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 LiCoO 2 layered cathode and by virtue of electrochemical tests and microstructure analysis, we systematically investigate the structural and chemical properties of LiCoO 2 SPTL under different cycling conditions and reveal that the LiCoO 2 SPTL does not contribute performance decay significantly. In comparison with Ni-contained LiNi 1-x-y Mn x Co y O 2 layered cathodes, LiCoO 2 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 LiCoO 2 SPTL is formed in the discharging process, particularly at lower discharge voltages. Our work deepens the understanding on the LiCoO 2 SPTL and its effect on the overall performance decay, which sheds new lights on the interfacial failures of layered cathodes. • Highlighted the difference of LiCoO 2 surface layer cycled at different voltages. • LiCoO 2 surface phase transition layer is formed during the discharge process. • LiCoO 2 surface layer is morphologically and structurally different from NMC. • LiCoO 2 surface phase transition layer has minor blocking effect to Li-ion. [ABSTRACT FROM AUTHOR]