Triallyl phosphite (TAPi) was used as an electrolyte additive to improve performance of LiNi0.6Co0.2Mn0.2O2/graphite pouch cells at elevated temperature. The high temperature storage and cycling performances of pouch cells were investigated by various electrochemical methods and microscopy techniques. The results show that TAPi can effectively enhance the discharge capacity retention ratio after high temperature storage test. In the storage experiment, compared with the cells without TAPi, the cells containing 0.5wt% and 1wt% TAPi exhibit excellent initial discharge capacity retention of 91.7% and 89.5%, respectively. Besides, for the high temperature cycling at 45°C, the cells containing 0.5wt% exhibit optimal cycling performance. The results of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) suggest that the interfacial film derived from TAPi is helpful for protecting the electrode surface at elevated temperature, thus promoting the capacity performance and retention during the high temperature cycling test. Overall, these results suggest that TAPi can be regarded as a candidate electrolyte addition for lithium-ion batteries with high-nick-content NCM cathode to accommodate elevated temperature condition.