As a facile and scalable approach to the surface modification of high-voltage cathode materials for lithium-ion batteries, direct UV-assisted conformal coating of poly(tris(2-(acryloyloxy)ethyl) phosphate) (PTAEP) gel polymer electrolyte on as-formed LiNi1/3Co1/3Mn1/3O2 (NCM) cathode is presented. The smooth and continuous PTAEP coating layer with nanometer-thickness (∼20 nm) is successfully introduced on the NCM surface without impairing electronic/ionic conduction pathways preformed in the NCM cathode. Owing to this structural uniqueness, the PTAEP-coated NCM cathode significantly improves the high-voltage (4.6 V) cycling performance and mitigates the exothermic reaction between the delithiated NCM and liquid electrolyte. This demonstrates that the conformal PTAEP nanocoating layer proposed herein, which is completely different from conventional inorganic material-based coating layers, acts as a new ion-conductive protective film that effectively suppresses unwanted interfacial side reactions between the high-voltage cathode materials and liquid electrolyte.