Sodium Ion Insertion/Extraction Properties of Sn-Co Anodes and Na Pre-Doped Sn-Co Anodes

Sodium-ion batteries (SIBs) are anticipated as promising alternatives to replace lithium-ion batteries. There are many reports on anode materials for SIBs, such as hard carbon and tin  [1,2]. However, there are hardly any reports on materials that provide both high capacity and stable cycle property. In this study, we focused on Sn-Co as an anode for SIBs. Sn-Co has been reported as a good anode for lithium ion batteries [3].

We investigated electrochemical properties of Sn-Co, and examined the correlation between the cycle performance and the binders of electrode component materials. A working electrode was prepared by mixing Sn-Co, Ketjen Black EC600JD, and polyvinylidene di fluoride (PVdF) or polyacrylic acid (PAA). Sodium metal was used for the counter electrodes.

Figure 1(a) shows the first discharge-charge curves of Na/Sn-Co cells incorporating PAA or PVdF as binder. The first capacities of electrode incorporating PAA and PVdF were 505 and 569 mAh/g, respectively. Na/Sn-Co cells using both binders showed two distinct plateaus (about 0.6 and 0.2 V), and the plateau regions were similar to a two phase equilibrium in Na-Sn alloy [2].

Figure 1(b) shows cycle properties of Na/Sn-Co cells incorporating PAA or PVdF as binder. The electrode incorporating PAA showed a better cycle property than the one incorporating PVdF. The discharge capacity of the former reached about 300 mAh/g after 30 cycles. This good cycle performance is attributed to buffering of the volume change during insertion and extraction of sodium ions because of the porous structure of PAA [4].

In addition, a large irreversible capacity loss in the first cycle in Na/Sn-Co cells (binder: PAA or PVdF) was observed. We tried sodium pre-doping to reduce the irreversible capacity and found that the pre-doping technique greatly reduced it. The detailed results will be shown at the meeting.

References

[1]    S. Komaba et al., Electrochem. Commun., 21, 65 (2012).

[2]    L. D. Ellis et al., J. Electrochem. Soc., 159, A1801 (2012).

[3]    N. Tamura et al., Electrochim. Acta, 49, 1949 (2004).

[4]    Y.-S. Park et al., J. Power Sources, 248, 1191 (2014).

Acknowledgement

We are grateful to Mitsubishi Materials Corp. for supplying Sn-Co.