Li4B4M3O12Cl (M = Al, Ga): An Electrochemically Stable, Lithium-Ion-Conducting Cubic Boracite with Substituted Boron Sites

A cubic boracite with substituted boron sites, Li4B4M3O12Cl (M = Al, Ga), derived by fully replacing the tetrahedral BO4 units in the parent lithium chloroboracite, Li4B7O12Cl, with AlO4 or GaO4 units, has been discovered. These substituted compounds have the largest unit cell dimensions of known boracites and are formed as the stable primary phase in highly crystalline glass-ceramics derived from the Li2O–B2O3–M2O3–LiCl quarternary system. The conductivity of Li4B4M3O12Cl glass-ceramics at room temperature was ∼10−5 S cm−1, an order of magnitude larger than the highest conductivity recorded for Li4B7O12Cl glass-ceramics. The Li4B4Al3O12Cl glass-ceramic is stable in contact with Li metal and exhibits a wide electrochemical window between 0 and 6 V vs. Li/Li+ and a Li+ ion transport number of ∼1.