All-solid-state thin film batteries are feasible by employing Al as anode and LiPON as electrolyte which are subsequently deposited by sputtering. The lithium ion conductivity of ∼ 10−6 S/cm for the thin film LiPON is in agreement with data reported for bulk material. The high voltage cathode Li2CoMn3O8 could be prepared by forming the compound by the combustion method andsubsequent e-beam evaporation of this material with the addition of 20 wt.-% LiNO3 at an oxygen partial pressure of 10−5 mbar. The thin film cells could be operated between 3 and 5 V vs. Al, LiAl. The chemical diffusion coefficient was found to be in the range from 10−13 to 10−12 cm2/s at room temperature by employing the GIT-technique for the composition x of Li2-xCoMn3O8 in the range from 0.1 to 1.6. Impedance studies of the complete battery system revealed a charge transfer resistance of 290 Θ, a double layer capacity of ∼ 45–70 μF for an electrode area of 6.7 cm2 and a rate determining chemical diffusion coefficient in the range from 10−12 to 10−11 cm2/s.