In recent years, global warming has become an increasingly serious problem. As part of efforts to address this issue, our laboratory is conducting research using polymer electrolyte fuel cell, in which CO₂ and H₂ gases are supplied and methane is produced through the electrolysis of CO₂ while simultaneously generating electricity. It is known that the charge/discharge capacity and thermal stability of lithium-ion secondary batteries decrease when used in low-temperature environmental. To clarify the underlying mechanisms, we investigate battery degradation mechanisms using differential capacity profiles and study the thermal stability of batteries using Accelerating Rate Calorimeter.