Structural and electrochemical characterization of radio frequency magnetron-sputtered LiCoO${}_2$ thin films

LiCoO${}_2$ thin films are grown by radio frequency magnetron sputter-deposition on platinum-coated Al${}_2$O${}_3$ (c-sapphire) substrates. Deposition parameters such as substrate temperature and the partial pressure ratio between O${}_2$ and Ar are varied to find suitable parameters for a cathode offering reversible intercalation and deintercalation of lithium ions. Structural characterization is carried out using X-ray diffraction and Raman spectroscopy, revealing that the orientation as well as the crystallinity of the deposited LiCoO${}_2$ films can be influenced by varying the O${}_2$/Ar ratio and simultaneously applying a sufficient substrate temperature during deposition. Furthermore, both the atomic composition and the atomic distribution are determined using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. Results show that the films posses surface contaminations of Li and oxygen-rich compounds but have a homogeneous atomic distribution throughout the depth profile. For electrochemical characterization, LiCoO${}_2$ thin films where charged and discharged against lithium metal while using a liquid electrolyte consisting of LiPF${}_6$ dissolved in a mixture of ethylen carbonate/dimethyl carbonate with a molar fraction of 1:1. Reversible discharge capacities in the order of up to 70 mAh g${}^{-1}$ or 32.40 µAh cm${}^{-2}$ µm${}^{-1}$ can be achieved for optimized LiCoO${}_2$ thin films. (© 2020 The Authors. Published by Wiley‐VCH GmbH)