Preparation and Improved Electrochemical Properties of PANI– Li3V2(PO4)3 composite As Cathode Material for Lithium Ion Batteries
In recent decades, lithium transition-metal phosphates such as LiMPO4 (M=Fe, Co and Mn) and Li3M2(PO4)3 (M=V, Fe and Ti) have been extensively studied as cathode materials for lithium ion batteries, due to high theoretical capacity, reversibility and better thermal stability than lithium transition metal oxides such as LiCoO2. 1-2
Monoclinic lithium vanadium phosphate, Li3V2(PO4)3, having structure similar to the open framework NASICON with corner shared VO6 octahedra and PO4 tetrahedra contains three independent lithium sites. The poly-anion instead of the O2− ions helps to stabilize the structure and allows a faster ion migration. The reversible cycling of all three lithium ions from Li3V2(PO4)3 would correspond to a theoretical capacity of 197 mAh/g in the voltage range of 3.0-4.8V, which is the highest reported of the phosphates. However, Li3V2(PO4)3 has poor conductivity, which restricts its practical application for lithium ion batteries. Much research has been devoted to increasing its conductivity by controlling particle size, carbon coating and metal doping in the V sites. 3-4
In this study, we tried to improve the electrochemical performance of the Li3V2(PO4)3cathode materials which have poor electronic conductivity by the wet ball-milling method and using PANI as conducting network.
Results and Discussion
Li3V2(PO4)3 particles were uniformly distributed within the PANI, forming a conducting network for easy movement of electrons. The highly porous and amorphous conducting polymer is also responsible for storing additional electrolyte near the surface of the electrode materials as represented in the SEM and TEM given in figure 1. This facilitated the increase in ionic and electronic conductivity, which in turn improved the electrochemical performance of the Li3V2(PO4)3. The content of the conducting polymer was optimized such that the charge-discharge mechanism of Li3V2(PO4)3 was not affected. The capacity retention of the PANI-Li3V2(PO4)3 composites were evaluated under various current rates. A considerable increase in capacity at high current rates was observed for the PANI-Li3V2(PO4)3 composite than the corresponding discharge capacity of pristine Li3V2(PO4)3 (figure 2). At 5C the capacity of PANI-Li3V2(PO4)3 composite was 122 mAh g-1 and the pristine Li3V2(PO4)3 was 102 mAh g-1. The reason behind the improvement in electrochemical properties of the PANI-Li3V2(PO4)3 composite substantiating with the physical and electrochemical studies will be discussed in detail.
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