Sn-Polyacrylate Composite Electrode for Advanced Na-Ion Batteries

Na-ion batteries are attractive rechargeable batteries as Li-free batteries. Sodium resources are abundant, inexpensive elements and unlimited everywhere, unlike lithium availability. Additionally, relatively low redox potential of sodium is slightly higher, 0.3 V, than that of Li/Li⁺. Therefore, many researchers extensively study about Na-ion batteries nowadays and the electrode performance is significantly improved, which is being competitive for Li-ion batteries. We have demonstrated approximately 300 mAh/g discharge capacity after 50 cycles by using hard-carbon electrodes in Na cells.^[1] In order to achieve higher energy battery, electrode materials with high reversible capacity is needed. Thus we have examined group 14 elements (C, Si, Ge, Sn, and Pb) as the potential negative electrodes materials for Na-ion batteries.^[2]  In our previous report, we demonstrated acceptable electrode performance with tin, approximately 500 mAh/g of reversible capacity after 30 cycles.^[2] However, reversible capacity was lower than that of expected capacity (847 mAh/g based on the formation of Na₁₅Sn₄).^[3]In this study, the factors affecting electrode performance of tin electrochemically alloying with Na are described; slurry conditions, cut-off voltages and so on. After the optimization of tin composite electrodes, finally, we have demonstrated more than 650 mAh/g of reversible capacity with good capacity retention.

  Nanosized tin powder (<150 nm, Sigma-Aldrich Co., Ltd), graphite, and sodium polyacrylate (PANa) were mixed with 80:10:10 (wt%) to prepare slurry with deionized water mixed with or without methanol (5 vol%) and were cast onto Al foil to make composite electrodes. The electrode was tested in aprotic Na cell with NaPF₆PC solution with FEC (2 vol%) additive. As shown in Fig.1, the higher reversible capacity (approximately 700 mAh/g) is obtained by addition of 5% methanol to the slurry. Approximately 200 mAh/g discharge capacity is increased compared with the slurry prepared without the addition of methanol. Change in the electrode morphology prepared with or without methanol in the slurry was observed by SEM (Fig. 2). SEM observation reveals that the uniform morphology and better dispersion of the active materials for the composite electrode prepared with methanol-added dispersant. It indicates that methanol additive is effective to improve the dispersion of tin and carbon materials in the aqueous slurry, resulting in better electrochemical properties in Na cells. Based upon these results, we will discuss the effects of cut-off voltages in relation to the changes in the surface of tin electrodes characterized by secondary ion mass spectroscopy and photoelectron spectroscopy in synchrotron facility.

References:

  [1]       M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa, S. Komaba, submitted.

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

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