Electrochemical Study on Porous Silicon Anodes in Sulfide-Based All-Solid-State Lithium-Ion Batteries: II
Authors: Okuno Ryota , Yamamoto Mari , Kato Atsutaka , Takahashi Masanari 
Published: May 2020

All-solid-state lithium-ion battery (LIB) with high safety and reliability is the dominant power source for electric and hybrid vehicles. Here, to meet the energy demands of near-future automobile technology, the anode materials should possess high energy density and long cycle life. Though Si with a high theoretical capacity (4200 mAh g-1) is the most attractive choice, there are few applications for the all-solid-state LIBs. This is because the large volumetric fluctuation (>300%) in lithiation/delithiation causes the drastic capacity loss and low coulombic efficiency. Very recently, sulfide-based all-solid-state LIBs with porous Si composite anodes have exhibited high capacity retention [1]. We proposed the following mechanisms to consistently explain the experimental results [2]: (a) the volumetric expansion is buffered by the shrinkage of Si pores and (b) the stress arising from the Si particles is relieved by the elastic deformation of solid electrolyte (SE). In the present study, to provide further insight into these findings, the electrochemical characteristics were evaluated for the porous Si composite anodes with different SEs.
Porous Si particles were prepared through air-oxidation demagnesiation of Mg2Si. The details have been described in our previous paper [1]. Anode composites were comprised of Si particles, SE, and acetylene black. Here Li3PS4 (LPS) and Li10GeP2S12 (LGPS) were adopted as sulfide-based SE. In an electric insulation tube, the anode composite and LPS powder were pressed to make two-layered pellet. As the counter electrode, Li-In foil was attached on the SE side. Finally, the three-layered pellet was compressed using stainless-steel disks. AC impedance of the half-cells thus prepared was measured in the range from 0.002 to 1 MHz.
Figure 1 shows the cycle characteristics of LPS- and LPGS-cells. The LPS composite anode cell exhibited an initial discharge capacity of 1240 mAh g-1. On the other hand, a discharge capacity of 924 mAh g-1, which was about 30% lower than that of LPS-cell, was observed in the LGPS-cell. The maximum discharge capacity was 1532 mAh g-1 for LPS-cell and 1269 mAh g-1 for LGPS-cells, respectively. As the results, the capacity retention of LPS-cell was calculated as 95% at 50th cycle, compared to 83% in LGPS-cell. It is worthy to note that these values were extremely larger than those of non-porous Si anode cells. As shown in Cole-Cole plots, Figure 2, in the LPS composite anode cell, there was few changes in the arc size between 1st and 5th cycles. On the other hand, the arc of LPGS-cell became large with the increase of cycle number.
The Young's modulus of LPS is known to be about 10% lower than that of LGPS [3]. Based on this fact, the above results are described as follows. Though the volumetric expansion is buffered by the shrinkage of pores, the porous Si particles slightly become large during lithiation. In LPS-cell, the stress should be relieved by the elasticity of surrounding SE. On the other hand, in LPGS with higher rigidity, voids are probably formed between the Si particles and the deformed SE, resulting in the lower capacity retention and higher interfacial resistance. In these considerations, we should note that LGPS with narrow potential window is prone to decompose in long cycles.
References
[1] R. Okuno, M. Yamamoto, Y. Terauchi and M. Takahashi, Energy Procedia, 156 (2019) 183-186.
[2] R. Okuno, M. Yamamoto, A. Kato, Y. Terauchi and M. Takahashi, IOP Conf. Series: Materials Science and Engineering, 625 (2019) 012012:1-5.
[3] A. Kato, M. Nose, M. Yamamoto, A. Sakuda, A. Hayashi and M. Tatsumisago, Journal of the Ceramic Society of Japan, 126 (2018) 719-727.

Figure 1