Electrochemical behavior of Sn-Fe alloy film negative electrodes for a sodium secondary battery using inorganic ionic liquid Na[FSA]-K[FSA]

Sn-Fe alloy thin films were prepared by electroplating of tin on iron substrates and annealing them at 533 K for 0 to 25 h. The charge-discharge behavior of the films as negative electrodes for sodium secondary batteries was evaluated using a Na[FSA]-K[FSA] (FSA = bis(fluorosulfonyl)amide) ionic liquid at 363 K. Although the Sn-Fe film without annealing (pure Sn film) showed a relatively high discharge capacity of 730 mAh (g-Sn)−1 in the first cycle, its capacity rapidly decreased to 30 mAh (g-Sn)−1 within 10 cycles. In contrast, Sn-Fe films annealed for 2 or 4 h retained higher discharge capacities of 191 or 82 mAh (g-Sn)−1 after 100 cycles, respectively, due to the formation of an FeSn2 alloy. Additionally, an FeSn2 film was prepared by removing the SnO layer of the Sn-Fe alloy film that was annealed for 25 h, and its charge-discharge behavior was investigated.