High-Entropy Doped KTiOPO₄-Type Vanadium-Based Fluorophosphate Cathodes for High-Energy Sodium-Ion Batteries

The development of high-energy-density and high-power cathode materials represents a critical requirement for advancing practical sodium-ion battery (SIB) technologies. In this work, a high-entropy-doped KTiOPO₄ (KTP)-type NaV_0.95(Fe, Mn, Ni, Al, Ca)_0.05PO₄F (HE-NVPF) cathode material is presented, designed to enhance reaction kinetics, operation voltage, and energy density through single-crystal phase formation and improved electronic/ionic conductivity. The high-entropy doping strategy enables the elimination of inductive nucleation agents while promoting single-crystal growth of HE-NVPF during low-temperature hydrothermal synthesis. The KTP-type crystal structure facilitates complete Na ion utilization and enables a solid-solution Na storage mechanism in HE-NVPF cathodes, accompanied by minimal lattice volume changes (4%). When tested in half cells in combination with Na metal anode, the HE-NVPF cathodes exhibit a remarkably high energy density of 532 Wh kg⁻¹ with an average operating voltage of 4.0 V, an exceptional long cycle life of 3 000 cycles, and high capacity retentions at 30 C (2 min per charge). Its practical feasibility is demonstrated in graphite//HE-NVPF full cells, which present power densities of above 10 000 W kg⁻¹ and energy densities of over 342 Wh kg⁻¹ for 1000 cycles. This work offers new insights into designing high-entropy doped cathode materials for long-life and fast-charging SIBs.