In this article we present the difference in thermal stability of Li/LiFePO4| half cells with liquid and solid polymer electrolytes. After two initial cycles, the cells were charged to two different state of charge (SOC) of 50 and 100%. The thermal stability of the half cells is assessed with an accelerating rate calorimeter, and the thermal runaway parameters are discussed for each experiment: dependence of self-heating rate on temperature, temperature of a first-detected exothermic reaction, and maximum cell temperature. The dependence of those parameters with respect to the SOC is also presented.