Effective thermal regulation is paramount for the performance, longevity, and safety of lithium-ion batteries. Their optimal operational window is notably narrow, typically between 20°C and 50°C. Deviation from this range precipitates accelerated degradation, while excursions beyond safe limits can trigger catastrophic thermal runaway. Consequently, the battery management system (BMS) must incorporate robust thermal management strategies. Among these, passive systems utilizing Phase Change Materials (PCMs) offer a compelling solution by absorbing heat generated during operation at a nearly constant temperature, thereby mitigating temperature rise. However, the widespread adoption of PCMs is hampered by their intrinsically low thermal conductivity, which severely limits heat transfer rates and creates localized hot spots. This review, from my perspective, synthesizes the significant progress in employing carbon-based additives—carbon nanotubes (CNTs), carbon fibers (CF), graphene, and expanded graphite (EG)—to engineer composite PCMs (CPCMs) with radically enhanced thermal conductivity. I will focus particularly on the transformative role of EG-based composites, their integration into battery management system architectures, and their emerging function in suppressing thermal runaway propagation.
1. The Imperative for Thermal Conductivity Enhancement in Storage Materials
The fundamental challenge with organic paraffins, fatty acids, and inorganic salt hydrates is their low thermal conductivity (κ), typically in the range of 0.2–0.5 W/(m·K). In a battery management system context, this results in slow thermal response and poor utilization of the material’s latent heat capacity, as heat remains concentrated near the battery surface instead of being rapidly dissipated throughout the PCM bulk. The primary objective is to embed a continuous, high-conductivity network within the PCM matrix to facilitate efficient heat spreading. Carbon allotropes are ideal candidates for this role due to their exceptional intrinsic thermal conductivity, chemical stability, and low density. The effectiveness of this network depends on the filler’s aspect ratio, dispersion quality, interfacial contact with the PCM, and its ability to form percolating pathways.
| Thermal Storage Technology | Mechanism | Key Material Property | Typical Energy Density | Limitation for BMS |
|---|---|---|---|---|
| Sensible Heat Storage | Temperature change | Specific Heat Capacity (cp) | Low (ρ cp ΔT) | Large mass/volume required; temperature varies. |
| Latent Heat (Phase Change) Storage | Isothermal phase transition | Latent Heat of Fusion (ΔHfus) | Moderate-High (ρ ΔHfus) | Low thermal conductivity (κ); possible leakage. |
| Thermochemical Heat Storage | Reversible chemical reaction | Enthalpy of Reaction (ΔHrxn) | Very High | Complex system control; kinetics and cycle stability. |
The governing equation for one-dimensional conductive heat transfer through a PCM or CPCM is Fourier’s law:
$$ q = – \kappa \frac{dT}{dx} $$
where \( q \) is the heat flux (W/m²), \( \kappa \) is the thermal conductivity (W/(m·K)), and \( dT/dx \) is the temperature gradient. Enhancing \( \kappa \) directly reduces the temperature gradient for a given heat flux, which is critical for maintaining uniform battery temperature in a battery management system.
2. Carbon-Based Composites: Mechanisms and Performance Limitations
2.1 One-Dimensional Enhancers: Carbon Nanotubes and Carbon Fibers
CNTs, with their theoretically ultra-high axial thermal conductivity (∼3000 W/(m·K)), promise significant enhancement. The typical approach involves dispersing CNTs into molten PCM via sonication or shear mixing. However, the observed improvements are often modest, rarely exceeding a factor of 2-3 over the base PCM. This discrepancy arises from several factors: (1) high interfacial thermal resistance (Kapitza resistance) between the CNT surface and the PCM molecules due to phonon scattering; (2) agglomeration of CNTs, which prevents the formation of a continuous network; and (3) increased viscosity hindering processing. The effective medium theory can be adapted to model this, but the interfacial resistance term is dominant:
$$ \kappa_{eff} = \kappa_{PCM} \frac{1 + 2\beta\phi}{1 – \beta\phi} \quad \text{where} \quad \beta = \frac{(\kappa_f/\kappa_{PCM} – 1)}{(\kappa_f/\kappa_{PCM} + 2)} $$
This classical Maxwell-Garnett model often overestimates \( \kappa_{eff} \) for nano-composites because it does not account for interfacial resistance, which becomes significant at the nanoscale.
CFs, while having lower intrinsic conductivity than CNTs, are often easier to process and align. Unidirectional CF mats can create anisotropic CPCMs where conductivity along the fiber direction is significantly higher than transverse to it. However, achieving high filler loading to create a robust network often comes at the cost of reduced latent heat storage capacity per unit mass. The primary limitation for both CNTs and CFs in a compact battery management system is the difficulty and expense of creating a volumetrically efficient, isotropic, and continuous 3D conductive network.
| Filler Type | Typical Loading (wt.%) | κPCM (W/(m·K)) | κCPCM (W/(m·K)) | Enhancement Factor | Key Challenge |
|---|---|---|---|---|---|
| Multi-walled CNTs | 1-5% | 0.24 (Paraffin) | 0.34 – 0.45 | ∼1.4 – 1.9 | Dispersion, interfacial resistance, agglomeration. |
| Carbon Nanofibers (CNFs) | 5-10% | 0.32 (Paraffin) | 0.45 – 0.55 | ∼1.4 – 1.7 | Network formation, latent heat dilution. |
| Aligned Carbon Fiber Mat | ~9% | 0.24 (Paraffin) | 0.77 (axial) | ∼3.2 (anisotropic) | Anisotropy, complex integration into pack. |
2.2 Two-Dimensional Enhancer: Graphene and Derivatives
Graphene nanosheets or nanoplatelets (GNPs) offer a two-dimensional pathway for heat transfer. Their large surface area provides extensive contact with the PCM, potentially reducing the interfacial resistance per unit volume compared to 1D fillers. Experimental results show variable success, with enhancement factors commonly between 1.5 and 3. Performance is highly sensitive to the sheet’s lateral size, thickness (number of layers), and functionalization. While thin sheets maximize surface area, they may also fold or crumple, creating additional interfaces. Thicker, plate-like GNPs can act as more effective lateral heat spreaders. Nevertheless, like CNTs and CFs, achieving a connected, isotropic 3D network with GNPs at low loadings remains non-trivial. The percolation threshold for thermal conductivity in such composites is a critical parameter:
$$ \kappa_{eff} \propto (\phi – \phi_c)^t \quad \text{for} \quad \phi > \phi_c $$
where \( \phi \) is the filler volume fraction, \( \phi_c \) is the percolation threshold, and \( t \) is a critical exponent. For randomly oriented 2D sheets, \( \phi_c \) can be relatively low, but reaching high absolute \( \kappa_{eff} \) often requires loadings that compromise other properties.
3. Expanded Graphite: A Paradigm Shift in Conductivity Enhancement
Expanded graphite (EG) represents a fundamentally different and more effective approach. It is produced by the rapid thermal expansion of acid-intercalated graphite flakes, yielding a worm-like structure of interconnected, porous graphite platelets. This unique microstructure is the key to its success. The material possesses a high intrinsic thermal conductivity along the graphite planes and, more importantly, forms a compressible, porous sponge with immense surface area.
3.1 Mechanism of Enhancement and Composite Fabrication
The enhancement process involves two critical steps: impregnation and compression. Molten organic PCM (e.g., paraffin) is readily absorbed into the open pores of the EG scaffold via capillary forces, creating a shape-stable composite powder. The non-polar nature of the graphite surface ensures good wettability and compatibility with organic PCMs. For inorganic salt hydrate PCMs, which are hydrophilic, the EG surface often requires modification using surfactants (e.g., Triton X-100) to render it hydrophilic, dramatically improving adsorption capacity from ~50% to over 80%.
The transformative second step is the compression of this powder into a monolithic block. This process mechanically re-orients and compacts the flexible graphite worms, forcing the individual graphite platelets into intimate contact and creating a continuous, percolating three-dimensional network throughout the PCM matrix. This network acts as a highly efficient “thermal highway,” bypassing the resistive PCM medium. The thermal conductivity of the resulting CPCM block follows a strong positive correlation with its compressed density (\( \rho_{CPCM} \)):
$$ \kappa_{CPCM} \approx \kappa_{EG,net} \cdot f(\rho_{CPCM}) + \kappa_{PCM} \cdot (1 – f(\rho_{CPCM})) $$
where \( \kappa_{EG,net} \) represents the effective conductivity of the compressed graphite network, which increases as the network becomes denser and better interconnected. This allows for tunable thermal properties tailored to a specific battery management system design.
| Composite System | PCM Type | κPCM (W/(m·K)) | Density (kg/m³) | κCPCM (W/(m·K)) | Enhancement Factor |
|---|---|---|---|---|---|
| Paraffin / EG | Organic (Paraffin) | ~0.25 | ~672 | 7.79 | > 30 |
| Sebacic Acid / EG | Organic (Fatty Acid) | ~0.37 | ~768 | 5.35 | > 14 |
| SAT-Urea / EG | Inorganic (Salt Hydrate) | ~0.5 | N/A | 9.05 | > 18 |
SAT: Sodium Acetate Trihydrate. Note the order-of-magnitude improvement achievable with compressed EG composites.
3.2 Dual-Function Thermal Storage: Phase Change and Thermochemistry
A groundbreaking advancement with salt hydrate/EG composites is their dual-function capability. Materials like Sodium Acetate Trihydrate (SAT) exhibit not only a solid-liquid phase change (endowing latent heat storage, ΔHfus ~ 225 kJ/kg) but also a reversible dehydration reaction at higher temperatures (endowing thermochemical heat storage, ΔHrxn ~ 568 kJ/kg):
$$ \text{NaC}_2\text{H}_3\text{O}_2 \cdot 3\text{H}_2\text{O}_{(s)} \xrightarrow[\text{Charge}]{\text{Discharge}} \text{NaC}_2\text{H}_3\text{O}_2_{(s)} + 3\text{H}_2\text{O}_{(g/l)} \quad \Delta H > 0 $$
This provides a two-stage thermal management strategy for a battery management system: the phase change handles normal operational heat, while the chemical reaction acts as a high-density thermal “shock absorber” to absorb the massive heat flux from a cell undergoing thermal runaway, potentially preventing propagation to neighboring cells. The high conductivity of the EG network ensures this heat is rapidly conducted away from the failing cell into the bulk of the composite material.
Integrating such high-performance CPCMs requires careful design within the overall battery management system architecture, balancing thermal, electrical, and mechanical requirements.
4. Application in Battery Thermal Management: From Cooling to Runaway Suppression
4.1 Conventional Thermal Management and System Integration
In standard operation, high-κ EG-CPCMs effectively flatten temperature distributions within battery modules. Experimental studies show that modules equipped with paraffin/EG blocks can maintain maximum temperatures below 50°C at moderate discharge rates, whereas modules with pure PCM or no PCM exceed safe limits. However, a purely passive PCM-based battery management system can saturate over time. Therefore, hybrid systems coupling CPCMs with active air or liquid cooling are often proposed. The CPCM acts as a primary buffer, absorbing peak heat loads and delaying the need for active cooling, thereby reducing the energy consumption of the BMS.
4.2 Addressing Practical Challenges: Flexibility and Insulation
A significant practical issue with compressed EG-CPCM blocks is their rigidity and high electrical conductivity, leading to high thermal contact resistance with curved cell surfaces and a risk of internal short circuits. A sophisticated solution is the creation of a flexible, insulating dual-network composite. By introducing a natural rubber (NR) matrix into the paraffin/EG mixture before compression, an independent, flexible, and electrically insulating network is formed. This NR network encapsulates the conductive EG network, drastically reducing the composite’s electrical conductivity (e.g., from 13.6 S/m to ~0.027 S/m) while largely preserving its thermal conductivity (e.g., 3.4 W/(m·K)). This yields a flexible CPCM sheet that conforms to battery surfaces, minimizes contact resistance, and enhances safety—a direct enabler for more reliable battery management system packaging.
4.3 Frontier Application: Arresting Thermal Runaway Propagation
This is where dual-function salt hydrate/EG composites show unparalleled promise. Standard insulation materials (e.g., aerogel) only delay heat transfer. In contrast, a block of SAT/EG composite placed between cells performs two functions during a runaway event in one cell: 1) Its high conductivity rapidly pulls heat from the failing cell, and 2) its enormous thermochemical storage capacity (∼568 kJ/kg) absorbs this heat endothermically. Experimental nail penetration tests demonstrate the stark difference: with aerogel, neighboring cells rapidly exceed 750°C and propagate runaway; with SAT/EG, the adjacent cells remain below 75°C, effectively quarantining the failure. This presents a paradigm shift for safety-focused battery management system design, moving from delay to active suppression.
5. Conclusion and Future Perspectives
The journey from low-conductivity PCMs to advanced carbon-based CPCMs has significantly advanced passive thermal management technology for lithium-ion batteries. Among carbon additives, expanded graphite stands out due to its unique ability to form a continuous, high-conductivity 3D network via simple impregnation and compression, delivering order-of-magnitude enhancements in thermal conductivity. When integrated into a battery management system, these materials provide effective temperature homogenization and peak shaving.
The future trajectory points toward multifunctional, intelligent materials. Key research frontiers include:
- Full-Temperature-Spectrum Management: Developing composite materials that integrate low-temperature heating (e.g., via conductive networks or embedded heating elements), mid-temperature phase change cooling, and high-temperature thermochemical runaway suppression into a single, flexible package.
- Advanced Multifunctional Composites: Further engineering of matrices like polymer/EG/rubber to achieve optimal and stable balances of high κ, flexibility, electrical insulation, flame retardancy, and self-healing properties.
- System-Level Integration and Intelligence: Coupling CPCM-based thermal management with real-time BMS algorithms. This involves modeling and predicting CPCM state-of-charge (i.e., melted fraction or dehydration degree) to proactively trigger active cooling or initiate safety protocols, creating a truly responsive and predictive thermal battery management system.
- Material Stability and Lifecycle: Intensive study of long-term cyclic stability—both thermal (for phase change) and chemical (for reversible reactions)—under realistic battery operating conditions is crucial for commercial viability.
In conclusion, carbon-enhanced composite thermal storage materials, particularly those based on engineered expanded graphite networks, have evolved from simple conductivity boosters to sophisticated, multi-storage-mechanism solutions. Their continued development is integral to building safer, more efficient, and more reliable battery systems, pushing the capabilities of the modern battery management system beyond mere monitoring into the realm of active and adaptive thermal governance.