We demonstrate herein that an ultra-thin alkylsilane self-assembled monolayer (SAM) coating can be used as a modifying agent at LiNi_0.5Mn_1.5O_4-d cathode/electrolyte interfaces in 5 V-class lithium-ion batteries. Bare LiNi_0.5Mn_1.5O_4-d cathode showed substantial capacity fading, with capacity dropping to 79% of the original capacity after 100 cycles at a rate of 1 C, which was entirely due to dissolution of Mn³⁺ from the spinel lattice via oxidative decomposition of the organic electrolyte. Capacity retention was improved to 97% on coating ultra-thin SAM onto the LiNi_0.5Mn_1.5O₄ cathode surface. Various characterizations revealed that such surface protection with highly ordered alkyl chains insulated the cathode from direct contact with the organic electrolyte, shift their valence band position, led to increased tolerance to HF.