Positive Li–Co–O films for all-solid-state thin-film lithium batteries were prepared by electron cyclotron resonance (ECR) sputtering using Li x CoO2 targets (x =1.0, 1.2, 1.7 and 2.0). The Li–Co–O films prepared using the x =1.0, 1.2 and 1.7 targets contained a Co3O4 impurity phase with high-temperature phase LiCoO2. The film prepared using the x =2.0 target was found to contain only a pure LiCoO2 phase by Raman spectrometry and the inductively coupled plasma/atomic emission spectrometry (ICP/AES) method, and a thin-film battery using this film exhibited good electrochemical properties as a result of the improved utilization of the positive film.