Phase Change Materials

Phase change materials (PCMs) are materials that change their physical state, from solid to liquid and vice versa, to store and release heat. A solid-phase PCM starts absorbing heat from the battery and, at a certain temperature (it depends on the specific PCM), the melting begins. As the PCM absorbs heat, the battery is maintained to a target temperature. When the PCM is cooled by the environmental changes, the phase changes to solid again, releasing energy in the form of latent heat. There are around 200 different PCMs that can target temperatures between −50 ◦C and 150 ◦C [54]. For electric vehicle BTMSs, it should be necessary to test PCMs in a smaller temperature range, but the starting costs are still high. This is one of the reasons why PCMs are not as common in BTMSs. On the other hand, PCMs are shown to be one of the most efficient coolants thanks to their high heat storage capacity. The operating costs are quite low, and the retention of properties in sequential

cycles is a very appreciable characteristic [55]. There are several kinds of PCMs, different in their composition and for the temperatures at which they change their phase. The most used are water-based PCMs, salt hydrates, paraffin, or innovative and eco-friendly plant-based PCMs. There are also innovative methods to improve PCMs' characteristics such as loading them with composite materials and metal foams. Samimi et al. studied innovative PCM loaded with carbon fiber [56] to increase the PCMs' thermal diffusivity (which is usually low) and, consequently, improving the temperature uniformity in the battery cells. Carbon-based materials have been chosen as proper conductive fillers because of their excellent thermal and electrical properties, their chemical stability, and their lower density than metals. For the experiments, different cooling media (air, wax as unloaded PCM, carbon-fiber-loaded PCM) have been compared during the discharge of a lithium-ion battery, provide evidence for the results of the research. It is shown that the higher the percentage of carbon fiber loading is, the lower the resulting cell temperature is. Then, PCMs not only act like heat sinks but, if loaded with carbon fiber, also lead to a better temperature distribution in the cells thanks to their improved thermal conductivity.
