The relentless pursuit of higher energy density and faster charging capabilities in modern Li-ion battery packs is intrinsically linked to the critical challenge of thermal management. The electrochemical performance, cycle life, and, most importantly, safety of a li ion battery are profoundly sensitive to its operating temperature. Excessive heat accumulation, particularly during high-rate discharge or fast-charge scenarios, can accelerate degradation mechanisms, induce thermal runaway, and create hazardous conditions. Conversely, operation at excessively low temperatures severely limits power capability. Therefore, maintaining the li ion battery pack within an optimal temperature range, typically 25–50 °C, and ensuring minimal temperature variation across cells, ideally below 5 °C, is paramount for reliability and longevity. This work explores the design, synthesis, and application of a novel hydrated salt-based phase change material (PCM) composite as a passive, efficient, and reliable solution for li ion battery thermal management.
Traditional active cooling methods, such as forced air or liquid cooling, add system complexity, consume additional power, and may struggle with spatial temperature uniformity. Passive thermal management using PCMs offers a compelling alternative. PCMs absorb and release large amounts of latent heat during phase transition (e.g., solid to liquid), effectively functioning as a thermal buffer. When integrated with a li ion battery, the PCM absorbs the generated heat, mitigating temperature rise during operation. While organic paraffin waxes have been widely studied, inorganic hydrated salt PCMs present distinct advantages: higher volumetric latent heat, superior thermal conductivity, non-flammability, and lower cost. However, their practical application is hindered by inherent issues like supercooling, phase segregation, and leakage in the molten state. This study addresses these challenges by formulating a eutectic hydrated salt mixture stabilized within a conductive matrix of expanded graphite (EG), creating a form-stable composite PCM (CPCM) tailored for li ion battery modules.
1. Material Design and Synthesis
The core PCM matrix is a ternary eutectic mixture designed to achieve a suitable phase change temperature aligned with li ion battery operational limits. Sodium acetate trihydrate (SAT) serves as the primary phase change constituent due to its high latent heat. Glycine is incorporated as a thickening agent and phase segregation inhibitor, effectively preventing the separation of salt and water during repeated melting-freezing cycles. Disodium hydrogen phosphate dodecahydrate (DSP) acts as a nucleating agent to mitigate supercooling. The optimal mass ratio for this eutectic hydrated salt (EHS) was determined as 85.15% SAT, 13.86% glycine, and 0.99% DSP. The composite is engineered by integrating Expanded Graphite (EG), a highly conductive, porous, and lightweight carbon material. EG serves a dual purpose: its porous network acts as a micro-container for the molten salt via capillary forces, preventing leakage and providing shape stability; simultaneously, it drastically enhances the thermal conductivity of the composite. The preparation involves melting the EHS components in a water bath at 78°C, followed by mechanical mixing with varying mass fractions of EG (5.0%, 10.0%, 15.0%). The mixture is then cooled in a mold to obtain the shape-stable PCM block. For enhanced encapsulation and mechanical robustness, a layer of thermally conductive potting compound is applied to the surface of select samples, resulting in the final CPCMs designated as CPCM-5, CPCM-10, and CPCM-15. Their compositions are detailed in Table 1.
| Sample Designation | Mass Fraction EHS (%) | Mass Fraction EG (%) | Mass Fraction Potting Compound (%) |
|---|---|---|---|
| PCM-5 | 95.0 | 5.0 | 0 |
| PCM-10 | 90.0 | 10.0 | 0 |
| PCM-15 | 85.0 | 15.0 | 0 |
| CPCM-5 | 47.5 | 2.5 | 50.0 |
| CPCM-10 | 45.0 | 5.0 | 50.0 |
| CPCM-15 | 42.5 | 7.5 | 50.0 |
2. Thermophysical Characterization of the Composite PCM
The efficacy of a PCM for li ion battery thermal management is governed by its key thermophysical properties: phase change temperature (Tpc), latent heat of fusion (ΔH), and thermal conductivity (k). Differential Scanning Calorimetry (DSC) was employed to measure Tpc and ΔH. The results, plotted in Figure 2 and summarized in Table 2, reveal that the pure EHS has a latent heat of 276.93 J/g and a phase change temperature of 48.14°C. Upon incorporation of EG, the phase change temperature shows a slight decrease to around 45.3°C, which is actually more favorable for li ion battery management as it aligns with the mid-range of the optimal window. The latent heat decreases proportionally with increasing EG content, as EG itself does not contribute to latent heat storage. The measured enthalpies for PCM-5, PCM-10, and PCM-15 are 201.26 J/g, 196.17 J/g, and 170.65 J/g, respectively. The theoretical enthalpy can be estimated assuming EG contributes no latent heat:
$$ \Delta H_{theoretical} = w_{EHS} \times \Delta H_{pure\ EHS} $$
where \( w_{EHS} \) is the mass fraction of the eutectic hydrated salt. The close agreement between measured and theoretical values for PCM-5 and PCM-10 indicates effective impregnation of the salt into the EG matrix without significant chemical interaction that would degrade energy storage capacity. The thermal conductivity, a critical parameter for rapid heat spreading within the li ion battery pack, was dramatically enhanced. Measurements showed the thermal conductivity of the pure EHS was approximately 0.51 W/(m·K). With the addition of EG, the CPCM samples exhibited significantly higher values, as shown in Table 2. The thermal conductivity follows a positive correlation with EG loading, reaching 1.60 W/(m·K) for CPCM-10. This represents over a threefold increase, which is crucial for ensuring the absorbed heat is quickly distributed throughout the PCM volume, preventing localized hot spots on the li ion battery surface.
| Sample | Phase Change Temperature, Tpc (°C) | Latent Heat of Fusion, ΔH (J/g) | Theoretical ΔH (J/g) | Thermal Conductivity, k (W/(m·K)) |
|---|---|---|---|---|
| Pure EHS | 48.14 | 276.93 | – | ~0.51 |
| PCM-5 | 45.45 | 201.26 | 263.08 | – |
| PCM-10 | 45.31 | 196.17 | 249.24 | – |
| PCM-15 | 45.19 | 170.65 | 235.39 | – |
| CPCM-5 | – | – | – | 1.21 |
| CPCM-10 | – | – | – | 1.60 |
| CPCM-15 | – | – | – | 1.84 |
3. Leakage Resistance and Form Stability
For reliable integration into a li ion battery pack over thousands of cycles, the PCM must maintain its structural integrity and contain the molten salt without leakage. A leakage test was conducted by placing the CPCM samples in a 60°C environment (above Tpc) for 3 hours. Visual inspection and mass loss measurement were performed. CPCM-5 showed minor surface efflorescence (dry salt crystals), indicating that the 5.0% EG content was insufficient to fully encapsulate all the salt. In contrast, CPCM-10 and CPCM-15 exhibited excellent form stability with no visible leakage or salt separation. The mass loss for all three sealed CPCMs was minimal, below 0.7%, confirming the effectiveness of the EG matrix combined with the surface potting compound in preventing leakage. This property is essential for ensuring long-term reliability and safety when the material is in direct contact with the li ion battery casing and electrical components.
4. Thermal Management Performance for Li-ion Batteries
The thermal regulation capability of the optimized composite (CPCM-10) was evaluated experimentally on both a single commercial 5 Ah LiNi0.5Mn0.3Co0.2O2/Graphite pouch li ion battery and a three-cell module. The CPCM was configured in a “sandwich” structure, with slabs placed on both large faces of the battery. A forced air cooling setup with equivalent spacing served as the baseline for comparison. Tests were conducted at room temperature (25°C) under constant-current charge/discharge cycles (0.5C charge, variable discharge rates). Surface temperatures at strategic points were monitored using K-type thermocouples.
4.1 Low to Moderate Discharge Rates (0.5C and 1.0C)
At a 0.5C discharge rate, the heat generation from the li ion battery is relatively low. The surface temperature of the single cell reached only about 32.5°C, which is below the phase change temperature of the CPCM. Consequently, the PCM operated primarily in sensible heat mode. The cooling performance of CPCM was similar to forced air cooling, with both methods maintaining the cell within a safe range. However, in a three-cell module, even at this low rate, a temperature gradient develops due to the limited heat dissipation from the central cell. The CPCM module demonstrated superior temperature uniformity, reducing the maximum temperature difference (ΔTmax) across the pack to 1.55°C, which was 33% lower than the ΔTmax observed with air cooling.
At a 1.0C discharge rate, the thermal load increases. For the single li ion battery, CPCM cooling reduced the peak temperature by approximately 3.4°C compared to air cooling. The more significant advantage was observed in the module test. With air cooling, the central cell’s temperature rose to 49.6°C, and the temperature spread within the pack reached 5.44°C, exceeding the recommended 5°C limit. In contrast, the CPCM pack effectively absorbed and distributed the heat. The central cell temperature was capped at 45.62°C, and the overall pack ΔTmax was controlled at 2.60°C, well within the safe limit and representing a 52% improvement over air cooling. This demonstrates the PCM’s ability to homogenize temperature within a li ion battery pack even under moderate stress.
4.2 High Discharge Rate (2.0C) Performance
The high-rate discharge test at 2.0C represents a stringent scenario where effective thermal management is critical for li ion battery safety and performance. The results, summarized in Table 3, highlight the exceptional capability of the CPCM system. For the single cell, CPCM cooling lowered the peak temperature from 55.5°C (air cooling) to 52.24°C. More impressively, it drastically reduced the surface temperature gradient on the cell itself (ΔTcell) from 2.02°C to just 0.21°C, an 89.6% reduction. This extreme uniformity prevents localized overheating on the li ion battery surface.
The performance for the three-cell module is even more compelling. Under air cooling, the system failed to manage the intense heat generation. The central cell temperature skyrocketed to 70.49°C, a dangerous level that accelerates degradation and poses safety risks. The temperature spread within that central cell was 8.83°C, and the overall temperature difference between the hottest and coldest points in the entire module reached 10.69°C. This level of non-uniformity leads to significant state-of-charge (SOC) imbalances between cells, reducing overall pack capacity and lifespan.
The CPCM-based thermal management system completely transformed this outcome. The peak temperature of the entire module was suppressed to 54.81°C, keeping the li ion battery pack firmly within the safe operational zone (<55°C). The maximum temperature differential across the module was only 2.41°C, a 77.5% reduction compared to air cooling. This exceptional control over both absolute temperature and temperature uniformity is attributed to the high latent heat absorption at the designed phase change point and the enhanced thermal conductivity provided by the EG network, which facilitates rapid lateral heat spreading from the hotter central cells to the cooler edge cells and into the PCM bulk.
| Configuration & Metric | Air Cooling | CPCM Cooling | Improvement |
|---|---|---|---|
| Single Cell | |||
| Maximum Temperature, Tmax (°C) | 55.50 | 52.24 | -3.26 °C |
| Cell Surface ΔT (°C) | 2.02 | 0.21 | -89.6% |
| Three-Cell Module | |||
| Maximum Pack Temperature, Tmax, pack (°C) | 70.49 (Cell 2) | 54.81 (Cell 2) | -22.2% |
| Max ΔT within Central Cell (°C) | 8.83 | 2.41 | -72.7% |
| Maximum Inter-Cell ΔT in Module (°C) | 10.69 | 2.41 | -77.5% |
The heat absorption process of the CPCM can be modeled conceptually. During the phase change period, the temperature of the li ion battery-PCM interface remains nearly constant at Tpc. The heat flux from the li ion battery (q”) is balanced by the latent heat absorption rate within the PCM:
$$ q” \cdot A_{interface} \approx \dot{m}_{PCM} \cdot \Delta H $$
where \( A_{interface} \) is the contact area between the li ion battery and PCM, and \( \dot{m}_{PCM} \) is the mass rate of PCM undergoing phase change. The high thermal conductivity of the composite ensures that heat penetrates deeper into the PCM slab, mobilizing more material for phase change rather than creating a thin melted layer at the interface.
5. Discussion and Implications
The developed hydrated salt/EG composite successfully overcomes the traditional drawbacks of hydrated salts while leveraging their advantages for li ion battery thermal management. The formulation effectively addresses supercooling and phase separation. The integration of EG solves the leakage problem and, most importantly, creates a thermally conductive network that is crucial for managing high heat fluxes from li ion batteries. The resulting CPCM-10 composite presents an optimal balance: a phase change temperature of ~45.3°C strategically positioned within the li ion battery’s optimal window, a substantial latent heat of ~196 J/g, and a thermal conductivity of 1.60 W/(m·K).
The experimental results unequivocally demonstrate its superiority over conventional forced air cooling, especially under high-stress conditions. The CPCM system excels not only in peak temperature suppression but, more significantly, in achieving exceptional temperature uniformity. This uniformity is critical for the health of a serially connected li ion battery pack, as it minimizes cell-to-cell variations in impedance, degradation rate, and capacity fade. The passive nature of the system eliminates the need for pumps, fans, or complex liquid plumbing, reducing parasitic power consumption, noise, and potential failure points. This makes it particularly attractive for applications where simplicity, reliability, and weight are concerns.
Future work could focus on further optimizing the composite for specific li ion battery chemistries with different heat generation profiles, scaling up the system for large battery packs, and investigating hybrid systems that combine CPCM with minimal active cooling for extreme scenarios. Long-term cycling tests to assess the stability of the composite over hundreds of phase change cycles in direct contact with li ion battery cells would also be valuable. Furthermore, modeling the thermal behavior using computational fluid dynamics (CFD) coupled with phase change physics could provide deeper insights for engineering design. The governing energy equation for the CPCM region during melting incorporates conduction and latent heat:
$$ \rho_{CPCM} c_{p, CPCM} \frac{\partial T}{\partial t} = \nabla \cdot (k_{CPCM} \nabla T) – \rho_{CPCM} \Delta H \frac{\partial f}{\partial t} $$
where \( \rho_{CPCM} \) is density, \( c_{p, CPCM} \) is specific heat, \( k_{CPCM} \) is thermal conductivity, and \( f \) is the liquid fraction. Solving this equation coupled with the li ion battery heat generation model can predict system performance under various operational profiles.
6. Conclusion
This work presents the development and validation of a high-performance form-stable composite phase change material based on a eutectic hydrated salt and expanded graphite for advanced thermal management of li ion batteries. The composite exhibits excellent thermophysical properties, including a suitable phase change temperature, high latent heat, significantly enhanced thermal conductivity, and reliable leakage resistance. In rigorous testing under high discharge rates (2.0C), the CPCM-based passive thermal management system demonstrated remarkable effectiveness. It successfully contained the maximum temperature of a li ion battery module below the critical 55°C threshold and reduced the maximum temperature difference within the pack to only 2.41°C, outperforming forced air cooling by a margin of over 77% in uniformity. By ensuring both safe operating temperatures and exceptional thermal homogeneity, this engineered hydrated salt CPCM offers a robust, efficient, and simple solution to a key challenge in advancing li ion battery technology, potentially enhancing the safety, performance, and cycle life of energy storage systems across automotive, aerospace, and stationary storage applications.