In situ Formation of Solid Electrolyte during Lithiation Process of MgCl₂ Anode in an All‐Solid‐State Lithium Battery

Abstract All‐solid‐state lithium batteries, which are free from the risk of liquid entanglement, are expected to have high energy densities and high safety. In this study, the omission of a solid electrolyte from the electrode and an increase in thickness of the electrode were investigated to improve the energy density of all‐solid‐state lithium batteries. We focused on MgCl₂, which reversibly self‐generates a solid electrolyte during the lithiation process, as an anode material that can operate without a solid electrolyte incorporated into the electrode mixture. Indeed, the electrochemical properties of the MgCl₂ electrode were approximately the same with or without a pre‐contained solid electrolyte, LiBH₄. X‐ray diffraction and X‐ray absorption spectroscopy analyses showed that lithiation produced Mg and LiCl, which were recovered to MgCl₂ by subsequent delithiation. The available electrode thickness was also investigated, and the thickness limit for the first lithiation was found to be ∼100 μm.