Electrochemical performance of hard carbon negative electrodes for ionic liquid-based sodium ion batteries over a wide temperature range

Abstract

Sodium ion batteries (SIBs) have been attracting much attention as promising next-generation energy storage devices for large-scale applications. The major safety issue with SIBs, which arises from the flammability and volatility of conventional organic solvent-based electrolytes, is resolved by adopting an ionic liquid (IL) electrolyte. However, there are only a few reports on the study of negative electrodes in ILs. Here, we report the electrochemical performance of a hard carbon (HC) negative electrode in Na[FSA]-[C3C1pyrr][FSA] (FSA=bis(fluorosulfonyl)amide, C3C1pyrr=N-methyl-N-propylpyrrolidinium) IL over a wide temperature range of −10°C to 90°C. High-temperature operation, which is realized for the first time by using an IL, can take full advantage of the high capacity of HC even at a very high discharge rate of 1000mA (g-HC)−1: the discharge capacity is 230mAh (g-HC)−1 at 90°C and 25mAh (g-HC)−1 at 25°C. Moreover, surprisingly stable cycleability is observed for the HC electrode at 90°C, i.e. a capacity retention ratio of 84% after 500 cycles. Finally, a high full-cell voltage of 2.8V and stable full-cell operation with Coulombic efficiency higher than 99% are achieved for the first time when using NaCrO2 as the positive electrode at 90°C.