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.