The global push towards carbon peak and carbon neutrality has elevated the development of green energy and the promotion of new energy vehicles to a strategic imperative. Within this transformative landscape, the power li ion battery stands as the cornerstone of innovation. Among various battery technologies, the li ion battery is favored for its high energy density, long cycle life, and lack of memory effect. However, the operational temperature of a li ion battery is a critical factor influencing its performance, safety, and longevity. Excessive heat generation during high-rate charge/discharge cycles, if not efficiently dissipated, can lead to accelerated degradation, thermal runaway, and catastrophic failure. Therefore, maintaining the temperature of a li ion battery within a safe operational window (typically 20°C to 50°C) through an effective Battery Thermal Management System (BTMS) is paramount.
Traditional cooling methods, such as air or liquid cooling, often add complexity, weight, and parasitic power consumption. In contrast, Passive Thermal Management Systems (PTMS) utilizing Phase Change Materials (PCMs) offer a compelling alternative. PCMs absorb and release significant amounts of latent heat during their phase transition (e.g., solid-liquid) at a nearly constant temperature. This property makes them ideal for stabilizing the temperature of a li ion battery. Despite their high latent heat, organic PCMs like paraffin (PA) and fatty acids (e.g., lauric acid, LA) suffer from inherent low thermal conductivity, which limits heat transfer rates. To overcome this, high-thermal-conductivity matrices like Expanded Graphite (EG) are incorporated to form Composite Phase Change Materials (CPCMs). This study focuses on the development and evaluation of a PA/LA/EG CPCM for the thermal management of a large-format pouch li ion battery, investigating its cooling performance under various ambient temperatures and discharge rates through combined experimental and numerical approaches.
1. Composite Phase Change Material Development and Characterization
The core of this passive thermal management strategy is a high-performance CPCM. The materials selected were Paraffin (PA, melting point 48-50°C), Lauric Acid (LA, melting point 44°C), and Expanded Graphite (EG, expansion volume 250 mL/g). The preparation process involved several steps to ensure homogeneity and optimal properties. Initially, PA and LA were mixed in specific mass ratios and melted in an oil bath at 60°C. After complete melting and magnetic stirring for one hour, varying mass fractions of EG were added to the homogeneous mixture and stirred for another hour. The resulting composite was then cooled, solidified, and pressed into sheets for experimental use.
The first stage of optimization aimed to find the optimal eutectic ratio of PA and LA. Three binary mixtures were prepared, as summarized in Table 1. Their thermal properties were characterized using Differential Scanning Calorimetry (DSC).
| Sample | PA Mass (g) | LA Mass (g) | EG Mass (g) | Phase Change Temp. (°C) | Latent Heat (J/g) |
|---|---|---|---|---|---|
| A1 | 4.00 | 6.00 | 0.00 | 37.6 | 169.8 |
| A2 | 5.00 | 5.00 | 0.00 | 36.6 | 223.1 |
| A3 | 6.00 | 4.00 | 0.00 | 36.5 | 167.4 |
The results indicated that the 1:1 mass ratio (Sample A2) yielded the highest latent heat of 223.1 J/g with a suitable phase change temperature of 36.6°C. This formulation was therefore selected as the base PCM for EG enhancement.
The second stage involved incorporating EG to dramatically improve the thermal conductivity. Four CPCMs with different EG loadings were prepared, as shown in Table 2. Thermal conductivity was measured, and DSC analysis was performed.
| Sample | PA:LA (1:1) Mass (g) | EG Mass (g) | EG wt.% | Phase Change Temp. (°C) | Latent Heat (J/g) | Thermal Conductivity (W/m·K) |
|---|---|---|---|---|---|---|
| B1 | 10.00 | 0.53 | 5% | 36.8 | 178.7 | 0.464 |
| B2 | 10.00 | 1.11 | 10% | 36.0 | 163.2 | 1.062 |
| B3 | 10.00 | 1.76 | 15% | 36.1 | 146.9 | 1.220 |
| B4 | 10.00 | 2.50 | 20% | 36.4 | 125.8 | 1.413 |
A clear trade-off is observed: increasing EG content enhances thermal conductivity but reduces the latent heat capacity due to the non-phase-changing nature of graphite. Sample B3 (15% EG) was chosen as the optimal compromise, offering a high thermal conductivity of 1.22 W/m·K (a >300% increase over the base PCM) while retaining a substantial latent heat of 146.9 J/g at a phase change temperature of 36.1°C. The DSC curve for this selected CPCM (B3) clearly shows distinct melting and solidification peaks.
2. Experimental Investigation of Battery Thermal Management
2.1 Experimental Setup and Procedure
The test subject was a large-format pouch-type li ion battery with a nominal capacity of 100,000 mAh (100 Ah). The experimental setup comprised the battery, the prepared CPCM modules, a battery cycler, a data acquisition system with T-type thermocouples, and an environmental chamber. The CPCM sheets were attached to both large surfaces of the pouch li ion battery. Five temperature measurement points were strategically placed on the battery surface: one at the center and two pairs near the positive and negative tabs to capture potential temperature gradients.
The testing protocol involved placing the battery (with or without CPCM) inside the environmental chamber until it reached a stable temperature at set points of 20°C, 25°C, 30°C, and 35°C. The li ion battery was first charged using a constant-current-constant-voltage (CC-CV) protocol. Discharge was performed at constant current rates of 0.5 C (50 A), 0.8 C (80 A), 0.9 C (90 A), and 1.0 C (100 A) until the cutoff voltage of 3.0 V was reached. Temperature data was recorded throughout the cycles.
2.2 Heat Generation Estimation and CPCM Mass Calculation
An initial estimation of the required CPCM mass can be derived from a simple energy balance. The total heat (Q1) generated by the li ion battery during a complete discharge must be absorbed by the CPCM (Q2). The energy balance is given by:
$$Q_2 = mc\Delta T + mH$$
$$Q_2 = Q_1$$
$$\Delta T = T_1 – T_2$$
Where m is the mass of CPCM, c is its specific heat capacity, H is its latent heat, T1 is its phase change temperature, and T2 is the ambient temperature. The battery’s heat generation can be approximated using its internal resistance. For the 1.0 C rate at 35°C ambient, with an internal resistance R of 38.15 mΩ and discharge time t, the heat is:
$$Q_1 = I^2 R t$$
Using these equations provides a first-order estimate. For the experiments, a CPCM mass of 92.17 g was used, which was sufficient to handle the heat loads across the tested conditions.
2.3 Results and Discussion: Thermal Performance
Baseline Performance (Natural Convection): The temperature rise of the li ion battery under natural convection at different ambient temperatures and discharge rates reveals critical insights. At a low rate of 0.5 C, the maximum temperatures reached 35.02°C, 37.93°C, 43.73°C, and 50.76°C for ambient temperatures of 20°C to 35°C, respectively. While the 50.76°C at 35°C ambient marginally exceeds the safe limit, the general trend shows that low-rate operation may not require active cooling. However, as the discharge rate increases, the situation changes dramatically. The temperature rise curves become steeper, almost linear at high rates. At 1.0 C, the maximum temperatures soared to 60.77°C, 61.66°C, 68.21°C, and 70.20°C for the same ambient range, far exceeding the safe operational limit and highlighting the imperative need for an effective BTMS for high-power li ion battery applications.
Performance with CPCM Cooling: The integration of the PA/LA/EG CPCM dramatically altered the thermal behavior of the li ion battery. The temperature profiles consistently exhibited a characteristic three-stage trend: a rapid initial rise, a temperature plateau, and a final rise. This plateau corresponds directly to the phase change interval of the CPCM (around 36.1°C), where the latent heat absorption effectively buffers the temperature increase. For instance, at a challenging condition of 30°C ambient and 1.0 C discharge, the CPCM reduced the peak temperature from 68.21°C (natural convection) to 47.21°C, a remarkable reduction of 21.00°C. The cooling effectiveness was quantified across all test matrix, as summarized in Table 3.
| Discharge Rate | Ambient 20°C | Ambient 25°C | Ambient 30°C | Ambient 35°C |
|---|---|---|---|---|
| 0.5 C | 0.82°C | -0.01°C | 7.06°C | 10.39°C |
| 0.8 C | 10.74°C | 14.36°C | 17.76°C | 19.31°C |
| 0.9 C | 14.31°C | 19.06°C | 19.59°C | 20.92°C |
| 1.0 C | 20.49°C | 19.51°C | 21.00°C | 20.24°C |
The data shows that the CPCM’s impact is most pronounced at higher discharge rates (0.8 C to 1.0 C), where temperature reductions of 14°C to 21°C are consistently achieved. At 0.5 C, significant cooling is only needed when the ambient temperature is already high (30°C, 35°C). Furthermore, the duration of the temperature plateau is inversely proportional to the discharge rate; higher rates deliver heat faster, melting the CPCM more quickly and shortening the plateau period. This underscores the importance of properly sizing the CPCM mass based on the worst-case heat generation scenario of the li ion battery.
3. Numerical Modeling and Simulation
To complement the experimental study and provide a tool for further design optimization, a three-dimensional transient thermal model of the li ion battery with CPCM was developed using ANSYS Fluent.
3.1 Model Assumptions and Governing Equations
Key assumptions were made to render the complex problem tractable: 1) Uniform heat generation within the li ion battery; 2) Homogeneous material properties; 3) Negligible internal convection and radiation; 4) Constant thermophysical properties; 5) Negligible volume change of the CPCM during phase change; 6) No liquid PCM flow (mushy zone model). The governing energy equation for the system is:
$$\rho c \frac{\partial T}{\partial t} = \lambda \left( \frac{\partial^2 T}{\partial x^2} + \frac{\partial^2 T}{\partial y^2} + \frac{\partial^2 T}{\partial z^2} \right) + q$$
Where \( \rho \), \( c \), and \( \lambda \) are density, specific heat, and thermal conductivity, respectively, and \( q \) is the volumetric heat generation rate. The heat generation in the li ion battery is calculated using a simplified Bernardi model that accounts for irreversible (Joule) and reversible (entropic) heating:
$$q = \frac{I}{V}\left(IR – T\frac{dU_0}{dT}\right)$$
Where \( I \) is current, \( V \) is battery volume, \( R \) is internal resistance, \( T \) is temperature, and \( dU_0/dT \) is the entropy coefficient.
3.2 Simulation Setup and Material Properties
The computational domain included the detailed geometry of the pouch li ion battery and the surrounding CPCM. The Solidification & Melting model was employed to simulate the phase change process. The critical thermophysical properties used in the simulation are listed in Table 4 and Table 5.
| Property | Value |
|---|---|
| Density | 2698.41 kg/m³ |
| Specific Heat | 1294.24 J/(kg·K) |
| In-plane Thermal Conductivity | 23.72 W/(m·K) |
| Through-plane Thermal Conductivity | 0.81 W/(m·K) |
| Property | Value |
|---|---|
| Density | 952.69 kg/m³ |
| Specific Heat | 1917.00 J/(kg·K) |
| Thermal Conductivity | 1.22 W/(m·K) |
| Latent Heat | 146.90 kJ/kg |
| Phase Change Temperature | 36.1°C |
3.3 Model Validation and Results
The simulation results were validated against experimental data. Figure X (Simulation vs. Experiment at 30°C) shows excellent agreement between the simulated and measured temperature profiles for various discharge rates. The characteristic three-stage trend is accurately captured by the model. The maximum error between simulation and experiment across all tested conditions was within 3.5°C, with the largest discrepancy occurring at the most severe condition (35°C ambient, 1.0 C rate). This small error validates the assumptions and the numerical model, confirming its utility for predicting the thermal behavior of a CPCM-managed li ion battery under diverse operating conditions.
The validated model was then used to simulate scenarios across the full parameter space. The simulated temperature contours clearly show how the CPCM layer maintains a more uniform and lower temperature distribution across the surface of the li ion battery compared to the case without cooling, effectively suppressing hot spots.
4. Conclusion
This comprehensive study demonstrates the significant potential of a tailored Composite Phase Change Material for the passive thermal management of high-capacity pouch li ion battery systems. The key findings are summarized as follows:
1. A high-performance CPCM was successfully formulated by creating a eutectic mixture of Paraffin and Lauric Acid (1:1 mass ratio) and enhancing it with 15 wt.% Expanded Graphite. This composite achieved an optimal balance, offering a latent heat of 146.9 J/g and a thermal conductivity of 1.22 W/m·K, which is crucial for efficient heat absorption and distribution.
2. Experimental results unequivocally proved the CPCM’s effectiveness. Under high discharge rates (0.8 C to 1.0 C) where a li ion battery is most vulnerable to overheating, the CPCM reduced the peak temperature by 14°C to 21°C, maintaining it within the safe operational limit. The thermal response exhibited a distinct “rapid rise – plateau – final rise” profile, with the plateau phase corresponding to the latent heat absorption.
3. The cooling performance is highly dependent on operational conditions. While low-rate (0.5 C) discharge may not require intervention except in high ambient temperatures, the CPCM is indispensable for high-power operation. The duration of the temperature plateau is inversely related to the discharge rate, dictating the required CPCM mass for a desired protection time.
4. A robust 3D transient thermal model was developed and validated against experimental data with a maximum error of less than 3.5°C. This model accurately captures the phase change dynamics and can serve as a powerful tool for optimizing the design of CPCM-based BTMS for specific li ion battery modules, including scaling up to multi-cell packs.
In conclusion, the PA/LA/EG CPCM presents a simple, reliable, and effective passive cooling solution that significantly enhances the thermal safety and performance consistency of li ion battery systems operating under strenuous conditions, contributing to the advancement of safer and more reliable energy storage for electric vehicles and other high-power applications.