Surface coating of composite electrode has recently received increasing attention and has been demonstrated to be effective in enhancing the electrochemical performance of lithium ion battery (LIB) materials. In this work, an electronic-insulating but ionic-conductive lithium carbonate (Li2CO3) is rationally selected as the unique coating material for commercial LiCoO2 (LCO) cathode. Li2CO3 is a well-known constitute in conventional solid electrolyte interface (SEI) layer, which can electrochemically protect the electrode. The carbonate coating layer is deposited on LCO composite electrodes via a facial magnetron sputtering approach. The sputtered Li2CO3 layer serves as an artificial SEI layer between the active material and electrolyte and can impede the formation of the primary SEI layer, which will permanently consume Li+ and reduce the reversible capacity of the electrode. After a 10 min Li2CO3 coating, the capacity retention of the composite electrode is improved from 64.4% to 87.8% when cycled at room temperature in the potential range of 3.0–4.5 V vs Li/Li+ for 60 cycles. The obtained discharge capacity is extended to 161 mAh g–1, which is 36% higher than the uncoated one (118 mAh g–1). When further increasing the charging potential up to 4.7 V, or elevating the operation temperature to 55 °C, the Li2CO3-coated LCO electrodes still display remarkably improved cycling stability.

Surface modification of LiCoO2 is an effective method to improve its energy density and elongate its cycle life in an extended operation voltage window. In this study, ZnO was directly coated on as-prepared LiCoO2 composite electrodes via radio frequency (RF) magnetron sputtering. ZnO is not only coated on the electrode as thin film but also diffuses through the whole electrode due to the intrinsic porosity of the composite electrode and the high diffusivity of the deposited species. It was found that ZnO coating can significantly improve the cycling performance and the rate capability of the LiCoO2 electrodes in the voltage range of 3.0-4.5 V. The sample with an optimum coating thickness of 17 nm exhibits an initial discharge capacity of 191 mAh g(-1) at 0.2 C, and the capacity retention is 81% after 200 cycles. It also delivers superior rate performance with a reversible capacity of 106 mAh g(-1) at 10 C. The enhanced cycling performance and rate capability are attributed to the stabilized phase structure and improved lithium ion diffusion coefficient induced by ZnO coating as evidenced by X-ray diffraction, cyclic voltammetry, respectively.

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