Potassium doping towards enhanced Na-ion diffusivity in a fluorophosphate cathode for sodium-ion full cells

Abstract

Improving the intrinsic Na⁺ ion diffusivity of the advanced cathode material Na₃V₂(PO₄)₂O₂F (NVPOF) has been a fundamental strategy to enhance its Na⁺ ion storage performance, while challenges remain in structural design and fabrication with specific features targeting the alleviation of migration energy barriers of the Na⁺ ions at different (de)sodiated states. Herein, K⁺ ions are introduced into the Na-sites in NVPOF to create Na⁺ ion vacancies, support the Na⁺ ion transport channels in the c axis and disrupt the continuity of the crystal structure. This strategy simultaneously reduces the electrostatic repulsive forces on the migrating Na⁺ ions from neighboring Na⁺ ions at a sodiated state and energy barriers originating from Na⁺ ion ordering at a desodiated state. Thus, the obtained NVPOF-K_0.05 cathode presents a dramatic enhancement in the rate capability up to 80C (49.1 mA h g⁻¹) and almost no decay in the long cycles at 10C up to 500 cycles (106.7 mA h g⁻¹). The NVPOF-K_0.05//Se@C sodium-ion full cell exhibits outstanding energy/power density (430.3 W h kg⁻¹ at 208.3 W kg⁻¹ and 310.5 W h kg⁻¹ at 3357.1 W kg⁻¹) and cycling stability (86.5% capacity retention of 107.1 mA h g⁻¹ after 1000 cycles at 10C).