Achieving stable cycling of LiCoO${}_2$ at 4.6 V by multilayer surface modification

LiCoO${}_2$, which was first proposed as a cathode in 1980 by Prof. John B. Goodenough, is still one of the most popular commercial cathodes for lithium-ion batteries. Tremendous efforts have been invested in increasing the capacity of LiCoO${}_2$ by charging to high voltage. However, a series of issues, such as structural instability and dramatic side reactions with electrolytes, can emerge as cut-off voltage above 4.5 V (vs Li/Li${}^{+}$). Here, a surface modification strategy with a multilayer structure is provided, involving a Zn-rich surface coating layer, rock-salt phase buffer layer and surface gradient Al doping layer, to overcome the detrimental issues and achieve stable cycling of LiCoO${}_2$ at 4.6 V. The complete coating of the modification layer restrains the interfacial side reactions with electrolyte and inhibits the impedance growth. The phenomenon of quasi-epitaxial growth demonstrates that the multilayer structure significantly reduces the lattice mismatch between host LiCoO${}_2$ and surface coating layer and enhances the stability of the Zn-rich outside layer, which promote the long-term effectiveness of the modification. Furthermore, the disordered rock-salt phase layer and Al surface doping also enhance the structural stability. All of these synergistically lead to the stable cycling of LiCoO${}_2$ at 4.6 V with a capacity retention of 65.7% after 500 cycles. (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)