Tailored Design of a Nanoporous Structure Suitable for Thick Si Electrodes on a Stiff Oxide-Based Solid Electrolyte.

Oxide-based all-solid-state batteries are ideal next-generation batteries that combine high energy density and high safety, but their realization requires the development of interface bonding technology between the stiff solid electrolyte and electrode. Even if the interface could be bonded, it is difficult to hold the interface, because only the electrode expands/contracts unilaterally during charge/discharge reactions. In particular, silicon (Si), which has eagerly awaited as a next-generation negative-electrode material for many years, changes in volume by several hundred percent. To solve these problems, in this work, highly porous silicon oxide (SiO_x) electrodes with different porous structures were fabricated on a stiff garnet-type Li₇La₃Zr₂O₁₂ solid electrolyte, the three-dimensional nanoporous structure was analyzed quantitatively, and the charge/discharge characteristics were investigated. The microscopic observation and electrochemical analysis revealed how we should control the porous structure, such as sizes of pores and SiO_x, size distribution, and porosity, for repeated and stable charge/discharge cycles. In addition, the resultant porous SiO_x electrodes demonstrated superior charge/discharge cycle performance even when it thickened to 5 μm, whereas non-porous SiO_x easily peeled off from the solid electrolyte when its thickness exceeded 0.1 μm. The thick SiO_x films greatly improved the energy density per unit area (mAh cm^-2). Nanosized fine pores with an interconnected open-pore architecture effectively mitigated the internal and interfacial stress upon expansion (charge)/contraction (discharge) of Si, and as a result, the thick and porous SiO_x electrode maintained the interfacial joint with the stiff solid electrolyte after repeated charge/discharge cycles. These results will provide useful insights for effectively designing more practical porous SiO_x powder effectively.