Thermal management is critical for lithium-ion batteries to maintain optimal performance, safety, and longevity. Passive cooling using phase change materials (PCMs) has emerged as an effective solution, leveraging latent heat absorption and release to regulate temperature without external energy input. PCMs absorb heat as they transition from solid to liquid and release it during solidification, stabilizing battery temperature within a narrow range.
The physics of PCM operation centers on latent heat capacity, the energy absorbed or released during phase transitions at nearly constant temperature. Unlike sensible heat storage, where temperature changes with energy input, PCMs maintain thermal equilibrium during phase change, making them ideal for battery applications. The effectiveness depends on thermophysical properties: melting temperature must align with the battery's optimal operating range (typically 20–50°C), while high latent heat capacity ensures sufficient energy absorption per unit mass.
Common PCMs include organic paraffins, inorganic salt hydrates, and eutectic mixtures. Paraffins, such as octadecane, are widely used due to their chemical stability, negligible supercooling, and melting points tunable by chain length (e.g., 25–40°C). Their latent heat ranges from 150–250 kJ/kg, but low thermal conductivity (0.2–0.4 W/m·K) limits heat dissipation. Salt hydrates like calcium chloride hexahydrate offer higher conductivity (0.5–1.0 W/m·K) and latent heat (200–300 kJ/kg), but suffer from phase segregation and corrosion risks. Composite PCMs address these tradeoffs by combining materials—for example, embedding paraffin in a metal foam matrix boosts conductivity to 5–20 W/m·K while retaining high energy storage.
Integration methods must accommodate PCM volume changes during phase transitions (typically 5–15% expansion). Encapsulation in microcapsules or macro-containers prevents leakage and improves compatibility. Microencapsulation, using polymer shells like polyurethane or silica, enhances heat transfer surface area but reduces PCM mass fraction. Macro-encapsulation in aluminum or graphite casings offers structural stability but increases system weight. Alternatively, PCMs are embedded in porous matrices (e.g., expanded graphite, carbon fibers), which provide conductive pathways and mitigate volume effects.
Hybrid systems combine PCMs with active cooling (air/liquid) to address high heat loads. In electric vehicles, PCM layers integrated between cells absorb peak heat during fast charging, while liquid cooling handles sustained loads. This reduces pump/fan energy by up to 30% compared to active-only systems. For example, a paraffin-graphite composite paired with mini-channel liquid cooling maintains cell temperatures below 40°C even at 3C discharge rates.
PCMs face inherent limitations. Low thermal conductivity restricts heat spreading, causing localized hot spots. Nano-enhanced PCMs, doped with carbon nanotubes (CNTs), graphene, or metal nanoparticles, improve conductivity (up to 10–30 W/m·K) without sacrificing latent heat. For instance, paraffin with 10% graphene nanoplatelets achieves 8.5 W/m·K. However, nanoparticle aggregation and cost remain challenges. Volume change during phase transitions can mechanically stress battery modules, requiring flexible encapsulation or void space design.
Recent advances focus on multifunctional PCM composites. Shape-stabilized PCMs, where paraffin is absorbed into polymer networks (e.g., polyethylene, styrene-butadiene), eliminate leakage and enhance mechanical strength. Bio-based PCMs like fatty acids (capric-lauric acid blends) offer renewable alternatives with comparable performance (latent heat ~180 kJ/kg). Phase change slurries, dispersing microencapsulated PCM in coolant fluids, enable simultaneous energy storage and convective cooling.
Material selection and system design depend on application specifics. Consumer electronics prioritize compactness, favoring thin PCM-graphite films. Grid storage systems use bulk salt hydrates for cost efficiency, while aerospace applications require lightweight composites with high specific energy. Ongoing research targets higher conductivity, cycling stability (PCMs degrade after ~1000 cycles), and fire resistance—critical for high-energy batteries.
In summary, PCM-based thermal management provides efficient, passive temperature control for lithium-ion batteries. By optimizing material properties and integration methods, PCMs mitigate thermal runaway risks and extend battery life. Hybrid systems and nano-enhanced composites represent the next evolution, balancing performance with practicality for diverse energy storage needs.