 14

17 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. ac b 15

18 c d e f 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 precipitation method; bar scale 300nm d CuO thermal annealing method; bar scale 1µm e CuO thermal 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 eV and eV of Cu 2p 3/2 and Cu 2p½ spin orbit coupling which are corresponding to Cu + and Cu 2+, respectively. [9]. Besides, we could also observe two small peaks. They are due to background satellite signals which can be ignored in 16

19 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 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 ev corresponds to Cu 2+ and ev corresponds to O 1s in CuO. a b c d 17

20 e 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: 18

21 Specific capacity = Step Time x Current / Active Material Mass 3600 The voltage vs specific capacity of prepared via all methods in 1 st, 2 nd, 10 th, 20 th and 40 th 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 V. By comparing the 1 st charge-discharge curve with the 2 nd 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 2 nd 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 19

22 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 1 st and 2 nd 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. 20

23 Sample CuO- 60 mag -1 MSM method CuO- 60 mag -1 Thin film 650 o C CuO- 240 mag -1 Thin film 650 o C CuO- 60 mag -1 precipitation method CuO- 240 mag -1 precipitation method Discharge capacity at 1 st cycle mah g -1 Charge Discharge Discharge Discharge Reversible Irreversible capacity at 1 st capacity at capacity at capacity at capacity Fading capacity cycle mah 2 nd cycle 10 th cycle 40 th cycle from 2-40 loss % g -1 mah g -1 mah g -1 mah g -1 cycle % Table 3: 1 st discharge, 1 st charge, 2 nd charge, 10 th charge and 40 th charge capacity, irreversible capacity loss, and reversible capacity fading of samples prepared by all methods 21

24 Fig. 6: Voltage V vs specific Capacity of sample CuO prepared by a CuO 60 mag -1 MSM method 350 o C b CuO 60 mag -1 Thin film 650 o C c CuO 240 mag -1 Thin film 650 o C d CuO 60 mag -1 precipitation method e CuO 240 mag -1 precipitation met 22

25 Figure. 7: Specific capacity vs Cycle number of CuO prepared by a CuO 60 mag -1 MSM method 350 o C b CuO 60 mag -1 Thin film 650 o C c CuO 240 mag -1 Thin film 650 o C 7. Conclusions We 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 V 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 V compared with current rate of 240mA/g within the voltage range of V. 23

26 8. Acknowledgment I 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. Reference 1. Guyomard, D., et al., New amorphous oxides as high capacity negative electrodes for lithium batteries: the LixMVO4 (M = Ni, Co, Cd, Zn; 1 < x 8) series. Journal of Power Sources, : p Poizot, P., et al., Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries. Nature, : p Sharma, Y., et al., Nanophase ZnCo2O4 as a High Performance Anode Material for Li Ion Batteries. Advanced Functional Materials, (15):p 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, : p 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: Amin G. ZnO and CuO nanostructures: low temperature growth, characterization, their optoelectronic and sensing applications. 1st ed. LiU-Tryck: Norrköping Sweden ; 2012 [SE ] 24

27 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: 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: Anisimov VI, Zaanen J, Andersen OK. Band theory and Mott insulators: Hubbard U instead of Stoner I. Phys Rev B 1991;44: 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: P. Verma, P. Maire and P. Novák, Electrochimica Acta, 55,

28 10. Appendix Macron size CuO Rietveld refinement pattern Intensity (Counts) 80,000 60,000 40,000 20, , Theta (Degrees) Tenorite % 80 Analysis Report Global R-Values Rexp : 1.32 Rwp : 6.35 Rp : 4.08 GOF : 4.81 Rexp`: 3.79 Rwp`: Rp` : DW : 0.18 Range Number : 1 R-Values Rexp : 1.32 Rwp : 6.35 Rp : 4.08 GOF : 4.81 Rexp`: 3.79 Rwp`: Rp` : DW : 0.18 Quantitative Analysis - Rietveld Phase 1 : Tenorite % Background Chebychev polynomial, Coefficient

29 Instrument Primary radius (mm) Secondary radius (mm) Corrections Cylindrical sample 2Th correction ur LP Factor 24 Structure 1 Phase name Tenorite R-Bragg Spacegroup C12/c1 Scale Cell Mass Cell Volume (Å^3) Wt% - Rietveld Crystallite Size Cry Size Lorentzian (nm) 83.0 Crystal Linear Absorption Coeff. (1/cm) Crystal Density (g/cm^3) Lattice parameters a (Å) b (Å) c (Å) beta ( ) Site Np x y z Atom Occ Beq Cu Cu O O