An electrolyte separator structure is provided. The electrolyte separator structure comprises a graded integral structure, wherein the structure comprises an ion-conducting first ceramic at a first end and an electrically insulating second ceramic at a second end, wherein the difference in the coefficient of thermal expansion of the ion-conducting first ceramic and the electrically insulating second ceramic is less than or equal to about 5 parts per million per degrees Centigrade, and wherein at least one of the first ceramic or the second ceramic comprises a strengthening agent. Method of making the ion-separator structure is provided. Electrochemical cells comprising the ion-separator structure and method of making the electrochemical cell using the ion-separator structure are also provided.