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 363K increased with consecutive cycles and reached 100mAhg-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.