As known, it is common for lithium ion battery (LIB) to be used under extreme circumstances, among the high temperature circumstance is included. Herein, a series of experiments were conducted at elevated temperatures of 50, 60, and 70 degrees C to examine the performance of LIB. Besides, the application of phase change material (PCM) under such condition was also investigated. Based on the experimental results, it is found that ambient temperature has a remarkable impact on the heat generation of battery, which decreases with the increasing ambient temperature. In addition, it is revealed that the inconsistency among batteries will deteriorate under high temperature circumstance by aggravating the differences of temperature and voltage. Moreover, the capacity decay rate of battery is demonstrated to be accelerated greatly by high temperature. After employing PCM into high temperature circumstance, it is presented that battery exhibits better performance including more stable temperature variation, better discharging performance and slower decay rate. And the performance of PCM is related to its thickness. (C) 2019 The Electrochemical Society.