Synthesis and electrochemical analysis of lithium vanadium(III) phosphate
Due to cost inefficiency and safety issues, scientists have studied the use of high capacity lithium vanadium(III) phosphate (LVP) as a possible substitute cathode material in rechargeable lithium batteries in place of lithium cobalt(III) oxide. Through research, three methods of synthesising LVP has been established but relationship between synthesis and electrochemical performance has yet to be studied. Thus the aim of the project was to synthesise LVP using all three known methods, understand their physical properties/morphology and test their electrochemical properties. The aim was to find out which method would yield LVP with the best electrochemical performance/best substitute cathode material.
Three different methods of synthesizing Lithium vanadium(V) phosphate (LVP) were experimented with. These three methods were polymer precursor method, carbothermal reduction (with ball-milling) method and carbothermal reduction (without ball-milling) method. The three samples of LVP synthesised were examined by scanning electron microscopy to identify key differences in their morphologies. The main difference observed is the porosity of the samples. Polymer precursor synthesized LVP sample has a less porous morphology while the other two samples which have porous plate-shaped and flake-like particles. Coin cells (CR2016) were fabricated using the LVP synthesised as cathode material. Coin cells then underwent electrochemical analysis like galvanostatic cycling (GC) and cyclic voltammetry (CV) in order to determine best method of synthesis. The capacity values and capacity fading were some of the main factors considered in the comparison. Cycling occurred at the potential range of 3.0-4.6V and at 17mAg-1. CV and GC cycling shows characteristic oxidation peaks at ~3.7 and ~4.1V vs. Li. The results also indicated that LVP synthesised through carbothermal reduction with ball-milling was most desirable with 143mAhg-1 and low capacity fading of ~2.1% over 30 cycles
My name is Kuan Tzu Hsiang and I am a junior college student in River Valley High School, Singapore. What I like doing the most is to observe, investigate and try to understand the things that happen to me everyday and to apply this knowledge to aid my community, friends and I during work and play.
2 years back I took part in my school's science mentorship program and the experience from this program actually ignited my passion in working in science research. Through the project I learnt about the scientific method and realised that te process of suggesting and procving ideas is fun. A take away from this lesson learnt was that it help me learn from my environment and aided me in developing my critical thinking skills allowing me to better observe and understand problems we face and even suggest solutions for these problems.
In college I might want to take a course in Physics and examine the topic of electronics in detail. Through this prject I have realised that electronics can impact our lives greatly and would be a great area to try and develop to make the world a better place as such I might also take a career as a researcher.
Winning the Google Science Fair would be a steping stone in accomplishing my dreams as a scientist and would also allow me to acquaint myself with researchers at CERN and understand more about the world of Physics and Science.
Nowadays, advancements in technology have rendered mobile devices ubiquitous in our societies. Just take a walk down the streets and you will see that almost everyone is using mobile devices like smart phones, tablets and laptops. But what do these devices have in common? They all contain a rechargeable battery. In fact vehicles like cars and planes and even medical implants carry rechargeable batteries and require them to operate. However, our current commercial rechargeable battery the lithium cobalt oxide battery face many serious issues such as safety and low cost-effectiveness rendering its future use unsustainable considering the fact the market for rechargeable batteries is so large. Lithium vanadium(III) phosphate (LVP) has been proven to be a viable substitute and scientists have made a breakthrough in this area of research, having established 3 different ways to synthesize LVP. But what is the best method to synthesize LVP? That is the question I aim to answer.
To quantify electrochemical performance capacity of the sample could be used. Capacity is a scalar quantity that can be calculated from current, time and weight which are all measurable physical quantities that can be easily obtained through electrochemical analysis/
Electrochemical property is quantified by capacity. Capacity is proportional to rate of redox couple reactions and rate of redox couple reactions is proportional to surface area of compound in contact with the electrolyte. Hence it can be inferred that the most porous sample/ sample with greatest surface area would have the best electrochemical performance.
Since the invention of the first battery, the Voltaic Pile, in the early 1800s, the idea of having a portable source of energy has revolutionized the way energy is utilized to benefit humans. A few decades back scientists have already observed that lead acid batteries were problematic and were expensive yet ineffective.
To overcome this problem research was carried out. Batteries investigated included nickel-cadmium batteries, phosphate-lithium ion batteries, nickel-metal-hydride batteries, lithium-ion batteries and even lithium-metal batteries (which were highly dangerous). Through trial and error it was discovered that lithium-ion batteries were best suited for the job as rechargeable batteries as it has high volumetric energy density and gravimetric energy density. This translates into lithium-ion batteries being the smallest and the lightest (a feature desired for modern day electronic devices). This research gave birth to the lithium-ion batteries (LIBs). LIBs were the choice batteries and the 1st generation LIBs had lithium cobalt oxide as cathode.
High costs and low thermal stability urged researchers to seek alternative materials. Scientists tried to replace cobalt with other metals such as manganese and nickel and this gave birth to the 2nd generation LIBs with general formula of Li(NixMnxCo1-2x)O2. This was an improvement but the problems of high costs and safety still plagued the LIBs. Scientists then began to look at iron and phosphate compounds as substitutes and discovered the 3rd generation cathode in the process – LiFePO4. Although safety has been improved the high costs of new batteries deterred producers from creating 2nd and 3rd generation LIB batteries thus the 1st generation LIBs are still in use today.
Through literature review I was able to identify the main problem of LIBs. High costs is the main factor in deterring the market from adapting any other batteries other than 1st generation LIBs since the other LIBs are relatively more expensive than the expensive LiCoO2 cathode.
With this in mind the key objective of my project was influenced. The aim was now to find a cathode material that is both cheaper and more effective than LiCoO2.
Synthesis of LVP
Polymer Precursor Method
The synthesis of LVP was based on following chemical reaction:
2NH4VO3 + 3LiCH3CO2 + 3NH4H2PO4 + C6H9NO à Li3V2(PO4)3 + by-products
Molar quantity of each chemical as indicated by the chemical equation is mixed together, the mixture was then dissolved in 100cm3 of deionised water and heated at 180°C under reflux for 12h. Excess water was then evaporated off and sample was then ground with mortar and pestle. Sample was then incubated in tube furnace at 750°C for 6h in argon (Ar) gas. After incubation, the sample was ground again to form a fine powder. During experimentation, the desired mass of reactants was 10g, as indicated in table 1. The reactant mixture yielded 3.115g of LVP.
Carbothermal Reduction Method
The carbothermal method was used to prepare cathode materials. In this project, a method similar to that proposed by Barker et al (2003)   was adopted to prepare LVP.
V2O5+3LiOH+3NH4 H2PO4+4C à Li3 V2 (PO4 )3+CO+ by products
Stoichiometric amount of chemicals were mixed well. For carbothermal synthesised LVP (with ball-milling), the mixture was mixed using ball miller (Spex Ball miller, USA) for 4h and for carbothermal synthesised LVP (without ball-milling),the mixture was mixed using a mechanical grinder for 20 min. Carbon acts as a reducing agent to reduce V5+ to V3+ in this reaction. After mixing, the samples were transferred to an alumina boat and heated at 750°C for 6 h in argon (Ar) gas. Ar gas is inert and would reduce vanadium from V3+ to V5+. Oxygen is unsuitable as reduction of vanadium may not occur. Approximately 5-6g of sample was synthesised for structural and electrochemical studies.
A slurry of the various LVP samples was prepared. The slurry was composed of 70% sample, 15% super P carbon and 15% polyvinylidene fluoride (PVDF) by mass. N-methyl pyrrolidone (NMP) was added as a solvent for PVDF subsequently. Using a magnetic stirrer the mixture was allowed to be mixed for 8h at room temperature and 2000 rounds per minute (rpm). Using a Doctor Blade, a thin film of slurry of approximately 20μm was spread on a sheet of aluminium (Al) foil. Al was the material of choice as it helps to prevent the oxidation of active material. The Al foil with the coat of slurry was then left in an oven to dry. Circular electrodes with diameter of 16mm were then cut out from the Al foil coated with slurry.
The anode of the cell was a circular disc of lithium metal; the electrolyte was a 1M solution of lithium hexafluorophosphate (LiPF6) dissolved in mixture of ethylene carbonate and dimethyl carbonate (1:1 ratio by volume).The cell parts were assembled in the following sequence: bottom cap, Li metal disc (anode), 10μL electrolyte, separator, 10μL electrolyte, cathode, spring, O-ring and cell cap. A stamping machine was then used to press the parts together.
Synthesis of LVP
During experimentation, it was discovered that from a same mass of reactants (10g), the carbothermal reduction method yielded ~5-6g of LVP while the polymer precursor method yielded ~3g of LVP, suggesting that the carbothermal reduction method has a higher yield than polymer precursor method.
The X-ray diffraction of the three LVP samples produced similar results [Figure 1] with lattice parameters as such a ≈ 8.2222Ǻ, b ≈ 8.5834 Ǻ, c ≈ 11.7709 Ǻ.
Since lattice parameter a ≠ b ≠ c, it can be inferred that LVP had the monoclinic structure [Figure 2] which is in agreement with findings of Wang et al (2012) .
From Figure 1, the purity of the LVP synthesised could also be assessed. As seen the experimental data (red line) follows the trend of the fitted data (green line) very closely. This suggests that there were few impurities within the synthesized samples and that the method of synthesis was successful.
SEM images [Figures 3.1a, 3.1b and 3.1c] showed that LVP particles were micron-sized and ~1μm. A comparison of SEM images also revealed that morphology of samples synthesised by carbothermal reduction (Figures 3.1b and 3.1c) differed from polymer precursor synthesised sample (Figure 3.1a). Polymer precursor sample had plate-like shaped particles and carbothermal synthesised samples had flake-like particles also existed within them. The morphology also revealed that all three samples were porous and with numerous pores of ~0.5μm in diameter on their surfaces. Comparison of SEM images obtained with those presented in past research by Wang et al in 2010 showed that LVP synthesised have similar morphologies .
Results from galvanostatic cycling agreed with other earlier research work by Jiang et al (2009)  and indicated that LVP indeed has a high capacity (theoretical capacity is 197mAhg-1) . It can also be observed that in the potential region of 3.0V-4.6V that LVP samples experienced three potential plateaus [as illustrated in Figures 4.1, 4.2 and 4.3]. Furthermore, results also suggest that LVP synthesised via polymer precursor method had the lowest performance level with capacity dropping to ~80mAhg-1 after the 1st cycle. The performance of carbothermal synthesised LVP samples were comparable with one another and capacity reached ~140mAhg-1. However, based on the maximum capacity, the LVP sample synthesised through carbothermal reduction method with ball-milling (max capacity = 143mAhg-1) proved to be better than the sample synthesised through carbothermal reduction without ball-milling (max capacity = 140 mAhg-1). Hence, it can be inferred that the carbothermal reduction method with ball-milling is a better method of LVP synthesis compared to the other two methods. Refinement of results indicated that LVP syntehsized via carbotermal reduction with ball-milling has least capacity fading of 2.1%.
LVP samples were successfully characterised through the various physical tests including XRD, BET, gas pycnometer and density. SEM imaging as well as computing software was also used to ascertain the structure, purity and structural integrity of LVP samples synthesised by comparing them with theoretical data from various databases. High compatibility of results from characterisation with findings previously published by Zheng et al in 2010  proves that methods of synthesis used were effective.
From experimental data it can be seen that LVP synthesised by carbothermal reduction method with ball-milling achieved the best electrochemical performance. It had the highest capacity of 143mAhg-1 and also the lowest capacity fading of ~2.1% after 30 cycles suggesting its potential role as a substitute cathode material for LIBs.
However, LVP sample synthesised by polymer precursor method was least promising. With a capacity of 80mAhg-1 which is much lower than theoretical capacity of 197mAhg-1 and its high capacity fading of ~7.5% over 30 cycles LVP sample synthesised via polymer precursor method is an unsuitable alternative cathode material for lithium cobalt(III) oxide used in most commercial batteries.
Results do not support my hypothesis as the surface area of the samples are similar. In light of my results, future research may be conducted.
Incompatibilities of theoretical data and experimental data of ~20% of XRD graphs indicate that methods of synthesis suggested have room for improvement.
In addition, the relationship between changes in crystal structure of LVP with oxidation and reduction of vanadium ions are unknown at the time of writing. For improved performance, further investigation can be carried out in this area to fully understand the redox couple reactions occurring during charge and discharge cycling of LVP batteries. This can be achieved by X-ray photo electron microscopy to observe morphology of samples during cycling. Furthermore, structural studies during charge cycle by X-ray diffraction to determine crystal structure of samples during cycling may also be executed.
Also the reasons for improved performance of LVP sample synthesised via carbothermal reduction method as compared to LVP sample synthesised via polymer precursor method is still not fully understood. Studies of how preparation techniques affect the morphology of samples synthesised may be carried out to investigate relationships between preparation methods and electrochemical performance of samples synthesised.
Further studies on current rate and long term cycling studies of LVP samples are also possible areas worth investigating as it may allow for determination of conditions necessary to optimise electrochemical performance of LVP.
I would like to thank Dr M. V. Reddy from NUS department of physics for his continual support and guidance throughout the project. Miss Elaine Phuar and Mr Chow Ban Hoe of River Valley High School for their kind guidance. Professor B. V. R. Chowdari and Professor K.P. Loh for their support and other staff and students at the Advanced Battery Lab for their kind assistance. I would also like to extend my gratitude to the Department of Physics, Faculty of Science, National University of Singapore, Organisers from Gifted Education Branch, Ministry of Education (MOE) for the opportunity to participate in the Science Research Programme.
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