Tin-Based Negative Electrodes for Sodium Secondary Battery Using Na[FSA]-K[FSA] Ionic Liquid Electrolyte

Abstract

Sodium secondary battery is promising as a large-scaled energy storage device owing to an abundant reserve of sodium and its large energy density. Our group has developed a new class of electrolyte for a sodium secondary battery: Na[FSA]–K[FSA] (FSA: bis(fluorosulfonyl)amide) ionic liquid with a wide electrochemical window (ca. 5 V) and practically low melting point (334 K) [1,2]. We have already reported that a Na/Na[FSA]–K[FSA]/NaCrO2 cell exhibits excellent cyclability and rate capability at 353–363 K [2,3]. However, a sodium metal negative electrode has its potential risk of dendritic sodium formation. Thus, the authors focused on tin as a new negative electrode material that has high theoretical capacity (847 mAh (g-Sn)–1) and safety [4,5]. In our previous study, we investigated the sodiation/desodiation process of tin thin films (ca. 10 µm thickness) electrodeposited on an Al foil at 363 K. Fig. 1 shows the initial charge–discharge curve of the film [4]. Three plateaus (a–c) were observed both during charging and discharging process in the 1st cycle. Each plateau corresponds to coexisting state of two Na–Sn alloy phases; (a)β-Sn/α-NaSn, (b)α-NaSn/Na9Sn4, (c)Na9Sn4/Na15Sn4. Although the initial charge (sodiation) and discharge (desodiation) capacities were 796 and 729 mAh (g-Sn)–1, respectively, the reversible capacity rapidly dropped below 100 mAh (g-Sn)–1 within 20 cycles, due to the large volume changes during sodiation/desodiation process (ΔV = 430% for Na15Sn4). In the present study, we used thinner tin films (ca. 1 µm thickness) electrodeposited on Cu foils for the improvement of its cyclability. Sn–Cu alloys inactive to sodium are expected to be formed at the Sn/Cu interfaces by the utilization of Cu foils as a current collector. These alloys simultaneously work as a buffer for the significant volume change and suppress the pulverization of the β-Sn layer reacted with sodium. Therefore, we fabricate various Sn–Cu films by annealing tin-coated copper foils at 463 K for 0–45 h. Fig. 2 shows the cyclability of the negative electrode made of annealed Sn–Cu thin films. Sn–Cu films without annealing (0h-annealed film) showed an initial reversible capacity of ca. 400 mAh (g-Sn)–1, equivalent to a half of the theoretical capacity of tin. This is originated from the low reactivity of Sn–Cu alloys to sodium. The films annealed for 10, 25 and 45h exhibited no β-Sn phase by XRD and much lower capacities than the other films. The best capacity retention was observed for the films annealed for 4 h. The film retained its reversible capacity of approximately 100 mAh (g-Sn)–1 for 100 cycles. This good cyclability would be attributed to the best mixed structure of active β-Sn and less active Sn–Cu alloys. Acknowledgments This study was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST), and MEXT program "Elements Strategy Initiative to Form Core Research Center" (since 2012), MEXT; Ministry of Education Culture, Sports, Science and Technology, Japan.