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

The intermediate product VPO₄/C is synthesized via sol-gel method, then it is mixed with other raw materials and calcined to prepare Li_1-xK_xVPO₄F/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 Li_0.99K_0.01VPO₄F/C exhibits the best electrochemical performance. With 2.9 wt% amorphous carbon uniformly wrapping the surface, Li_0.99K_0.01VPO₄F/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.