Synthesis And Electrochemical Properties Of Li₃V₂(PO₄)₃/C And LiV₃O₈ Cathode Materials For Lithium-ion Batteries

Monoclinic Li3V2(PO4)3 is an attractive cathode material for the application in electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to its high theoretical capacity (197mAh -1), high operate voltage, and structure stability during the cycling. Layered lithium vanadate oxide LiV3O8,is also regarded as a promising cathode material in rechargeable lithium batteries because of its potentially high specific capacity and low cost. The main objective in this research is to overcome the drawbacks of Li3V2(PO4)3 and LiV3O8 by several methods, in order to improve their electrochemical performance.Li3Vi(PO4)3/C cathode materials were synthesized by carbon-thermal reduction method using polystyrene, stearic acid and ascorbic acid as the carbon sources, respectively. When employed polystyrene as the carbon source, we mainly focused on the effects of carbon source and carbon content on the electrochemical performance of Li3V2(PO4)3/CO After comparison, the residual carbon produced by the pyrolysis of PS produced fine particle sizes and uniform carbon distribution on the Li3V2(PO4)3 particle surface; better than in composite with acetylene black. In the potential range of 3.0-4.3 V, the lower polystyrene added Li3V2(PO4)3/C with a thin carbon coating possesses the highest initial discharge capacity at lower current densities. However, at high current densities, the higher polystyrene added Li3V2(PO4)3/C with a thicker carbon coating (23-27 nm) shows best electrochemical performance. By using stearic acid as a carbon source, it is found that the Li3V2(PO4)3/C composite synthesized at 700℃shows the best electrochemical performance. The Li3V2(PO4)3/C shows a high initial discharge capacity of 130.6 mAh g-1between 3.0 and 4.3 V, and 185.9 mAh g-1 between 3.0 and 4.8 V at 0.1 C, respectively. Even at a charge-discharge rate of 15 C, the Li3V2(PO4)3/CO still can deliver a discharge capacity of 103.3 and 112.1 mAh g-1in the potential region of 3.0-4.3 V and 3.0-4.8 V, respectively. It is improper for to adopt carbon-thermal reduction method to prepare Li3V2(PO4)3/CO composite by using ascorbic acid as a carbon source. When the precursor was treated by hydrothermal method, the Li3V2(PO4)3/CO with fine particles and well carbon coating can be obtained, exhibiting good electrochemical performance.cathode material was synthesized by carbon-thermal reduction method using polyvinyl alcohol (PVA-124) as a carbon source. The electrochemical properties of the Li.V2(PO4)3/C material at various temperatures (-20.0.25,40 and 65℃) were tested. At-20℃the Li3V2(PO4)3/C electrode presents an high initial discharge capacity of 84.3 mAh g-1 between 3.0 and 4.3 V, and 118.9 mAh g-1 between 3.0 and 4.8 V at 0.1 C, respectively. However, the electrode can only deliver small discharge capacities at -20℃at 10 C rate. At higher temperatures, the capacity increases with the temperature between 3.0 and 4.3 V. but decreases between 3.0 and 4.8 V. EIS analysis reveals that the Ra is considered to be a predominant factor to influence the capacity of the electrode at low temperatures. In the potential range of 3.0-4.8 V. the lower discharge capacity would be mainly resulted from the larger crystal structural distortion and non-uniformity of SEI layer at high temperatures.Spherical porous Li3V2(PO4)3/CO composites were synthesized by a soft chemistry route using hydrazine hydrate as the spheroidizing medium. This porous structure can provide sufficient contact between active materials and electrolyte, thus the electrochemical performance of Li3V2(PO4)3/C composites are enhanced. The spherical porous Li3V2(PO4)3/C electrode shows a high discharge capacity of 129.1 mAh g"1 between 3.0 and 4.3 V, and 183.8 mAh g-1 between 3.0 and 4.8 V at 0.2 C, respectively. Even at a charge-discharge rate of 15 C, this material can still deliver a discharge capacity of 100.5 and 121.5 mAh g-'in the potential region of 3.0-.3 V and 3.0-4.8 V, respectively. Plate-like Li3V2(PO4)3/C composite was synthesized via a solution route followed by CTR by using glycine as the morphology control agent. At a charge-discharge rate of 3 C, the plate-like Li3V2(PO4)3/C exhibits an initial discharge capacity of 125.2 and 133.1 mAh g-1in the voltage ranges of 3.0-4.3 V and 3.0-4.8 V, respectively. After 500 cycles, the electrodes still can deliver a discharge capacity of 111.8 and 97.8 mAh g'correspondingly, showing a good cycling stability.Rod-like LiVsOs composites were fabricated by using a carbamide-assisted rheological phase reaction method. These one-dimensional (ID) materials have been considered as an effective way for achieving high-rate capability and enhancing power performance because they can provide efficient one-dimensional electron transport pathways and accommodate the volume changes during charge/discharge processes. The rod-like LiV3O8 calcined at 500℃has the optimal performance, delivering an initial discharge capacity of 273.6 and 250.4 mAh g-1between 2.0 V and 4.0 V at a current density of 50 and 120 mA g-1, respectively. After 60 cycles by applying 50 mA g-1a discharge capacity of 213.0 mAh g-1is obtained, showing a good cycling performance.Wafer-liked porous xLiV3O8-VyLi0.3V2O5 (Li-V-O) composites are synthesized by a facile self-assembled synthesis using a glycine-assisted solution route followed by a low temperature reaction. The compound synthesized at lower temperature shows low cry stall inity. The higher calcining temperature will result in good crystallinity which leads to the compound have a slightly lower a value, indicating the preferred orientation along the (100) plane and a slightly smaller interlayer spacing in the structure which would lead to a longer diffusion path for the lithium ions and thus depress the electrochemical performance. Among these Li-V-0 composites, the one synthesized at 400℃, which has 27.06 wt.%Li0.3V2O5, exhibits the highest initial discharge capacities of 265.7 and 237.0 mAh g-1at current densities of 50 and 120 mA g-1between 2.0 and 4.0 V, respectively. Even at a high current density of 480 mA g-1 it still can deliver a discharge capacity of 144.7 mAh g"1. The good electrochemical performance of the as-synthesized composite can be attributed to the porous structure, thus highly improves the specific surface area, enhances the contact with electrolyte, and decreases the impedance of Li+migration through surface-passivating layer. In addiction, the diffusion coefficients of Li ions in this composite determined by galvanostatic intermittent titration technique are in the region of 10-'4 to 10-9 cm2 s-1 in the charge/discharge processes.