Si-Based Composite Anodes with Long Cycle Life for Lithium-Ion Batteries Achieved By Synergistic Effects of Elastic and Stiff Silicides

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A composite electrode comprising LaSi2 and pure Si exhibits good charge–discharge cycling performance, even with a degrading capacity after repeated cycles. Before cycling, a metallographic structure was formed in which the Si phase was finely dispersed in the LaSi2 matrix phase; hence, the elastic LaSi2 relieves the stress generated by Si and suppresses electrode disintegration.1 In contrast, prior to capacity degradation, the metallographic structure transformed into a form in which the LaSi2 phase was surrounded by the Si matrix phase; the positional relationship between the two phases was reversed, and LaSi2 could not relieve the stress from Si. In the case of a composite electrode with CrSi2, which possesses stiff mechanical properties to withstand the stress generated by Si, the metallographic structural changes after cycling were suppressed, resulting in good cycling stability. Herein, we considered that the addition of stiff silicides as a third phase to the LaSi2/Si composite could further improve the cycle life by suppressing microstructure inversion. Composite comprising elastic LaSi2, stiff MSi2 (where M = Cr, Mo, Nb, Ta, Ti, or W), and elemental Si were prepared by mechanical alloying method, and the composite electrodes fabricated using gas-deposition (GD) method. The prepared GD electrode was mounted in a coin-type cell as the working electrode. We used a Li metal sheet and glass fiber filter as the counter electrode and separator, respectively. Additionally, we used 1mol dm–3 lithium bis(fluorosulfonyl)amide dissolved in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide as an ionic liquid electrolyte. Galvanostatic charge-discharge tests were conducted in constant current mode with a charge capacity limitation of 1000 mA h g(Si)–1. In this study, the capacity of the silicides was ignored: the silicide-alone and Si-alone electrodes stored Li, whereas the silicide in the silicide/Si electrodes did not exhibit the reversible capacity.2 Figure 1 shows the cycle life of the LaSi2/MSi2/Si electrodes with the Vickers hardness of MSi2. The cycle life was shortest when WSi2, with the stiffest mechanical properties (hardest Vickers hardness), was used. The cycle life improved as the stiffness of MSi2 lowered. When NbSi2 was added, superior cycling performance was achieved, maintaining a reversible capacity of 1000 mA h g(Si)–1 for approximately 1800 cycles. This exceptional performance was achieved based on the synergistic effects of stiff and elastic silicides. In contrast, the cycle life was adversely decreased when TiSi2, which possessed the lowest stiffness, was used. This trend was observed when LaSi2 was used as the elastic silicide, and the same trend may not be obtained when combined with other elastic silicides. This study provides new insights into the effects of silicide functions on the charge–discharge characteristics of composite electrodes.1) Y. Domi, H. Usui, K. Nishikawa, and H. Sakaguchi, ACS Appl. Nano Mater., 4, 8473 (2021).2) Y. Domi, H. Usui, K. Nishikawa, H. Sakaguchi, et al., Electrochemistry, 88, 548 (2020). Figure 1