Revealing the minor Li-ion blocking effect of LiCoO₂ surface phase transition layer

In-depth understanding interfacial failures and differentiating their contributions to the overall performance decay are in urgent need for cell design and optimization. Herein, with the focus on the frequently observed surface phase transition layer (SPTL) of LiCoO2 layered cathode and by virtue of electrochemical tests and microstructure analysis, we systematically investigate the structural and chemical properties of LiCoO2 SPTL under different cycling conditions and reveal that the LiCoO2 SPTL does not contribute performance decay significantly. In comparison with Ni-contained LiN1-x-yMnxCoyO2 layered cathodes, LiCoO2 SPTL shows two distinctive features. One is the porous morphology due to severe surface corrosion/dissolution and the other is the spinel-like lattice structure. The porous surface layer enables a percolating surface diffusion and the spinel structure offers a bulk diffusion for Li ions, which jointly leads to a minor blocking effect of Li ion during its intercalation/deintercalation. We experimentally verified that the LiCoO2 SPTL is formed in the discharging process, particularly at lower discharge voltages. Our work deepens the understanding on the LiCoO2 SPTL and its effect on the overall performance decay, which sheds new lights on the interfacial failures of layered cathodes.