Characterization of all-solid-state secondary batteries with LiCoO2 thin films prepared by ECR sputtering as positive electrodes

We fabricated all-solid-state lithium secondary batteries consisting of LiCoO2 thin films prepared by electron cyclotron resonance (ECR) sputtering LiPON and metallic lithium films, and investigated the influence of the sputtering target composition on the performance of the batteries and LiCoO2 films. We found that the LiCoO2 film sputtered with a stoichiometric LiCoO2 target included many impurities (mainly Co3O4) and these impurities were eliminated by adding an excess of Li source to the sputtering target to achieve a Li/Co atomic ratio of 2.0 elsewhere. The LiCoO2 film sputtered with a Li2.0 target exhibited a larger discharge capacity and a high performance level for large current operation. However, the capacity of a battery employing LiCoO2 film sputtered with a Li2.0 target decreased more rapidly than that with a Li1.0 or Li1.7 target in a charge–discharge cycle test. We also investigated the cycle performance of LiCoO2 films in an ordinary liquid electrolyte by using beaker type cells. We found that the decrease in capacity during the cycle tests was caused by the deterioration of the LiCoO2 film, because the dependence of the target composition on the cycle performance in the beaker type cells was similar to that in the all-solid-state cells. We consider the capacity decrease to be caused by the deterioration in the crystallinity of the LiCoO2 film when using the Li2.0 target and caused by the formation of a Co3O4 layer on the surface of the LiCoO2 film when using a Li1.7 target on basis of the results of X-ray diffraction analysis and Raman spectroscopy.