Structural and Electrochemical Characterization of Radio Frequency Magnetron‐Sputtered LiCoO₂ Thin Films

LiCoO₂ thin films are grown by radio frequency magnetron sputter‐deposition on platinum‐coated Al₂O₃ (c‐sapphire) substrates. Deposition parameters such as substrate temperature and the partial pressure ratio between O₂ 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 LiCoO₂ films can be influenced by varying the O₂/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, LiCoO₂ thin films where charged and discharged against lithium metal while using a liquid electrolyte consisting of LiPF₆ 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 LiCoO₂ thin films.