Crystalline maricite NaFePO₄ as a positive electrode material for sodium secondary batteries operating at intermediate temperature

Abstract

Maricite NaFePO₄ (m-NaFePO₄) was investigated as a positive electrode material for intermediate-temperature operation of sodium secondary batteries using ionic liquid electrolytes. Powdered m-NaFePO₄ was prepared by a conventional solid-state method at 873 K and subsequently fabricated in two different conditions; one is ball-milled in acetone and the other is re-calcined at 873 K after the ball-milling. Electrochemical properties of the electrodes prepared with the as-synthesized m-NaFePO₄, the ball-milled m-NaFePO₄, and the re-calcined m-NaFePO₄ were investigated in Na[FSA]-[C₂C₁im][FSA] (C₂C₁im⁺ = 1-ethyl-3-methylimidazolium, FSA⁻ = bis(fluorosulfonyl)amide) ionic liquid electrolytes at 298 K and 363 K to assess the effects of temperature and particle size on their electrochemical properties. A reversible charge-discharge capacity of 107 mAh g⁻¹ was achieved with a coulombic efficiency >98% from the 2nd cycle using the ball-milled m-NaFePO₄ electrode at a C–rate of 0.1 C and 363 K. Electrochemical impedance spectroscopy using m-NaFePO₄/m-NaFePO₄ symmetric cells indicated that inactive m-NaFePO₄ becomes an active material through ball-milling treatment and elevation of operating temperature. X-ray diffraction analysis of crystalline m-NaFePO₄ confirmed the lattice contraction and expansion upon charging and discharging, respectively. These results indicate that the desodiation-sodiation process in m-NaFePO₄ is reversible in the intermediate-temperature range.