Trimethyl phosphate (TMP) which is a non-flammable solvent is a good candidate to replace flammable carbonates in battery electrolytes. In the case of Li-ion battery using high voltage cathode such as LiNi0.5Mn1.5O4 (LNM), the use of TMP based electrolyte leads to high cycle performance (96% capacity retention after 50 cycles). Also gas generation drastically decreased. According to careful observation of the LNM surface by scanning electron microscopy (SEM) after cycles, it has been found that TMP decomposes itself and forms polymer based film on the electrode surface suppressing further electrolyte decomposition. This work leads us to propose new electrolytes for Li-ion batteries using high voltage cathode.