Unlocking the Sodium Storage Potential in Fluorophosphate Cathodes: Electrostatic Interaction Lowering Versus Structural Disordering

Abstract

Electrostatic interaction and Na⁺-ordering are identified as two possible kinetic constraints in determining the Na⁺ diffusivity in Na₃V₂(PO₄)₂O₂F (NVPOF), a representative polyanionic-based cathode material for sodium-ion batteries. As both factors are compositionally related and intertwined, isolating individual factors to pinpoint the dominant one is essential yet challenging for achieving the full electrochemical potential of NVPOF. Herein, NVPOF doped with Zn²⁺ or Mg²⁺ is developed to study the relative influence of the electrostatic interaction and structural disordering on the Na⁺ diffusivity and thus Na⁺ storage performance. The crystal structural analysis and theoretical modeling reveal that a limited amount (0.6 at% of Na) of Zn²⁺ doped at the Na-site with Na-vacancies created, while a ten-fold higher Mg²⁺ doped at both the Na- and V-site, which introduces additional Na⁺ for charge compensation. As a result, compared to the Zn²⁺ doped counterpart, the Mg²⁺ doped NVPOF cathode shows a Na⁺ diffusivity up to 3 times higher even encountering larger repulsive forces, and a much enhanced Na⁺ storage property. This work demonstrates the superiority of regulating the degree of order in the framework to address the defect formation energy of NVPOF, which is realized via doping and can be extendable to other polyanionic-based cathode materials.