Controllable preparation and electrochemical properties of In-situ annealed LiCoO2 films with a specific crystalline orientation on stainless steel substrates

Abstract

The crystallization characteristics of LiCoO2 (LCO) thin films are key factors that affect the electrochemical properties of films. In this paper, in-situ annealed LCO films with a specific crystalline orientation were prepared on stainless steel (SS) substrates by radio-frequency (RF) magnetron sputtering. The effects of the high-temperature pretreatment of the SS substrate, substrate temperature and sputtering power on the crystallization and electrochemical properties of the LCO films were investigated in detail. It was found that the structure of the SS substrate was transformed from the γ phase to the δ phase after the high-temperature pretreatment, and the crystallization orientation of the LCO films could be induced by the different SS phase structures. The crystallization characteristics of the LCO films were also clearly influenced by the substrate temperature. The crystallization orientation changed from the (003) orientation to the (101) orientation when the temperature increased to a high degree, but too high of a substrate temperature (such as 600 °C) ruptured the films. The discharge specific capacity of the LCO films increased with an increasing substrate temperature, showing a capacity of 53.8 μAh cm−2 μm−1 at 550 °C and exhibiting good cycling stability. At this temperature, the crystal orientation could be controlled by adjusting the sputtering power to avoid rupturing the thin films, thereby achieving high-quality LCO films with a specific orientation. The LCO films prepared at 120 and 200 W showed obvious (101) and (003) preferred orientations, respectively. The galvanostatic charge-discharge tests showed that the (101) preferred orientation film exhibited a large initial discharge capacity of 57 μAh cm−2 μm−1, but its cycling performance was poor. The (003) preferred orientation delivered a superior cycling performance with an initial discharge capacity of 55.6 μAh cm−2 μm−1 and a capacity retention of 72% after 150 cycles. Cyclic voltammetry and electrochemical impedance spectroscopy measurements showed that the (101) preferred orientation was more conducive to the full infiltration of the electrolyte and the transport of lithium ions, but the stability of the structure and surface was poor and the impedance clearly increased during cycling. The (003) preferred orientation exhibited better structural and surface stability and exhibited better lithium-ion diffusion after repeated electrochemical processes.