An ab initio study of high-voltage fluorine-rich cathodes for potassium ion batteries

Abstract

High-potential cathode materials are desirable for alkali metal ion batteries. Four fluorine-rich phosphate-based hosts including two monoclinic and two orthorhombic phases were explored through ion substitutions based on experimentally synthesized polyanionic compounds. These polymorphs were proved thermodynamically and dynamically stable by first-principles calculations. Although the KVPO₄F₂ family shares the same nominal chemical composition, the open circuit voltages (OCVs) varying from 4.57 to 5.78 V vs. K⁺/K have a strong dependence on the number of F⁻ ions present in the VO_6−xF_x polyhedron; the more the F⁻ ions bonded to the V atom, the higher the OCV that the V⁵⁺/V⁴⁺ redox pair in the framework could provide. Moreover, the layered phase crystalizing in the Pca2₁ space group exhibits an impressively low K⁺ ion migration barrier of 0.07 eV, which may endow the bulk material with fast-charging capability. Based on the Pca2₁-structured prototype, we tuned the cell potential by replacing F⁻ ions with Cl⁻ ions or substituting SO₄²⁻ for PO₄³⁻ anion groups. The resulting KVSO₄F₂ could deliver an average voltage of 4.71 V vs. K⁺/K and demonstrate a diffusion barrier of 0.12 eV, making it a promising high-energy and high-power cathode for practical applications. Our work may provide new insights into the discovery and design of high-voltage electrode materials for KIBs.