Ultrathin reference electrode and electrochemical devices including the same

A method of forming a battery cell comprising an anode, a cathode, and a reference electrode, wherein the reference electrode is interposed between the anode and the cathode, wherein the reference electrode comprises an active material layer disposed on a current collector, and wherein the active material layer comprises lithium, a lithium aluminum alloy, sodium, a sodium potassium alloy, a sodium calcium alloy, a sodium-lithium-magnesium alloy, a sodium-lithium-aluminum alloy, a sodium lead alloy, a sodium silicon alloy, a sodium antimony alloy, or a sodium zinc alloy, the method comprising forming the active material layer on the current collector.