Improved Electrochemical Performance of NaVOPO4 Positive Electrodes at Elevated Temperature in an Ionic Liquid Electrolyte

Abstract

Sodium secondary batteries operating in a wide temperature range are attractive as large-scale energy storage devices, and ionic liquid electrolytes are suitable for this purpose. In this study, NaVOPO4 has been investigated as positive electrode material for Na secondary batteries, and its electrochemical performance has been examined in the Na[FSA]–[C3C1pyrr][FSA] ionic liquid (C3C1pyrr = N-methyl-N-propylpyrrolidinium and FSA = bis(fluorosulfonyl)amide) at 298 and 363 K. The NaVOPO4 electrode exhibits a reversible capacity of 60 and 101 mAh g−1 at 298 and 363 K, respectively. Acceptably good rate capability is achieved at 363 K, as 76% of the maximum capacity is maintained at 5 C rate. Cyclability tests prove good reversibility of the material, in which 74% of the initial specific capacity maintains over 300 cycles at 363 K. XRD measurements reveal that the charge–discharge process of NaVOPO4 involves a single-phase reaction. Galvanostatic intermittent titration technique (GITT) analysis highlights a 3–5-fold increase of the apparent Na chemical diffusion coefficient in NaVOPO4 upon increasing the temperature from 298 to 363 K, which is reflected in the superior electrochemical performance at 363 K than at 298 K.