Graphene encapsulation is an attractive surface-coating technology that can simultaneously improve the rate capability and cycle stability of nickel-rich LiNi_xCo_yMn_1-x-yO₂ (NCM). Here, we describe carbon encapsulation with the addition of polyethylenimine (PEI) and polyhedral oligomeric silsesquioxane (POSS), which can effectively suppress access to and generation of harmful factors in the electrolyte to maximize the rate performance and cycle stability of nickel-rich NCM. The PEI-POSS/carbon layer not only facilitates electron and lithium-ion transport on the NCM surface, but inhibits side reactions with the electrolyte during repeated electrochemical reactions. In addition, it provides mechanical support that suppresses the formation of microcracks related to anisotropic volume change of nickel-rich NCM secondary particles and inhibits irreversible phase transitions on surface structures by mitigating electrolyte wettability. As a result, PEI-POSS/carbon-encapsulated NCM exhibits a higher rate capability (84 mAh g^-1 at 5 C) and cycle stability (93.5% for 100 cycles at 1 C) compared with bare NCM (0 mAh g^-1 at 5 C and 78.4% for 100 cycles at 1 C). In a cycle test at 45 ℃, it achieves a capacity retention of 72.6% for 100 cycles at 1 C, which is a 323% improvement in performance over that of bare NCM (22.5%).