Electrochemical Sodiation-desodiation of Maricite NaFePO₄ in Ionic Liquid Electrolyte

Abstract

Maricite NaFePO₄ (m-NaFePO₄) has long been regarded as an electrochemically inactive material because Na+ cannot overcome the activation energy of its diffusion pathways for sodium extraction and insertion in the structure. In this study, the charge-discharge behavior of m-NaFePO₄ ball-milled to the nano-size level was investigated in Na[FSA]-[C₃C₁pyrr][FSA] (C₃C₁pyrr = N-methyl-N-propylpyrrolidinium and FSA = bis(fluorosulfonyl)amide) ionic liquid electrolyte at 298 and 363 K. The charge-discharge performance was improved upon elevating the operational temperature, possibly because of the enhanced Na+ diffusion in the maricite structure and in the electrolyte, and improved electrode reaction. The reversible capacity at 363 K increased with consecutive cycles and reached 100 mAh g−1 with nearly 100% coulombic efficiency in the 120th cycle at the C/2 rate. Ex-situ XRD results confirmed the preservation of the maricite phase after cycling, which may indicate that the practical capacity for maricite NaFePO₄ without amorphization. By reconsideration of electrochemically inactive materials for intermediate temperature operation, this study suggests the possibility of extending the range of available positive electrode materials for sodium secondary batteries.