Solid electrolyte materials as well as their applications and methods of manufacture are disclosed. In one embodiment, a solid electrolyte material has a formula of A3+δCl1−δBδO, where δ is greater than 0. In the above formula, A is at least one of Li and Na, and B is at least one of S, Se, and N. In another embodiment, a solid electrolyte material is a crystal structure having the general formula A3XO, where A is at least one of Li and Na. Additionally, X is Cl, at least a portion of which is substituted with at least one of S, Se, and N. The solid electrolyte material also includes interstitial lithium ions and/or interstitial sodium ions located in the crystal structure.