Charge–discharge behavior of tin negative electrode for a sodium secondary battery using intermediate temperature ionic liquid sodium bis(fluorosulfonyl)amide–potassium bis(fluorosulfonyl)amide

The charge–discharge behavior of a Sn negative electrode for use in a sodium secondary battery was investigated in an intermediate temperature ionic liquid, NaFSA–KFSA (xNaFSA = 0.56, xKFSA = 0.44, FSA = bis(fluorosulfonyl)amides), at 363 K. A Na/Sn half-cell was prepared using a Sn-film (10–12 μmt) electrode. The charge–discharge curves at 0.619 mA cm−2 with cut-off voltages of 0.005 and 1.200 V exhibited three potential plateaus in both the charging (alloying) and discharging (de-alloying) processes, indicating the formation of various alloy phases. Charge and discharge capacities as high as 790 and 729 mA h (g-Sn)−1, respectively, were obtained for the 1st cycle; however, the capacities decreased rapidly due to volume change during the alloying and de-alloying processes. The best cycleability was achieved when the lower and higher cut-off voltages were set to 0.005 and 0.200 V, respectively. In this case, a reversible capacity of about 300 mA h (g-Sn)−1 was obtained for the initial 15 cycles. ► Charge–discharge of Sn negative electrode was tested in Na/NaFSA–KFSA/Sn at 363 K. ► Three potential plateaus appeared during both charge and discharge cycles. ► 1st cycle charge and discharge capacities were 790 and 729 mA h (g-Sn)−1. ► Cycleability and capacity dropped due to volume change during alloying/de-alloying. ► Reversible capacity was 300 mA h (g-Sn)−1 for 15 cycles with most negative plateau.