The multiple effects of potassium doping on LiVPO4F/C composite cathode material for lithium ion batteries

Abstract

The intermediate product VPO4/C is synthesized via sol-gel method, then it is mixed with other raw materials and calcined to prepare Li1-xKxVPO4F/C (x = 0, 0.005, 0.01, 0.02). Potassium with large ionic radius partially substitutes the lithium site and slightly increases the unit cell volume. The potassium dopant suppresses the formation of Li3V2(PO4)3 and reduces the particle agglomeration. The doped samples possess lower polarization and higher discharge plateau than the pristine LiVPO4F/C, and Li0.99K0.01VPO4F/C exhibits the best electrochemical performance. With 2.9 wt% amorphous carbon uniformly wrapping the surface, Li0.99K0.01VPO4F/C delivers a reversible capacity of 140.9 mA h g-1 at 0.12 C and maintains 98.3 mA h g-1 at the charge/discharge rate of 10 C. Its specific discharge capacity with a retention of 96.13% decreases from 131.9 to 126.8 mA h g-1 after 125 cycles at the charge/discharge rate of 1 C. The improvement of rate and cycling performance is ascribed to the decrease of charge transfer resistance (88.47 Ω) and the increase of Li+ diffusion coefficient (6.75 × 10−13 cm2 s−1), indicating that potassium doping facilitates the migration and diffusion of Li+ due to the expansion of Li+ pathway.