Thermal management is the principal reliability challenge in lithium battery packs. Cell temperature during fast charging can reach 55–65 °C, and the temperature differential between cells in a module must be kept below 5 °C to avoid accelerated degradation and capacity imbalance. Thermal interface materials (TIM) — silicone gap fillers, potting compounds, and thermally conductive adhesives — fill the air gaps between cells, between the cell array and the cooling plate, and between the pack and the vehicle chassis.
TIM Functions in Battery Pack Architecture
A typical EV battery pack uses three categories of TIM:
Cell-to-cooling-plate gap filler: The largest volume TIM application. Gap filler is dispensed between the bottom of the prismatic or pouch cell array and an aluminum cooling plate carrying glycol/water coolant. Required thermal conductivity: 3–8 W/m·K. Required compliance: soft enough (Shore 00 10–30) to accommodate 0.5–2 mm dimensional variation across the cell array without imposing significant force on cells. Cured gap filler must withstand 10–15 years of thermal cycling between -40 °C and +85 °C.
Structural potting compound: Used to encapsulate module interconnects and bus bars. Electrical insulation (dielectric strength > 15 kV/mm) plus moderate thermal conductivity (1–3 W/m·K). Typically a two-component RTV-2 silicone or epoxy.
Thermal grease for BMS heat sink: The battery management system generates moderate heat from power MOSFETs. CPU-grade thermal grease (1–6 W/m·K) applied between BMS PCB and its aluminum heat sink.
Filler Materials and Thermal Conductivity
| Filler | λ (W/m·K) | Typical Loading | Application |
|---|---|---|---|
| Spherical Al₂O₃ | 25–40 | 75–85 wt% | Gap filler 1–4 W/m·K |
| Angular Al₂O₃ | 20–35 | 65–78 wt% | Cost-optimized potting |
| Boron nitride (BN) | 250–300 | 35–55 wt% | Premium gap filler 6–12 W/m·K |
| AlN | 150–220 | 55–70 wt% | High-end, moisture-protected |
| MgO | 30–60 | 68–78 wt% | Mid-range cost/performance |
| ZnO | 50–60 | 55–70 wt% | Secondary filler, low cost |
Bimodal particle size distribution (large particles D50 = 50–80 μm mixed with fine particles D50 = 1–5 μm) achieves higher packing density and lower viscosity at a given thermal conductivity target, reducing dispensing pressure and improving flow into narrow gaps.
Silicone vs Epoxy Selection
Silicone-based TIM (gap fillers and potting): preferred for battery applications because of wide temperature range (-55 °C to +200 °C), low modulus, and long-term reliability without brittleness. Silicone outgasses siloxanes at low levels — a concern for electrical connectors, but well within limits for battery pack environments.
Epoxy-based potting: higher rigidity, better adhesion to aluminum and steel structures, lower cost per kg. Risk of thermal cycling fatigue cracking after 3–5 years in -40 °C to +85 °C cycling regime. Reserved for structural bonds and bus bar encapsulation where rigidity aids retention.