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

Abstract

The electrochemical properties of Sn-Co were investigated to show the correlation between the cycle performance and the binders of the electrode component materials. Sn-Co electrodes with polyacrylic acid (PAA) exhibited a better cycle property (about 300 mAh/g up to 30 cycles) than those with polyvinylidene difluoride (PVdF). This better cycle property with PAA was due to the slight change in the volume of the electrode that occurred during cycling as revealed by in-situ light microscopy. In addition, Na pre-doping in Sn-Co electrodes improved the average coulombic efficiency from 95.4% to 99.9% at 2-10 cycles. Lithium-ion batteries (LIBs) are used for power storage in electrical products and electric vehicles, and the demand for them is likely to increase. However, since lithium is not an abundant metal, it is expensive. On the other hand, because sodium is abundant and cheap; and interest in sodium-ion batteries (SIBs) has been growing.The materials that have been studied for use as SIB anodes include hard carbon 1-4 and tin. [5][6][7] Hard carbon can be cycled more than 100 times, but the capacity is only about 250 mAh/g. 1 In addition, the capacities of sodium cells with hard carbon electrodes are smaller than equivalent lithium cells. 4 On the other hand, the capacity of tin is about 500 mAh/g, but tin electrodes have the drawback of poor cycle performance (only several cycles) due to the large expansion and contraction of the volume of tin electrodes (about 5.3 times larger than hard carbon) 6 that accompanies Na-ion insertion (alloying) and extraction (dealloying). These alloying/dealloying processes of Sn with Na are similar to those of Sn with Li. Therefore, a key factor is that tin-based electrodes inhibit the change in volume during cycling. As reported in Ref. 6, the cycle properties of tin electrodes for SIBs can be improved by adopting a polyacrylic acid (PAA) binder. In that study, the capacity of Sn electrodes remained about 500 mAh/g after 20 cycles. Thus, a binder is one of the most important component materials in an electrode.LIBs using Sn-Co anodes were first commercialized by Sony Corporation.8 The Sn-Co anodes exhibit good cycle performance because cobalt does not alloy with lithium and cobalt buffer the change in volume of electrode during cycling. [9][10][11][12][13] This paper evaluates the electrochemical properties of Sn-Co electrodes for SIBs to reveal the correlation between cycle performance and binder.Sn-Co electrodes were prepared with polyvinylidene difluoride (PVdF) or PAA as binders. The electrochemical properties of Sn-Co electrodes with a PAA binder were examined by performing galvanostatic discharge-charge experiments, and the results were compared with those obtained with PVdF as the binder. Additionally, the change in volume of a Sn-Co electrode with PAA or PVdF during Na-ion insertion (alloying)/extraction (dealloying) was evaluated by in-situ light microscopy for the first time. The crystallographic structural change of Sn-Co during cycling was characterized by X-ray d...

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