Thermal management is critical in battery systems to maintain optimal operating temperatures, ensure safety, and prolong lifespan. Thermal interface materials (TIMs) play a vital role in enhancing heat transfer between battery cells and cooling systems by filling microscopic air gaps that would otherwise act as thermal insulators. The effectiveness of TIMs depends on their thermal conductivity, mechanical properties, and long-term reliability under thermal cycling conditions.

TIMs can be classified into four primary categories based on their composition and form: thermal greases, gap fillers, phase change materials, and graphite sheets. Each type has distinct thermal and mechanical properties that determine its suitability for battery applications.

Thermal greases are viscous, paste-like materials composed of silicone or hydrocarbon oils filled with thermally conductive particles such as aluminum oxide, zinc oxide, or boron nitride. They offer high thermal conductivity, typically ranging between 1 and 5 W/mK, and are applied in thin layers to minimize thermal resistance. However, greases can suffer from pump-out effects under thermal cycling, where the material migrates away from the interface, reducing effectiveness over time.

Gap fillers are elastomeric pads or sheets filled with ceramic or metal particles to enhance thermal conductivity, which can range from 1 to 10 W/mK depending on filler content. These materials are compressible, allowing them to accommodate variations in gap thickness between battery cells and cooling plates. Silicone-based gap fillers are common due to their flexibility and electrical insulation properties, but they may degrade under prolonged exposure to high temperatures.

Phase change TIMs (PCM-TIMs) transition from solid to semi-liquid states at elevated temperatures, improving conformability and interfacial contact. These materials typically have thermal conductivities between 2 and 8 W/mK and are advantageous in applications where consistent pressure cannot be maintained. However, their performance may decline if the phase change process leads to material displacement over repeated cycles.

Graphite sheets are lightweight, highly conductive materials with in-plane thermal conductivities reaching up to 1500 W/mK, though through-plane conductivity is significantly lower (5-20 W/mK). They are often used in thin, flexible forms and provide excellent electrical insulation when laminated with polymer layers. However, graphite sheets require high compressive forces to achieve optimal thermal contact, which can be challenging in large battery packs.

Selecting the appropriate TIM involves evaluating multiple criteria beyond thermal conductivity. Compressibility is crucial for accommodating manufacturing tolerances and preventing excessive mechanical stress on battery cells. Materials with low hardness values (below 50 Shore 00) are preferred for gap-filling applications. Electrical insulation is another critical factor, particularly in high-voltage battery systems where conductive fillers could create short-circuit risks. Most silicone-based TIMs provide sufficient dielectric strength, whereas metallic filler composites require additional insulating layers.

Long-term stability under thermal cycling is a major concern, as repeated expansion and contraction can lead to material degradation, delamination, or drying out. Accelerated aging tests reveal that silicone-based TIMs generally exhibit better durability than hydrocarbon-based alternatives, though filler settling can reduce performance over time. Phase change materials must be evaluated for their ability to re-solidify uniformly after melting, as non-reversible phase separation can increase thermal resistance.

Application techniques also influence TIM performance. Dispensing methods for greases must ensure uniform coverage without excessive thickness, which can impede heat transfer. Pre-formed gap filler pads require precise cutting and placement to avoid air pockets, while graphite sheets often need adhesive backing or mechanical clamping to maintain contact pressure. Automated dispensing and robotic placement are increasingly used in high-volume battery manufacturing to improve consistency.

Recent advancements focus on nanocomposite TIMs that combine high thermal conductivity with improved mechanical stability. For example, boron nitride nanosheets dispersed in polymer matrices achieve conductivities above 10 W/mK while maintaining electrical insulation. Hybrid materials incorporating carbon nanotubes and graphene offer anisotropic heat transfer properties, beneficial for directing heat toward cooling channels. Another development involves elastomeric TIMs with self-healing properties, where reversible chemical bonds allow the material to recover from mechanical damage during thermal cycling.

Performance degradation mechanisms in TIMs include filler particle settling, polymer matrix degradation, and interfacial delamination. Thermal cycling tests show that materials with higher crosslink density in the polymer matrix resist degradation better but may sacrifice conformability. Oxidation and moisture absorption can also reduce thermal conductivity, particularly in non-silicone-based formulations.

In high-power battery packs, where heat generation is substantial, TIMs must maintain low thermal resistance under continuous load. Nanocomposites with aligned filler structures demonstrate improved performance by creating efficient heat conduction pathways. Additionally, TIMs with low thermal resistance at high pressures (below 0.5 cm²K/W at 50 psi) are preferred for compact battery designs where space constraints limit cooling system size.

The evolution of TIM technology continues to address the demands of next-generation battery systems, balancing thermal performance, reliability, and manufacturability. As battery energy densities increase and fast-charging capabilities expand, the role of advanced TIMs in maintaining thermal stability will remain a key focus in battery pack design.