Preparation and Improved Electrochemical Properties of PANI– Li3V2(PO4)3 composite As Cathode Material for Lithium Ion Batteries

Introduction

 

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.

References

 

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