(Trimethylsilyl)methanesulfonate (TMSOMs), functionalized with task-specific chemical moieties, is proposed as an interface-stabilizing additive to improve the electrochemical performance of 5V-class layered over-lithiated oxides (OLOs). TMSOMs offers a great opportunity to enhance the interfacial stability of an OLO material by providing an effective protective layer composed of SO3 and OSi functional groups after its electrochemical oxidation over 4.0 V (vs. Li/Li+), which remarkably reduces the internal pressure of the cell associated with electrolyte decomposition. As a result, the cell employing TMSOMs affords excellent capacity retention (92.8% at 100 cycles) together with considerable rate performance, negligible transition metal dissolution, and stable high temperature performance based on its enhanced interfacial stability. These results are attributed to the synergistic effects of the SO3 and OSi functional groups that once the sulfonic ester-based protective layer is developed on the electrode surface, it effectively mitigates decomposition of the electrolyte, while the OSi functional groups readily scavenge fluoride species in the electrolyte, leading to outstanding interfacial stability for the OLO material. On the basis of spectroscopic evidence, a comprehensive mechanism for the action of TMSOMs is suggested considering the specific role of each functional group.