Enhancing Li-Ion Transport in Solid Electrolytes by Confined Water

Developing new oxide solid electrolytes with fast Li-ion transport and high stability is an important step to realize high-performance solid-state Li-ion batteries. Hydrate materials containing confined water widely exist in nature or can be easily synthesized. However, they have seldom been explored as Li-ion solid electrolytes due to the stereotype that the presence of water limits the electrochemical stability window of a solid electrolyte. In this work, it is demonstrated that confined water can enhance Li-ion transport while not compromising the stability window of solid electrolytes using Li-H-Ti-O quaternary compounds as an example system. Three Li-H-Ti-O quaternary compounds containing different amounts of confined water are synthesized, and their ionic conductivity and electrochemical stability are compared. The compound containing structural pseudo-water is demonstrated to have an ionic conductivity that is 2-3 order of magnitude higher than the water-free Li4Ti5O12 and similar stability window. A solid-state battery is made with this new compound as the solid electrolyte, and good rate and cycling performance are achieved, which demonstrates the promise of using such confined-water-containing compounds as Li-ion solid electrolytes. The knowledge and insights gained in this work open a new direction for designing solid electrolytes for future solid-state Li-ion batteries. Broadly, by confining water into solid crystal structures, new design freedoms for tailing the properties of ceramic materials are introduced, which creates new opportunities in designing novel materials to address critical problems in various engineering fields.