Size and shape optimization of silicon anodes for all-solid-state batteries

All-solid batteries are expected to be the next generation of rechargeable batteries due to their high energy density and enhanced safety[1]. This technology should enable the use of lithium metal anodes. However, the reactivity between the lithium and the sulphide-based electrolyte and the growth of lithium dendrites limit its use[2]. Silicon (Si) can be a promising alternative as it is an abundant, non-toxic and affordable material. In this study, we investigate the most appropriate silicon morphology for efficient cycling in an all-solid-state battery. Three Si materials are compared: silicon microparticles (µSi), nanoparticles with carbon coating (SiNPsC) and silicon nanowires (SiNWs) synthesized in the laboratory by a chemical growth process[3]. The Si material is mixed with a solid electrolyte, Li6PS5Cl (LPSCl), and vapor-grown carbon fibers (VGCF) to form a composite electrode that is tested in half-cell configuration with a Li-In alloy counter electrode. The Si material reaching the highest specific capacity at the first cycle is µSi with 2700mAh/gSi on average. The analysis of the galvanostatic curves and their derivatives (dQ/dV curves) shows that it forms the crystalline Li15Si4-c phase at the first lithiation, corresponding to the most lithiated state of silicon[4], while silicon nanoparticles and nanowires do not. However, it shows the lowest capacity retention, with 53% at the 15th cycle. This can be explained by the formation of Li15Si4-c which creates higher mechanical stresses during cycling, and therefore possible pulverization of the silicon particles[5]. EDX imaging of µSi composite electrodes also shows reduced contact between the silicon particles and the electrolyte, so contact losses may occur during cycling. SiNPsC and SiNWs offer a better trade-off between high initial specific capacity and cycling stability. At the first cycle, a specific capacity of 2000mAh/gSi and 2500mAh/gSi respectively and a capacity retention of 69% and 63% at the 15th cycle are achieved. Electrochemical Impedance Spectroscopy (EIS) measurements in the lithiated and delithiated states provide a better understanding of the current limitations and possible mechanisms occurring during cycling.