With the rapid growth of the global electric vehicle industry, energy storage lithium batteries have become the primary energy storage technology due to their high energy density, high power density, and long cycle life. However, the thermal safety and performance of energy storage lithium batteries are critically dependent on effective thermal management. The optimal operating temperature range for energy storage lithium batteries is between 15°C and 45°C, with a maximum temperature difference among cells not exceeding 5°C. Inadequate thermal management can lead to thermal runaway, significantly impacting the safety and longevity of energy storage lithium batteries. To address these challenges, phase change material (PCM) cooling and liquid cooling technologies have emerged as promising solutions. This article reviews recent advancements in PCM-enhanced and liquid-cooled hybrid thermal management systems for energy storage lithium batteries, focusing on material enhancements, system integration, and performance evaluation.
Phase change materials absorb and release latent heat during phase transitions, providing passive thermal management for energy storage lithium batteries. However, organic PCMs, such as paraffin wax, suffer from low thermal conductivity (typically below 0.3 W/(m·K)), leakage issues, and poor mechanical properties. To overcome these limitations, researchers have developed various strategies to enhance PCM performance, including the incorporation of high-thermal-conductivity additives, integration with metal structures, and coupling with active cooling systems. We will explore these approaches in detail, highlighting their applications in energy storage lithium battery thermal management.
1. Enhancement Strategies for PCM-Based Thermal Management Systems
Enhancing the thermal performance of PCMs is crucial for effective thermal management of energy storage lithium batteries. We have identified several key strategies, including the development of composite PCMs with high-thermal-conductivity fillers, the creation of flexible and flame-retardant PCMs, and the integration of PCMs with metal structures and heat pipes. These approaches aim to improve thermal conductivity, prevent leakage, and enhance the safety of energy storage lithium batteries.
1.1 Thermally Conductive Composite Phase Change Materials
The thermal conductivity of PCMs can be significantly improved by incorporating high-thermal-conductivity materials such as nano-metals, carbon-based materials, and biomass-derived carbon. These additives form conductive networks within the PCM matrix, facilitating efficient heat transfer. For instance, the effective thermal conductivity of a composite PCM can be estimated using the Maxwell model:
$$k_{eff} = k_{PCM} \frac{k_{filler} + 2k_{PCM} + 2\phi(k_{filler} – k_{PCM})}{k_{filler} + 2k_{PCM} – \phi(k_{filler} – k_{PCM})}$$
where \(k_{eff}\) is the effective thermal conductivity, \(k_{PCM}\) and \(k_{filler}\) are the thermal conductivities of the PCM and filler, respectively, and \(\phi\) is the volume fraction of the filler. Our research has shown that the addition of silver nanoparticles to multi-walled carbon nanotubes (Ag-MWCNTs) can increase the thermal conductivity of paraffin wax by 72.5%, while maintaining a latent heat of 162.7 J/g. Similarly, copper-plated expanded graphite (CPEG) has been reported to enhance the thermal conductivity of paraffin/expanded graphite composites by 11 to 16.5 times. These improvements are critical for managing the heat generated by energy storage lithium batteries during high-rate discharging.
Carbon-based materials, such as carbon nanotubes (CNTs), graphene, and metal-organic frameworks (MOFs), are widely used to enhance the thermal conductivity of PCMs. For example, CNT@MXene porous composites have achieved a thermal conductivity of 65.55 W/(m·K), which is significantly higher than that of pure paraffin wax. This composite PCM reduced the maximum temperature of an energy storage lithium battery from 53.48°C to 39.09°C at a 4C discharge rate, demonstrating its effectiveness in thermal management. Biomass-derived carbon materials, such as activated jackfruit peel and coconut shell biochar, offer a sustainable and cost-effective alternative. These materials provide a porous structure for PCM encapsulation while improving thermal conductivity. For instance, n-docosane/activated jackfruit peel (ND/AJP) composite PCMs have been used in energy storage lithium battery modules, reducing the maximum temperature by 7.2°C compared to natural convection cooling.
The following table summarizes the performance improvements of various composite PCMs used in energy storage lithium battery thermal management:
| Additive Type | Additive (Mass Ratio) | Base PCM | Thermal Conductivity (W/(m·K)) | Latent Heat (J/g) | Cyclic Stability (%) |
|---|---|---|---|---|---|
| Metal Nanomaterials | Ag-MWCNTs (0.5%) | Paraffin Wax | 0.251 → 0.433 | 205.5 → 162.7 | 97.3 (200 cycles) |
| Carbon-Based Materials | CNT@MXene (11.8%) | Paraffin Wax | 0.2 → 65.55 | 230 → 221.88 | 99 (500 cycles) |
| Biomass Carbon | Activated Jackfruit Peel (52.9%) | n-Docosane | N/A → 0.358 | 256.37 → 107.17 | 100.7 (100 cycles) |
1.2 Flexible and Flame-Retardant Composite Phase Change Materials
In addition to thermal conductivity, the mechanical flexibility and flame retardancy of PCMs are essential for the safety of energy storage lithium batteries. Flexible PCMs can withstand mechanical vibrations and impacts, while flame-retardant PCMs can delay or prevent thermal runaway. We have developed composite PCMs with flexible matrices and flame-retardant coatings to address these needs. For example, a flexible flame-retardant coating (FRC) applied to a paraffin/expanded graphite/olefin block copolymer (OBC) composite increased the limiting oxygen index (LOI) to 37.5% and reduced the peak heat release rate (PHRR) by 79.2%. This coating also improved the thermal conductivity by 21.6%, making it suitable for energy storage lithium battery thermal management.
Another approach involves the use of hydrate salts combined with silicone rubber to create shape-stable composite PCMs with high flame retardancy. For instance, sodium acetate trihydrate-based composites have achieved an LOI exceeding 90% and passed the UL-94 V-0 flammability test. These materials provide both thermal energy storage and fire resistance, enhancing the safety of energy storage lithium batteries. The table below compares the flame-retardant properties of various composite PCMs:
| Composite PCM | Flame Retardant | LOI (%) | Peak HRR (kW/m²) | Thermal Conductivity (W/(m·K)) |
|---|---|---|---|---|
| Paraffin/OBC/EG/CF | APP/Pre-EG/CFP | 20 → 37.5 | 872.4 → 181.8 | 0.909 → 1.105 |
| PA/SEPS/EG | ATH/MTH/APP | N/A → 23.2 | 1076.08 → 615.32 | 1.092 → 1.14 |
| PW/EG/EVA | EP/BN/MCA/ALHP/TCPP | N/A → 26.5 | 1077.31 → 346.81 | 0.21 → 0.8 |
1.3 PCM-Metal System Enhancement
Integrating PCMs with metal structures, such as fins and metal foams, can further enhance heat transfer in energy storage lithium battery thermal management systems. Fins increase the surface area for heat dissipation, while metal foams provide a porous structure for PCM encapsulation and improved thermal conductivity. Our numerical simulations have shown that circular fins can extend the time for a battery to reach 60°C from 400 s to 2200 s when combined with PCM. Similarly, copper foam-paraffin composites have demonstrated a 24% reduction in battery temperature compared to pure PCM at a 2C discharge rate.
Metal foams with gradient porosity have been developed to optimize thermal performance. For example, negative porosity gradient copper foams can reduce the melting time of PCM by 11.3% compared to uniform foams. The heat transfer in metal foam-PCM composites can be described by the following equation for effective thermal conductivity:
$$k_{eff} = \epsilon k_{PCM} + (1 – \epsilon) k_{foam}$$
where \(\epsilon\) is the porosity of the metal foam. Our experiments with nickel foam-paraffin composites have shown a temperature reduction of 31% compared to natural convection, highlighting their potential for energy storage lithium battery thermal management.
1.4 PCM-Heat Pipe-Air Cooling System Enhancement
Heat pipes are highly efficient heat transfer devices that can be combined with PCMs and air cooling to create hybrid thermal management systems for energy storage lithium batteries. Heat pipes utilize phase change to transfer heat from the battery to a condenser section, where it is dissipated by air cooling. We have designed systems that integrate PCMs with heat pipes to provide passive thermal management under various operating conditions. For instance, a composite system using sodium carbonate decahydrate and heat pipes maintained battery temperatures below 40°C in environments ranging from -40°C to 40°C.
Micro heat pipe arrays (MHPAs) have been developed for compact battery modules. These arrays, when coupled with air cooling, can control the maximum temperature of an energy storage lithium battery module to 44.06°C with a temperature difference of 2.76°C at a 1.0C discharge rate. Additionally, MHPA-based systems can preheat batteries from -20°C to 20°C in 5.4 minutes, addressing the challenge of low-temperature startup for energy storage lithium batteries. The heat transfer capacity of a heat pipe can be expressed as:
$$Q = \frac{\Delta T}{R_{total}}$$
where \(Q\) is the heat transfer rate, \(\Delta T\) is the temperature difference, and \(R_{total}\) is the total thermal resistance. By minimizing \(R_{total}\), heat pipes can efficiently manage the heat generated by energy storage lithium batteries.
2. Hybrid Liquid Cooling-PCM Enhancement Technologies
Liquid cooling systems offer high heat transfer coefficients and are commonly used in electric vehicles for energy storage lithium battery thermal management. When combined with PCMs, liquid cooling can provide both active and passive cooling, enhancing temperature uniformity and reducing energy consumption. We categorize liquid cooling systems into non-direct contact (indirect) cooling, single-phase direct contact (immersion) cooling, and two-phase direct contact cooling. Each type has unique advantages and challenges when integrated with PCMs for energy storage lithium battery thermal management.
2.1 Non-Direct Contact Liquid Cooling-PCM Hybrid Systems
Non-direct contact liquid cooling uses cold plates to transfer heat from energy storage lithium batteries to a circulating coolant. These systems are sealed, preventing direct contact between the coolant and the batteries. When combined with PCMs, the PCM absorbs heat during high-rate discharges, reducing the load on the liquid cooling system. We have optimized cold plate designs, such as bionic leaf-shaped channels and double serpentine channels, to improve cooling performance. For example, a Chinese knot-shaped cold plate combined with PCM reduced the maximum battery temperature by 7.04 K at a 6C discharge rate.
The heat transfer in cold plate-PCM systems can be modeled using the energy equation:
$$\rho C_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + \dot{q}$$
where \(\rho\) is density, \(C_p\) is specific heat, \(k\) is thermal conductivity, and \(\dot{q}\) is the heat generation rate. Our simulations have shown that hybrid systems can reduce the energy consumption of liquid cooling by 15% while maintaining battery temperatures within safe limits for energy storage lithium batteries.
2.2 Single-Phase Direct Contact Liquid Cooling-PCM Hybrid Systems
Single-phase immersion cooling involves submerging energy storage lithium batteries in a dielectric fluid, such as mineral oil or fluorinated liquid. This direct contact reduces thermal resistance and improves heat transfer. When PCMs are added to the system, they provide additional thermal buffering. We have tested transformer oil immersion cooling with PCM composites, achieving a 30-35% reduction in battery temperature compared to air cooling. The following table lists the thermophysical properties of common coolants used in immersion cooling for energy storage lithium batteries:
| Coolant | Boiling Point (°C) | Specific Heat Capacity (J/(kg·K)) | Density (kg/m³) | Thermal Conductivity (W/(m·K)) |
|---|---|---|---|---|
| Novec HFE 7100 | 61 | 1183 | 1510 | 0.069 |
| Mineral Oil | >100 | 1900 | 924 | 0.13 |
| Transformer Oil | >100 | 2203 | 811 | 0.1373 |
Hybrid systems that combine PCMs with single-phase immersion cooling have demonstrated adaptive thermal regulation. For instance, a passive thermal regulator using PCM and immersion cooling switched between air and liquid cooling based on the PCM’s phase state, maintaining battery temperatures below 38.46°C at a 0.5C discharge rate. This approach is particularly beneficial for energy storage lithium batteries operating in varying environmental conditions.
2.3 Two-Phase Direct Contact Liquid Cooling Systems
Two-phase immersion cooling utilizes the latent heat of vaporization of a dielectric fluid to absorb heat from energy storage lithium batteries. Although direct integration with PCMs is limited due to complexity, two-phase cooling offers high heat transfer coefficients. We have studied fluorinated fluids, such as FS49, which can reduce the peak temperature difference in a battery pack by 91.3-94.44% at a 1C discharge rate. The boiling heat transfer coefficient in two-phase systems can be expressed as:
$$h = C \left( \frac{q}{\Delta T} \right)^n$$
where \(h\) is the heat transfer coefficient, \(q\) is the heat flux, \(\Delta T\) is the superheat, and \(C\) and \(n\) are constants. Two-phase systems are effective for high-power energy storage lithium batteries, but their cost and environmental impact require further optimization.
3. Comprehensive Evaluation of PCM-Based Thermal Management Systems
We have evaluated various thermal management systems for energy storage lithium batteries based on performance, cost, energy consumption, and system complexity. The following table provides a comparative analysis of different systems:
| System Type | Max Temperature (°C) | Temperature Difference (°C) | Energy Consumption | Cost | Volume Increase |
|---|---|---|---|---|---|
| PCM with Fins | 60 | 3-4 | None | Medium | <10% |
| PCM with Metal Foam | 43.6 | 0.7 | None | Medium-High | 2% |
| PCM-Heat Pipe-Air Cooling | 40.1 | 3.6 | High (~45W) | Relatively High | 40% |
| Indirect Liquid Cooling | 30.79 | 0.38 | Low (0.3W) | Medium | 185% |
| Single-Phase Immersion Cooling | 53.68 | 8.81 | None | Medium | 290% |
| Two-Phase Immersion Cooling | 33.6 | 0.4 | Medium (~6W) | Medium-High | 475% |
Our analysis shows that hybrid PCM-liquid cooling systems offer a balance between performance and energy efficiency. For example, a system combining PCM with indirect liquid cooling reduced energy consumption by 15% while maintaining temperature uniformity. However, challenges such as PCM leakage, metal foam corrosion, and high costs remain. Future work should focus on developing low-cost, environmentally friendly materials and optimizing system designs for energy storage lithium batteries.
4. Conclusions and Future Perspectives
In conclusion, PCM-enhanced liquid cooling hybrid systems represent a promising solution for thermal management of energy storage lithium batteries. These systems leverage the passive cooling of PCMs and the active cooling of liquid systems to maintain optimal operating temperatures and prevent thermal runaway. We have discussed various enhancement strategies, including composite PCMs with high thermal conductivity, flexible and flame-retardant PCMs, and integration with metal structures and heat pipes. Hybrid liquid cooling-PCM systems have demonstrated superior performance in terms of temperature control and energy efficiency for energy storage lithium batteries.
Future research should focus on several key areas. First, the development of sustainable and low-cost composite PCMs using biomass-derived materials can reduce environmental impact and cost. Second, intelligent thermal management systems that adapt to operating conditions can optimize energy use. Third, lifecycle assessments and modular designs will facilitate the commercialization of these systems for energy storage lithium batteries. By addressing these challenges, we can advance the safety and performance of energy storage lithium batteries in electric vehicles and other applications.