Capacity retention of succinonitrile (SCN)-plasticized solid polymer electrolyte membrane (PEM) in lithium iron phosphate (LiFePO4) half-cells has been investigated at an elevated temperature of 60°C with or without lithium bis(oxalato)borate (LiBOB) modification. The electrochemical and chemical stabilities of several symmetric cells and half-cells were examined under different thermal and electrochemical conditions. At ambient temperature, the unmodified PEM in the LiFePO4 half-cell appeared stable up to 50 cycles tested. Upon cycling at 60°C, the capacity declined rapidly while the cell resistance increased. Spectroscopic characterizations on the chemical compositions of the solid PEM surfaces on both cathode and anode sides reveal possible occurrence of nucleophilic side reactions on the unmodified PEM surface of the lithium anode side. This reaction product is seemingly mobile, capable of shuttling between the Li anode and the LiFePO4 cathode during the charge/discharge cycling, which eventually has led to drastic capacity fading in the half-cell as well as reduction in Coulombic efficiency. This side reaction can be effectively suppressed upon doping LiBOB additive (0.2–1.0wt%) into the PEM, contributing to capacity retention improvement at 60°C. Plausible mechanisms of the high temperature side reactions and suppression of the side reactions by LiBOB have been proposed. It may be inferred that the present chemical probing methodology on both sides of the PEM surfaces is feasible only because of the ‘solid’ nature of the polymer electrolyte membrane as opposed to their liquid counterpart.