In this work, we analyzed the thermal stability of polycarbonate: lithium trifluoromethane sulphonylimide (LiTFSI) systems. Polyethylene carbonate (PEC) and polypropylene carbonate (PPC) undergo depolymerization to ethylene carbonate (EC) and propylene carbonate (PC), respectively, in the presence of Li salt under annealing treatment. The decomposition reaction can explain the surprising high ionic conductivity recently reported for solid polymer electrolyte (SPE) polycarbonate-based systems. Because this phenomenon can strongly impact the results of SPE research, it is essential to control the depolymerization of polymers to avoid inaccurate electrochemical performance. In addition, we observed that the standard drying procedure for the preparation of high-salt-concentration polymer (polymer-in-salt) SPEs traps a high amount of residual solvent, e.g., acetonitrile (ACN), due to the strong bond between the solvent and ionic species. This finding represents another factor influencing the remarkably high ionic conductivity.