Understanding the Electrochemical Mechanism of the New Iron-Based Mixed-Phosphate Na4Fe3(PO4)2(P2O7) in a Na Rechargeable Battery

Abstract

Compounds with a mixed polyanion framework have recently gained attention as a new class of compounds for material exploration. The potential tunability of the structure by using various combinations of polyanions can potentially lead to a novel cathode. However, the redox reaction in complex structures often involves complex structural evolutions during the electrochemical reaction, which require careful analysis. We investigated the electrochemical mechanism of Na x Fe 3 (PO 4 ) 2 (P 2 O 7 ) (1 ≤ x ≤ 4), which was recently proposed as a promising mixed-polyanion cathode for Na rechargeable batteries, using first principles calculations and experiments. We discovered that the de/sodiation of the Na x Fe 3 (PO 4 ) 2 (P 2 O 7 ) electrode occurs via a one-phase reaction with a reversible Fe 2+ /Fe 3+ redox reaction and accompanies an exceptionally small volumetric change of less than 4%. Na ion intercalation usually induces a large volumetric change in conventional systems; therefore, this small volume change is unusual and was attributed to the open framework of polyanion compounds with P 2 O 7 dimers that are capable of rotating and distorting to accommodate the structural change. Structural robustness was further observed at even highly charged states at temperatures as high as 530 °C from in situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC). We believe that the improved understanding of the electrochemical mechanism provided here will expedite the optimization of the new Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) electrode for Na rechargeable batteries.