High-performance NaFePO₄ formed by aqueous ion-exchange and its mechanism for advanced sodium ion batteries

Room-temperature sodium ion batteries (SIBs) have attracted tremendous attention recently as cheaper alternatives to lithium ion batteries (LIBs) for potential application in large-scale electrical energy storage stations. Among the various classes of iron phosphate cathodes used in SIBs, olivine NaFePO₄ is one of the most attractive host materials for advanced sodium ion batteries owing to its electrochemical profile and high theoretical capacity. As an alternative to the organic-based electrochemical ion-exchange process which is disadvantaged by sluggish dynamics and co-intercalation of Li⁺, we investigated an aqueous-based, electrochemical-driven ion-exchange process to transform olivine LiFePO₄ into highly pure olivine NaFePO₄, which shows superior electrochemical performance. Using a combination of ab initio calculations and experiments, we demonstrate that the mechanism is attributed to the much faster Na⁺/Li⁺ ion-exchange kinetics of NaFePO₄ at the aqueous electrolyte/cathode interface compared to the organic electrolytes. Operando Fe K-edge XANES and XRD were also carried out to study the staged evolution of phases during the sodiation/desodiation of NaFePO₄ nanograins.