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

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 1000 mA (g-HC)⁻¹: the discharge capacity is 230 mAh (g-HC)⁻¹ at 90 °C and 25 mAh (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 2.8 V 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