Anode Properties of Si-Based Composite Electrodes Containing Two Silicides with Different Functions for Lithium-Ion Batteries

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A LaSi2/Si composite electrode has longer charge-discharge cycle life than a pure Si electrode. However, the capacity of the composite electrode decreases gradually over repeated cycles. We identified the microstructure of the electrode to ascertain factor of capacity fading by transmission electron microscopy. Before cycling, the pure Si phase was highly dispersed in the LaSi2 matrix phase. In such microstructure, the elastic LaSi2 could alleviate the stress caused by Si volume change during lithiation and delithiation; and hence it suppressed electrode disintegration.1 On the other hand, prior to capacity decline, the positions of LaSi2 and pure Si were reversed. The LaSi2 phase was surrounded by the pure Si matrix phase and the LaSi2 could not relax the stress. Hence, we added stiff silicide which withstands the Si-generated stress into the LaSi2/Si. As a result, the LaSi2/NbSi2/Si exhibited superior cycle life. Herein, we investigated the effect of difference in elastic silicides on cycle life. The MSi2/CrSi2/Si (M = Ni, La, or Sm) powder was prepared by mechanical alloying method. The composite powder was deposited on the Cu substrate using a gas-deposition (GD) method. The fabricated GD electrode was mounted in a 2032-type coin-cell as the working electrode. Li metal foil and glass fiber filter were also used as the counter electrode and separator, respectively. In addition, we used 1 mol dm–3 lithium bis(fluorosulfonyl)amide dissolved in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide as an ionic liquid electrolyte. We conducted galvanostatic charge-discharge tests in constant current mode with a charge capacity limitation of 1000 mA h g(Si)–1. In the present study, we did not consider the capacity of the silicides: the Si-alone and silicide-alone electrodes stored Li, while the silicide in the silicide/Si electrodes did not show the reversible capacity2. Figure 1 shows the cycle life of the MSi2/CrSi2/Si electrodes with the different indentation work rates of MSi2. The indentation work rate means an indicator of elasticity: it indicates that how much the MSi2 layer returns to its original shape after deformation. The cycle life was the shortest when NiSi2 with the lowest indentation work rate was used. In contrast, when SmSi2 was used, it achieved the longest cycle life with high reversible capacity of 1000 mA h g(Si)–1 over 1900 cycles. We predicted that the cycle life improved using a more elastic silicide. However, the cycle life adversely lowered when LaSi2 with the highest elasticity was used. Thus, the excellent performance should be achieved based on the synergistic effects of stiff and elastic silicides.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