A process for sputter deposition of RuOx films with high volumetric Li capacities and good cyclability was developed, demonstrating the potential for use of RuOx films as cathodes in high-performance thin-film microbatteries. Using a stoichiometric RuO2 sputtering target, the effects of oxygen content within the Ar/O2 plasma on the stoichiometry of thin films deposited at room temperature was investigated and correlated with electrochemical performance. The oxygen content of the films was found to increase with the O2 to Ar ratio of the sputtering gas. RuOx thin films with x ≥ 1.92 delivering capacities of ∼550 μAh/cm2μm in the 0.75–3.6 V vs. Li/Li+ voltage range, which is much higher than that of under-stoichiometric RuOx (320 μAh/cm2μm). The capacities obtained for RuOx films significantly exceed the capacity of LiCoO2 films (∼60 μAh/cm2μm) typically used in thin-film microbatteries. Given that high capacity RuOx films can be obtained using only room temperature processing, their use in thin-film microbatteries will also allow integration with silicon-based integrated circuits.