Lithium-ion batteries, due to their high energy densities and efficiencies, are highly suitable energy storage devices for a wide range of applications that include portable electronics and electric vehicles. However, there is a rising concern for the safety implications of current battery technology. Solid polymer electrolyte (SPE) based batteries can be used for stretchable and flexible applications and offer advantages in safety, thermal stability, and manufacturability. In this study, the thermo-electrochemical mechanisms in a battery based on SPE were investigated. Initially, current models for conventional Li-ion cells were examined and verified in COMSOL Multiphysics. Current thermal runaway simulations utilize conventional organic liquid electrolyte 18650 Li-ion cells. Parametric analysis was performed to enhance various model inputs and assess the implications of modeling SPEs. A new model was developed to incorporate flexible, thin-film batteries that utilize a SPE, polyethylene oxide (PEO). Simulation results show that thermal runaway is suppressed in a battery based on SPE, despite the presence of electrode decomposition events. A sensitivity analysis was also performed to assess various parameter influences on the model.