Hiroyuki Usui

Silicon (Si)-based anodes are promising candidates for all-solid-state lithium-ion batteries (ASSLIBs) because of their high theoretical capacity; however, poor cycling stability remains a critical challenge. Herein, we systematically evaluated the charge–discharge properties of two Si-based active materials (phosphorus (P)-doped Si and iron disilicide (FeSi2)/P-doped Si composites) as ASSLIB anodes. Both materials have previously demonstrated superior electrochemical performance in liquid electrolyte cells. The Si reference electrode delivered an initial reversible capacity of ∼2440 mA h g(Si)–1; however, the capacity faded rapidly, with ∼13% of the initial capacity being lost within 10 cycles. Conversely, the P-doped Si electrode exhibited ∼10% higher initial capacity and superior capacity retention. The FeSi2/P-doped Si electrode maintained >90% of its capacity after 100 cycles. Particle size homogenization via sieving further improved charge–discharge cycling stability. Micro-Raman spectroscopy findings indicated that the high electronic resistivity of pure Si electrodes may give rise to more spatially variable Raman responses, potentially reflecting less uniform lithiation across the electrode surface. The reduced resistivity of P-doped Si and FeSi2/P-doped Si appeared to promote more spatially consistent Raman features, consistent with more uniform lithiation–delithiation behavior. The superior performance of the FeSi2/P-doped Si electrode was further attributed to, its order-of-magnitude, higher electronic conductivity in comparison with Si. Silicide incorporation reduced the molding and stack pressures required during cell fabrication, showing the potential of silicide-based electrodes to enable pressure-free ASSLIB construction. These findings collectively establish rational design strategies for Si-based anodes and identify silicide/Si composites as a viable pathway toward the practical realization of ASSLIBs.

Li-ion batteries with LiNi0.5Mn1.5O4 cathodes offer high energy density; however, they face technical challenges due to side reactions related to metal dissolution and gas emissions caused by electrolyte decomposition, which hinder their practical applications. To address these issues, we introduced (Ta and P)-coating on LiNi0.5Mn1.5O4 particles using an aqueous solution containing Li, P, and Ta ions. Float gas evolution results demonstrated that the (Ta and P)-coating could effectively suppress gas generation on the LiNi0.5Mn1.5O4 cathode. Electrochemical impedance spectroscopic measurements before and after the float test revealed that the coating also mitigated increases in interfacial resistance, indicating the formation of a more stable interface between the coating layer and the electrolyte. Together, these effects enhance active material utilization and extend cycle life. Our study demonstrates the potential of Ta and P coatings to improve the high-temperature stability and long-term durability of LiNi0.5Mn1.5O4-based lithium-ion batteries.

We prepared rutile-type TiNbO₄ by a sol-gel method to investigate the relationship between the calcination temperature in the synthesis and its nanostructure and evaluated the influence of the nanostructure on the electrode properties as a Na-ion battery anode. The crystallite size of TiNbO₄ and the size of the diffusion path along its c-axis were increased with increasing calcination temperature. We revealed that the TiNbO₄ was comprised of aggregated single-crystalline primary nanoparticles to form mesoporous secondary particles of spherical marimo shape. At a high current rate of 5C, the electrode of TiNbO₄ calcined at 700 °C exhibited the highest reversible capacity of 170 mA h g^-1. It is suggested that TiNbO₄ at 700 °C has a relatively wide diffusion path to allow high-speed Na⁺ insertion-extraction and that its smaller crystallite size improves the utilization rate of the active material.

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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