A Review of Phase Change Materials for the Thermal Management and Isothermalisation of Lithium-Ion Cells

Abstract

Abstract Li-Ion batteries will play an important role in reaching emission targets by sustaining the further integration of renewable energy technologies and Electric Vehicles (EVs) in society. Their performance however is quite sensitive to temperature, leading to capacity fade, acceleration of ageing effect and possible thermal runaway. A Thermal Management System (TMS) should maintain a battery at an operating temperature within an optimal range and maximise temperature uniformity, i.e. approaching an isothermal condition. Many studies have experimentally investigated the electrical performance of Li-Ion batteries under controlled environmental temperatures. Notably however, these controlled conditions do not impose a uniform temperature or a controlled rate of cooling, as a TMS would. From a review of the relevant literature a ratio of the heat generation to the power production is proposed, i.e. quantifying an equivalent electro-chemical efficiency to advance research in this technological area and as additional TMS design metric. Overall, there is enough evidence that 25–30 °C is the best temperature range to minimise the ageing effect while 25–40 °C is typically reported as the general Li-Ion cells operating range. No specific temperature is identified to optimise the cycle electro-chemical efficiency and minimise the ageing effect. Therefore, a TMS should keep Li-Ion batteries within a specific temperature range according to the need for either higher electro-chemical efficiencies (i.e. higher powers and lower heat generation rates) or higher operating life. There are four main thermal management approaches of Li-Ion batteries: air-cooling, liquid-cooling, boiling and Phase Change Materials (PCM). Air cooling is preferred for safety reasons but is less efficient as the rate of heat transfer achievable is relatively low. Forced air cooling can effectively keep the temperature at a preferred level but fails to guarantee a uniform temperature. Liquid cooling is better in terms of heat transfer performance, but it is less safe and can still result in significant thermal gradients within the pack. Boiling effectively keeps Li-Ion cells temperature constant and uniform but can be quite complex to operate and control. Phase Change Materials (PCMs) as a passive cooling approach are proposed as an effective and low-cost isothermalisation technique. However, when Li-Ion batteries are operated under extreme conditions (high ambient temperature, high discharge rates), PCM are not able to recover all latent energy potential during solidification and this leads to possible thermal runaway. Overall, it is clear that no TMS alone is holistically better than others and the choice between air cooling, liquid cooling, boiling and latent heat PCM systems is entirely linked to the specific combination of temperatures, heat rates, cells capacity and geometry. Active PCM systems however, mainly a combination of liquid cooling and passive PCM, show promising results towards an ideal isothermal condition. Also, they introduce the potential to store the thermal energy and use it as needed, converting a Li-Ion cell from an Electrical Energy Storage System (EESS) to a Combined Heat and Power (CHP) system.