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

Improving the intrinsic Na+ ion diffusivity of the advanced cathode material Na3V2(PO4)(2)O2F (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).

Automated summary

This study focuses on improving the Na+ ion diffusivity of the cathode material NVPOF by introducing K+ ions, which leads to enhanced rate capability and cycling stability in sodium-ion full cells.