Lithium manganese oxide spinel (LiMn2O4) batteries show catastrophic capacities fading after extended storage and being work at 55°C. In view of electrolyte, the performance deterioration of LiMn2O4 cathode mainly origins from acidic impurity HF from the decomposition of LiPF6 salt in the presence of trace water at 55°C, which is believed to accelerate electrolyte decomposition. For that reason, the effect of p-toluenesulfonyl isocyanate (PTSI) as electrolyte additive on the stability of the electrolyte and cycling performance of LiMn2O4/Li cell at 55°C was studied. To verify that PTSI suppress Mn3+ dissolution mechanism of LiMn2O4 cathode, diethyl carbonate (DEC)-based electrolyte using PTSI is newly evaluated as novel film-forming electrolyte additive. Comparative analysis of the electrolyte with and without PTSI by linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Based on these results, PTSI can be used as effective electrolyte additive for formation of a good solid electrolyte interphase (SEI) layer on the LiMn2O4 electrode. The capacity fading of LiMn2O4/Li cells after extended storage and being work at 55°C were obviously inhibited, which is mainly originated from the suppression of carbonate solvents electrolyte decomposition and the enhancement of SEI conductivity.