Electrochemical Study on Porous Silicon Anodes in Sulfide-Based All-Solid-State Lithium-Ion Batteries

Abstract

All-solid-state lithium-ion battery (LIB) with high safety and reliability is the dominant power source for electric and hybrid vehicles. 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, we have found that sulfide-based all-solid-state LIBs with porous Si anodes exhibited high capacity retention [1,2]. In the present study, the electrochemical impedance was measured to provide further insight into the stable cyclability. Porous Si particles were prepared through air-oxidation demagnesiation of Mg2Si. The details have been described in our previous paper [1]. Commercially available non-porous Si particles were also used as a reference active material. Anode composite was comprised of Si particles, Li3PS4 solid electrolyte (SE), and acetylene black. We adopted two mixing methods, ball milling and mortar milling, to vary the dispersion degree of Si particles. In an electric insulation tube, the anode composite and SE 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 2×10-3 to 1×106 Hz. The ball-milled porous Si anode cells exhibited the capacity retention over 80% up to 150 cycles. In contrast, the lower retention, 40%, was observed for the mortar-milled porous Si anode cells. The ball-milled and mortar-milled non-porous Si anode cells possessed the poorer cycle stability with the capacity retention of 10% or less. As shown in the Cole-Cole plots, in the ball-milled porous Si anode cell, there was few changes in the arc size between 1st and 20th cycles. On the other hand, the arc of the mortar-milled porous Si anode cell became gradually large with the increase of cycle number. In addition, huge arcs were observed in the non-porous Si anode cells, regardless of the mixing method. The experimental results described above can be explained as follows. By utilizing the ball milling, Si particles are highly dispersed in the SE matrix. In this case, a lot of pores on the porous Si surface are exposed to the electrolyte. As the result, surface electrolyte interface (SEI) is easily formed at 1st cycle. However, SEI become hardly thick with the increase of charge/discharge cycle. This is because SE possesses high stability and low fluidity. In addition, the volumetric expansion is buffered by the shrinkage of Si pores. Therefore, the slight stress arising from the Si particles is relieved by the elastic deformation of SE. These findings strongly suggest that almost constant interfacial resistance causes the cycle stability of the ball-milled porous Si anode cells.