The evolution of redox potentials in pyrochlore-based intercalation anodes for potassium-ion batteries

Tungsten oxides and related compounds have been known as negative electrode materials for metal-ion batteries for decades. Despite their structural flexibility, most studies has largely focused on lithium-based energy storage systems. Here, we investigated KM_xW_2−xO₆ (M = Ta, Nb, Ti, Cr, Al) oxides adopting a defect pyrochlore structure as intercalation-type anodes for potassium-ion batteries. Crystal structure, chemical composition, and thermal behavior of all representatives were comprehensively characterized. Electrochemical testing in K half-cells revealed average operating potentials of the considered pyrochlores to be in the range of ∼1.1–1.3 V vs. K⁺/K, consistent with density functional theory predictions. The variation in the electrochemical performance among the KM_xW_2−xO₆ was correlated with differences in electronic conductivity of M dopants, as validated by M density of states calculations. Through regression analysis of Rietveld-refined structural data, a strong linear dependence (ρ = 0.96) between the average intercalation potential of KM_xW_2−xO₆ and the intrinsic parameters (ionic radius and electronegativity) of the M metal was established. This work not only advances fundamental understanding of tungsten-based oxygen-deficient pyrochlores but also paves the way for their development as potassium-ion intercalation hosts.