Compositional Tuning of Mixed Chromium Monopotassium Phosphates by Ni 2+ /Co 2+ Substitution for Energy Storage Applications

This study synthesized a solid-solution series of KCo1–xNixCr2(PO4)3 phosphates with the α-CrPO4-type three-dimensional framework, in which edge-sharing CrO6 octahedra and PO4 tetrahedra create tunnels hosting K+ cations. Systematic Ni2+/Co2+ substitution was used to tune dielectric and charge-transport properties. All compounds showed stable paraelectric behavior; increasing Co content enhanced polarizability and dielectric permittivity while maintaining low dielectric loss. When used as electrode materials in graphite-based supercapacitors, higher Co content led to improved electrochemical performance, with the Co1Ni0 composition delivering a specific capacitance of 447 F/g, an energy density of 48.18 Wh/kg, and a power density of 1752 W/kg. The results demonstrate that Ni2+/Co2+ substitution is an effective strategy for designing advanced supercapacitor electrodes that combine high charge-storage capacity (due to increased permittivity) with improved power capability (due to higher conductivity).