High-Entropy Doping Enabling Ultrahigh Power Density for Advanced Sodium-Ion Batteries.

Abstract

Sodium-ion batteries (SIBs), owing to the plentiful sodium resources, are considered a viable large-scale energy storage substitute for lithium-ion batteries. Recently, Na3V2(PO4)2F3 (NVPF) has been increasingly investigated as an SIBs cathode material. However, the development of this cathode material is hindered by low intrinsic electronic conductivity, poor cycling stability at high rates, and low energy density. This work proposes a high-entropy strategy using multielement low-concentration doping to modulate vanadium sites' morphology, band structure, and coordination environment. Density functional theory (DFT) calculations and advanced analysis show that the d orbitals of transition metals introduce additional energy levels, narrowing the band gap from 1.59 to 0.68 eV and enhancing electronic conductivity. Moreover, the high-entropy effect induces fluorine vacancies, V-O bond contraction, sodium-ion rearrangement at Na3 sites, and particle diameter reduction, collectively improving sodium-ion diffusion kinetics and mitigating detrimental phase transitions. As a result, the high-entropy Na3V1.9Fe0.02Ni0.02Co0.02Mg0.02Cr0.02(PO4)2F3 cathode material exhibits a superior energy density of 460.6 W h kg-1 at 0.5C, an exceptional power density of 15.3 kW kg-1 at 100C, and a capacity retention of 70.5% at 50C after 12,000 cycles. More importantly, the insights obtained here represent significant scientific and technological advancements for the next generation of advanced SIBs.