A solid state cell based on metal oxides is described that is capable of electrochemically generating lithium vapour in UHV. The cell consists of a solid lithium ion conductor, Li3.75Si0.75P0.25O4, located between the intercalation electrode, LiMn2O4, and a metal gauze. By using an interphase of graded composition, the often-encountered problem of establishing a stable contact between the metal oxide electrode and electrolyte is avoided. Cyclic voltammetry, chronoamperometry and a.c. impedance indicate that lithium is generated by a process of nucleation and growth on the electrode surface. A potential application of this new cell in the synthesis of n-type diamond films is explored.