The 58th Battery Symposium in Japan / The 21st International Symposium on Batteries, Fuel Cells and Capacitors/Effects of high Na<sup>+ </sup>fraction on sodium secondary battery performance in the Na[N(SO<sub>2</sub>F)<sub>2</sub>]-[C<sub>1</sub>C<sub>1</sub>im][N(SO<sub>2</sub>F)<sub>2</sub>] system