Rational design of chloride ion transport channels in an open borate framework†

Chloride-ion solid electrolytes have been widely investigated as the core components of membranes, gas sensors, and all-solid-state batteries with high energy densities. Several fast chloride-ion solid electrolytes operating at ambient-to-intermediate temperatures have been reported, ranging from inorganic metal chlorides and metal–organic compounds to complexes. However, these chlorides have the major drawback of being thermally and chemically unstable, and design strategies for chloride-ion conductors have not yet been developed compared with those for cation conducting solids, which restricts their practical applications. Here, we report that water-insoluble and thermally stable borate chloride (Ca_1−xLa_x)₂B₅O₉Cl_1+2x exhibits one-dimensional chloride-ion transport defined by the dimensionality of an open borate framework. Experimental and first-principles molecular dynamics simulations indicate that the predominant chloride-ion conduction is attributed to cooperative diffusion through interstitial chloride sites. This conduction is also strongly influenced by the association with La species. These results show that the rational design of chloride-ion channels and conduction paths based on the dimensionality of the borate framework would provide a new direction for the development of the variety and conducting properties of chemically and thermally stable chloride-ion solid electrolytes.