Over five years, we have been conducting R&D of sodium secondary batteries using amide ionic liquids as electrolytes. The new batteries intrinsically have high safety due to the nonflammability and nonvolatility of ionic liquids. Moreover, they can be operated in a wide temperature range of 273-363 K. We found that several types of FSA-based (FSA: bis(fluorosulfonyl)amide) ionic liquids exhibit superior properties for the application to sodium secondary batteries.

In the present paper, we first investigated the fundamental electrolyte properties for the binary Na[FSA]-[C₃C₁pyrr][FSA] (C₃C₁pyrr: N-methyl-N-propylpyrrolidinium) ionic liquid system. Then, NaCrO₂ and hard carbon (HC) were selected as positive electrode and negative electrode active materials, respectively, and their fundamental charge–discharge characteristics were examined with coin-type half-cells. A 1.5 mAh coin-type full cell composed of HC/NaCrO₂ exhibited excellent cycle properties, maintaining 90% of its initial capacity after 1000 cycles at 333 and 363 K. On the basis of these results, the 1.5 mAh full cell was fabricated and its performance was tested. It showed a high charge–discharge energy efficiency of 97.5% at 363 K and C/5 rate. Finally, we constructed a large-sized prismatic HC/NaCrO₂full cell with a capacity of 27 Ah by a mass production process. We confirmed that its performance is comparable with that of the 1.5 mAh coin-type cell.

Acknowledgements

This study was partly supported by Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST) and the Japanese Ministry of Education Culture, Sports, Science and Technology (MEXT) program “Elements Strategy Initiative to Form Core Research Center” (since 2012).