The urgent need for higher energy density of aqueous Li-ion batteries (ALBs) cannot only be satisfied by electrolyte modifications, the utilization of layered oxide cathodes is another efficient strategy, and particularly Li[NixCoyMn1-x-y]O₂ (NCM) materials are of high interest due to their high specific capacities. Concerning the H⁺-Li⁺ exchange side reaction of layered cathode in water solution, however, whether proton contamination degrades NCM-type cathodes in highly-concentrated aqueous electrolyte is an unclear but meaningful point. In this work, the underlying mechanisms responsible for degradation of NCM622 | aqueous/non-aqueous hybrid electrolyte |TiO₂/LiTi₂(PO₄)₃ (P/N=1.2:1) full-cells are explored by comprehensive studies involving in the evolution of electrochemical impendence and lattice structure changes after cycling within different operating voltage ranges. It is found that proton co-intercalation into the layered structure still takes place in high concentration aqueous/non-aqueous hybrid electrolytes, and the NCM622 cathode quickly shows degradation after being charged to higher cut-off voltage owing to severe protonation. The introduced proton can increase the diffusion barrier for Li⁺ ions, which in turn hinders lithiation of the de-lithiated cathode.