Lithium-rich layered oxide cathode is a promising high-capacity cathode material for high-energy lithium-ion batteries. However, to achieve its high capacity, the development of high-voltage electrolytes is essential. In this work, trimethyl phosphite (TMP) is investigated as an electrolyte additive for high-voltage lithium-ion batteries using Li1.2Mn0.54Ni0.13Co0.13O2 cathode. When 1% TMP is introduced into the electrolyte, cycling performance and rate capability of the Li1.2Mn0.54Ni0.13Co0.13O2 cathode are improved significantly. The Li/Li1.2Mn0.54Ni0.13Co0.13O2 cell with the TMP-containing electrolyte exhibits high capacity retention of 81.3% after 100 cycles and good rate capability of 150 mA h g−1 at 5 C and 90 mA h g−1 at 10 C, while the capacity of the cell without TMP is 48 mA h g−1 at 5 C. Based on the measurements on impedance spectra and thermal stability, it is concluded that TMP can effectively deactivate the catalyzing effect of some transition metal ions on the surface of the Li1.2Mn0.54Ni0.13Co0.13O2 cathode.