Application of Multi-Layer Graphite Paper (MLGP) in New Energy Vehicle Battery Pack Thermal Management

I. Physical Properties and Thermal Dissipation Mechanism of Multi-Layer Graphite Paper

    Multi-layer graphite paper is manufactured through calendaring and lamination processes using high-purity expanded graphite. Its core advantage stems from the anisotropic thermal conductivity inherent to the graphite crystal structure. The in-plane thermal conductivity of a single graphite paper layer can reach 1500-2000 W/(m·K), significantly exceeding copper (~400 W/(m·K)) and aluminum (~237 W/(m·K)). Conversely, its through-plane thermal conductivity is engineered to be much lower, typically 5-10 W/(m·K). This characteristic enables highly efficient lateral heat spreading within the battery pack while restricting longitudinal heat transfer, thus preventing excessive heat accumulation towards the battery pack casing.

    By stacking graphite paper layers with varying porosity and thickness (typically 3-5 layers), a gradient thermal pathway is constructed. For example:

• Contact Layer (0.05mm): Utilizes high-density graphite paper (porosity <5%) for direct interfacing with cell surfaces, ensuring interfacial contact thermal resistance below 0.1 K·cm²/W.

• Spreading Layer (0.1-0.2mm): Employs medium-porosity graphite paper (10%-15%) to accelerate lateral heat conduction towards the pack edges.

• Buffer Layer (0.3mm): Features low-density graphite paper (porosity >20%), providing both thermal energy absorption (thermal buffering) and stress buffering functions.

    Experimental data indicates that an MLGP system utilizing a sandwich structure can reduce the internal temperature delta within a battery module from 8-10°C in conventional liquid cooling solutions to below 3°C (Source: CATL 2024 Technology Whitepaper).

II. Engineering Application Cases and Performance Advantages

1. Thermal Runaway Mitigation in Tesla 4680 Battery Packs: Tesla implemented a composite solution incorporating 5-layer graphite paper + silicone thermal interface material within the Cybertruck battery pack, replacing 30% of liquid cooling tubes. Finite Element Analysis (FEA) simulations and physical testing validated that this design reduces peak cell temperature during 10C fast charging from 65°C to 52°C. Crucially, the thermal runaway propagation time was extended beyond 30 minutes, significantly exceeding the industry standard requirement of ≥15 minutes.

2. Lightweighting Breakthrough in CATL's Qilin (Kirin) Battery: CATL integrated multi-layer graphite paper with Phase Change Material (PCM) in its third-generation Cell-to-Pack (CTP) technology. This approach boosted the gravimetric energy density of the battery pack to 255 Wh/kg, achieving a 12% weight reduction compared to traditional designs. Key innovations included:

• Using ultra-thin MLGP (total thickness 0.5mm) to replace aluminum-based heat spreaders.

• Enhancing interfacial adhesion between the graphite paper and PCM via laser micro-drilling (aperture 50-100µm).

3. Low-Temperature Performance Improvement for BYD Blade Battery: BYD utilized modified graphite paper incorporating carbon nanotubes (CNTs) (thermal conductivity enhanced to 2200 W/(m·K)) in its low-temperature version Blade battery. This modification increased the effective discharge capacity at -30°C to 85% of the nominal capacity, up from only 72% in conventional designs.

III. Industrialization Challenges and Innovation Directions

Despite significant advantages, the large-scale application of MLGP faces challenges:

• Long-Term Reliability: Delamination between graphite paper layers under cyclic thermal stress can increase contact thermal resistance. Current solutions involve:

• Developing polyimide (PI)-based adhesives with operating temperatures up to 300°C.

• Applying nanoscale alumina coatings (<100nm thick) via magnetron sputtering to reduce interfacial wear.

• Cost Pressure: High-purity expanded graphite (≥99.95%) constitutes ~60% of material costs. Industry initiatives include:

• Utilizing alternative biomass carbon sources (e.g., palm kernel shells, achieving ~40% cost reduction in graphite precursor preparation).

• Implementing continuous calendering processes, exemplified by Toray's integrated roll-pressing/sintering equipment, tripling production efficiency.

• Recycling Lag: Waste MLGP containing organic binders is difficult to recycle directly. EcoGraf's (UK) supercritical CO₂ delamination technology enables ~99% graphite recovery, but the processing cost remains high at ~USD 1200/ton.

IV. Future Trends: From Material Innovation to System Integration

• Material Hybridization: Combining graphite paper with materials like aerogels or metal nanowires to create intelligent materials offering both thermal conduction and insulation functionalities. For example, NASA is testing graphite paper/silica aerogel sandwich structures capable of maintaining battery pack integrity under extreme conditions up to 2000°C.

• Intelligent Structural Design: Utilizing Digital Twin technology to optimize MLGP layer count, thickness, and arrangement. BMW Group's AI-driven design platform (developed with ANSYS) reduces thermal solution iteration time from 6 months to 2 weeks.

• Policy-Driven Standardization: The EU's prospective "Technical Specification for Graphite-based Thermal Management Materials in Vehicles" (scheduled for 2025) will mandate battery pack thermal management system carbon emission intensity ≤5kg CO₂/kWh (currently 8-10kg CO₂/kWh), compelling adoption of low-carbon materials like graphite paper.

Conclusion

    The application of MLGP in NEV battery pack thermal management signifies a transformative leap for graphite materials from traditional industrial sectors into high-end manufacturing. Continuous advancements in material modification, intelligent manufacturing processes, and circular economy models position this technology to potentially penetrate over 80% of global NEV battery packs by 2030, becoming a core driver for achieving industry carbon neutrality goals. Moving forward, the primary focus for academia, research, and industry will be balancing the critical trade-offs between performance, cost, and sustainability.