The relentless pursuit of sustainable transportation has positioned lithium-ion batteries (LIBs) as the cornerstone of modern electric vehicle (EV) technology. Their high energy density, long cycle life, and declining cost make them indispensable. However, the thermal behavior of LIB packs remains a critical challenge for battery management system (BMS) designers. During operation, especially under high current charge/discharge cycles, irreversible heat generation can lead to rapid temperature rise. An efficient battery management system must maintain the pack within an optimal temperature window, typically 25°C to 40°C, and ensure a minimal temperature difference, ideally below 5°C, between cells to prevent accelerated degradation, capacity fade, and catastrophic thermal runaway.
Traditional thermal management solutions within a battery management system, such as forced air or liquid cooling, add complexity, weight, and parasitic power consumption. Phase Change Material (PCM)-based cooling presents an elegant, passive alternative. PCMs absorb substantial amounts of heat as latent heat during their phase transition, effectively isothermalizing the system. Among organic PCMs, Polyethylene Glycol (PEG) is particularly attractive due to its high latent heat, chemical stability, non-toxicity, and moderate cost. Yet, two inherent drawbacks hinder its direct application in a robust battery management system: low thermal conductivity, which limits heat spreading, and leakage in the molten state, which poses safety and reliability risks.
This work addresses these dual challenges by developing a novel composite PCM (CPCM) specifically engineered for integration into lithium-ion battery thermal management systems. We propose a synergistic “joint-intercrossed network” utilizing the distinct porous structures of diatomite and expanded graphite (EG) to encapsulate PEG. Diatomite, a natural mineral with a highly porous microstructure, acts as a primary micro-container, while EG, with its worm-like, interconnected graphitic network, provides a three-dimensional pathway for rapid heat conduction and additional capillary force for retention. This design philosophy aims to maximize PEG loading for high energy storage while ensuring complete shape stability and significantly enhanced thermal transport—key attributes for an effective BMS.
The performance of a battery management system is directly linked to its ability to handle heat. The total heat generation rate in a cell can be approximated by:
$$
q_{\text{gen}} = I^2R_{\text{total}}
$$
where \(q_{\text{gen}}\) is the volumetric heat generation rate, \(I\) is the operational current, and \(R_{\text{total}}\) represents the total internal resistance (sum of ohmic and polarization resistances). Since heat generation scales with the square of the current, high-current scenarios (e.g., fast charging or aggressive acceleration) present the greatest thermal challenge for the battery management system. A CPCM integrated into the BMS acts as a heat sink, with its cooling capacity governed by:
$$
Q_{\text{PCM}} = m_{\text{PCM}} \left[ c_{p,s} (T_m – T_i) + \Delta h + c_{p,l} (T_f – T_m) \right]
$$
where \(m_{\text{PCM}}\) is the mass of the active PCM, \(c_{p,s}\) and \(c_{p,l}\) are the specific heats of the solid and liquid phases, \(T_i\), \(T_m\), and \(T_f\) are the initial, melting, and final temperatures, and \(\Delta h\) is the latent heat of fusion. Our objective was to maximize \(m_{\text{PCM}}\) and \(\Delta h\) within a leak-proof structure with high effective thermal conductivity \(k_{\text{eff}}\).
1. Material Synthesis and Characterization Methodology
We employed a vacuum impregnation method to synthesize the PEG/Diatomite/EG composites. PEG-1000 was used as the core PCM. Diatomite (SiO₂ content >85%) and expandable graphite (80 mesh) were the supporting materials. The expandable graphite was first thermally treated at high temperature to obtain worm-like expanded graphite (EG) with a volumetric expansion ratio of 200-300.
The synthesis process was systematic: Predetermined masses of PEG, diatomite, and EG were weighed. Diatomite and EG powders were first dry-mixed to achieve homogeneity. PEG was melted at 80°C and then gradually poured into the mixed powders under continuous mechanical stirring at 80°C for 20 minutes. This slurry was transferred to a glass dish and placed in a vacuum oven at 80°C under a pressure of 0.08 MPa for 2 hours to facilitate deep infiltration of molten PEG into the porous networks. Finally, the sample was cooled to room temperature to solidify. We prepared a matrix of 15 samples with varying compositions to map the relationship between structure and properties, as summarized in Table 1.
| Sample ID | PEG | Diatomite | EG | Primary Investigated Variable |
|---|---|---|---|---|
| S1-S7 | 30-90 | 70-10 | 0 | PEG/Diatomite ratio (No EG) |
| S8, S10, S11 | 70, 80, 90 | 30, 20, 10 | 3 | Fixed EG (3%), varying matrix |
| S11-S14 | 90 | 10 | 3, 4, 5, 6 | Fixed PEG/Diatomite, varying EG |
| S15 | 94 | 0 | 6 | Control: PEG/EG only |
Characterization techniques included Scanning Electron Microscopy (SEM) for morphology, X-ray Diffraction (XRD) for crystallinity and chemical compatibility, Differential Scanning Calorimetry (DSC) for phase transition properties (temperature and enthalpy), and Transient Plane Source (TPS) method for thermal conductivity. A critical qualitative leakage test was performed by placing compressed discs of each CPCM on qualitative filter paper and heating at 50°C for 5 hours.
2. Morphology, Structural Stability, and Leakage Prevention
SEM analysis revealed the fundamental mechanisms behind the CPCM’s performance. Pure EG exhibited a classic worm-like, accordion-shaped structure with numerous interlinked macro- and micro-pores, ideal for both adsorbing liquid and providing thermal highways. Diatomite particles showed a disk-like morphology peppered with a vast number of nano- and micro-scale pores (∼500 nm), creating an immense surface area for capillary adsorption. In the optimal composite (Sample S14), the microstructure clearly showed EG worms embedded within and interconnected by the diatomite-PEG matrix. The PEG was thoroughly adsorbed within the pores of both materials, forming a cohesive solid. No separate PEG pools were observed, indicating successful encapsulation.
XRD patterns confirmed physical, not chemical, integration. The characteristic peaks of PEG, diatomite, and EG were all present in the composite without shift or new peaks, proving the components were physically blended without reaction, ensuring chemical stability crucial for long-term BMS operation.
The leakage test was the definitive proof of concept for a reliable battery management system component. Samples with only diatomite (S1-S7) showed increasing leakage as PEG content rose above 50%. Sample S7 (90% PEG, 10% diatomite) failed catastrophically. Introducing just 3% EG (S8, S10, S11) dramatically improved retention, but S11 still showed minor seepage. Incrementally increasing the EG content to 6% (Sample S14) resulted in a completely leak-free disc even after prolonged heating, with the highest achievable PEG content of 90%. The control sample S15 (94% PEG, 6% EG, no diatomite) showed leakage, underscoring the indispensable role of diatomite’s nano-porous structure in the joint network for ultimate leakage prevention. This synergy is vital for safety in a battery management system.
3. Thermophysical Properties: Tuning for BMS Requirements
DSC measurements provided the core thermal storage data. The melting temperature (\(T_m\)) and latent heat (\(\Delta H_m\)) are critical for BMS design, as they define the operating temperature buffer. Results are consolidated in Table 2.
| Sample ID | PEG (wt.%) | \(T_m\) (°C) | \(\Delta H_m\) (J/g) | \(k_{\text{eff}}\) (W/m·K) | Leakage at 50°C |
|---|---|---|---|---|---|
| Pure PEG | 100 | ~33.2 | ~160.0* | ~0.26 | Severe |
| S7 | 90 | 33.19 | 129.4 | N/A (Liquid) | Severe |
| S11 | 90 | 32.74 | 127.7 | 1.28 | Minor |
| S14 | 90 | 33.09 | 124.1 | 2.70 | None |
*Theoretical value for pure PEG-1000.
The data shows that the melting point remains consistently near 33°C, ideal for lithium-ion battery temperature regulation. The latent heat scales almost linearly with PEG content. Most importantly, Sample S14 retains 124.1 J/g, which is 77.6% of the theoretical value for pure PEG—an excellent retention rate for a fully shape-stable composite. Thermal cycling tests (50 melt-freeze cycles) on S14 showed negligible changes in \(T_m\) and \(\Delta H_m\), demonstrating the durability required for a battery management system over its lifespan.
The enhancement in thermal conductivity (\(k_{\text{eff}}\)) was profound, as also shown in Table 2. The incorporation of EG created percolating paths for phonon transport. The \(k_{\text{eff}}\) increased from 1.28 W/(m·K) for S11 (3% EG) to 2.70 W/(m·K) for S14 (6% EG). This represents a >10-fold increase over pure PEG, drastically improving the heat spreading capability across the battery pack—a key function of the thermal management subsystem within the broader battery management system.
4. Integration and Performance in a Simulated Battery Management System
To evaluate practical efficacy, we constructed a module simulating a critical function of a battery management system: active thermal regulation. A pack of four 18650 Li-ion cells (2600 mAh, LiCoO₂ chemistry) connected in series was embedded within a block of Sample S14 CPCM. The pack was subjected to various dynamic stress tests representing real BMS scenarios.
Experimental Conditions: Two discharge/charge rates (1C = 2.6A, 2C = 5.2A) and two initial ambient temperatures (20°C, 30°C) were combined to create eight distinct operational modes for the BMS to handle. Cell surface temperatures were monitored using thermocouples. The performance was benchmarked against an identical pack without CPCM.
4.1 Temperature Regulation and Peak Suppression
The core thermal regulation data is summarized in Table 3. The CPCM-based BMS strategy was exceptionally effective under high-stress conditions.
| BMS Test Mode | Max. Temp. Bare Pack (°C) | Max. Temp. with CPCM (°C) | Peak Temp. Reduction (\(\Delta T\)) | Max. Cell-to-Cell ΔT (°C) |
|---|---|---|---|---|
| 1C, Start @20°C | ~30.4 | ~28.5 | ~1.9 | <2.0 |
| 2C, Start @20°C | ~47.0 | ~34.0 | ~13.0 | ~2.0 (vs. ~5.0 bare) |
| 1C, Start @30°C | ~40.4 | ~32.5 (Plateau) | ~7.9 | <2.0 |
| 2C, Start @30°C | ~51.0 | ~39.0 | ~12.0 | ~2.0 (vs. ~8.0 bare) |
The most significant result is for the 2C, 30°C start condition—a scenario simulating fast driving on a hot day. The bare pack exceeded 50°C, entering a dangerous temperature zone where degradation accelerates rapidly. The CPCM-integrated module, however, kept the maximum temperature below 40°C, a critical safety threshold. The temperature plateau during the phase change process was clearly visible, demonstrating the latent heat absorption at work. Furthermore, the CPCM excelled at homogenizing temperature, reducing the maximum cell-to-cell temperature difference from 8°C to about 2°C. Maintaining such low temperature gradients is a primary goal of any sophisticated battery management system to ensure balanced aging and performance.
Another less obvious but valuable function emerged: thermal buffering in cold environments. During the constant-voltage (CV) charge phase after high-current charging, the bare pack cooled rapidly, often falling below the initial ambient temperature. The CPCM module cooled much more slowly, providing a “thermal mass” effect that could be beneficial for maintaining optimal battery temperature in cooler climates, reducing the need for auxiliary heating—a significant energy drain managed by the BMS.
4.2 Enhancement of Discharge Efficiency
The ultimate metric for a battery management system is the efficient delivery of energy. Discharge efficiency (\(\eta_d\)) is defined as:
$$
\eta_d = \frac{I_d \cdot t_d}{C_{\text{nominal}}} \times 100\%
$$
where \(I_d\) is discharge current, \(t_d\) is discharge time to cutoff voltage, and \(C_{\text{nominal}}\) is the rated capacity. The CPCM’s role in maintaining optimal temperature directly improved this efficiency, as shown in Table 4.
| Discharge Mode | Efficiency (Bare Pack) | Efficiency (with CPCM) | Absolute Gain | Relative Improvement |
|---|---|---|---|---|
| 1C, Start @20°C | 77.78% | 90.28% | 12.50% | 16.07% |
| 2C, Start @20°C | 66.12% | 69.10% | 2.98% | 4.50% |
| 1C, Start @30°C | 85.42% | 91.67% | 6.25% | 7.32% |
| 2C, Start @30°C | 63.45% | 69.27% | 5.82% | 9.17% |
The most dramatic improvement occurred at 1C from a 20°C start, where efficiency jumped by over 16%. This highlights how the CPCM prevents excessive cooling during moderate operation, keeping the battery closer to its efficiency peak near 30°C. Even under the harshest condition (2C, 30°C), the CPCM provided a meaningful 9% relative improvement. This data conclusively shows that integrating this advanced CPCM into the thermal management strategy of a battery management system not only enhances safety but also directly improves the operational economy and range of the vehicle.
5. Conclusion and Perspective for BMS Development
We have successfully developed and characterized a novel PEG/Diatomite/EG composite phase change material engineered to address the critical thermal challenges in lithium-ion battery packs. The joint-intercrossed porous network of diatomite and EG synergistically provides unparalleled shape stability (leak-proof up to 90% PEG content) and dramatically enhanced thermal conductivity (2.70 W/m·K). This combination of high latent heat (124.1 J/g) and high thermal diffusivity is rare among passively cooled systems.
When integrated into a simulated battery management system, this CPCM demonstrated exceptional performance: It effectively suppressed peak temperatures, preventing a pack from exceeding 40°C even under a demanding 2C discharge at a 30°C ambient start—a scenario that caused an uncontrolled pack to surpass 50°C. It drastically improved temperature uniformity, reducing cell-to-cell differentials to approximately 2°C, a key factor for longevity managed by the BMS. It also provided beneficial thermal inertia, buffering against low temperatures. Most importantly, it directly increased the electrical discharge efficiency by up to 16%, translating to more usable energy from the battery pack.
This work underscores the significant potential of advanced material science in augmenting the capabilities of traditional battery management system architectures. The presented CPCM is a compelling drop-in solution for enhancing the thermal management subsystem of a BMS, offering a passive, reliable, and high-performance method to increase safety, longevity, and efficiency of lithium-ion battery packs for electric vehicles and energy storage systems. Future work will focus on scaling up the production process, optimizing the CPCM module design for specific cell formats, and testing its long-term stability under real-world driving cycle profiles within a full-scale battery management system.