Thermal degradation mechanisms of solid electrolyte interphases (SEIs) on graphite and SiO electrodes are examined at moderately elevated temperatures (60-130 degrees C). Of the two possible degradationmechanisms, the attack of phosphorus pentafluoride (PF5), which is generated by thermal decomposition of lithium hexafluorophosphate (LiPF6) used as the lithium salt, dominates over the thermal decomposition of the SEI layer itself over this temperature range. Once the SEI layer is damaged, electrolyte decomposition and film deposition takes place on the newly exposed electrode surfaces due to the loss of passivating ability; this is a repair process for the damaged SEI. Such damage/repair of the SEI layer continues until the Li+ ions/electrons are exhausted from the negative electrodes. An undesired feature of this process is the steady increase in SEI thickness, which causes electrode polarization and eventual capacity fading. (C) 2017 The Electrochemical Society. All rights reserved.