In situ X-ray absorption near edge structure studies and charge transfer kinetics of Na₆[V₁₀O₂₈] electrodes

Polyoxometalates (POMs) have been reported as promising electrode materials for energy storage applications due to their ability to undergo fast redox reactions with multiple transferred electrons per polyanion. Here we employ a polyoxovanadate salt, Na₆[V₁₀O₂₈], as an electrode material in a lithium-ion containing electrolyte and investigate the electron transfer properties of Na₆[V₁₀O₂₈] on long and short timescales. Looking at equilibrated systems, in situ V K-edge X-ray absorption near edge structure (XANES) studies show that all 10 V⁵⁺ ions in Na₆[V₁₀O₂₈] can be reversibly reduced to V⁴⁺ in a potential range of 4-1.75 V vs. Li/Li⁺. Focusing on the dynamic response of the electrode to potential pulses, the kinetics of Na₆[V₁₀O₂₈] electrodes and the dependence of the fundamental electron transfer rate k₀ on temperature are investigated. From these measurements we calculate the reorganization energy and compare it with theoretical predictions. The experimentally determined reorganization energy of λ = 184 meV is in line with the theoretical estimate and confirms the hypothesis of small values of λ for POMs due to electrostatic shielding of the redox center from the solvent.