Li-ion Battery Thermal Management Technology

At present, the theory of battery thermal management is perfected and the technology is continuously innovated. On the one hand, start with the battery's own materials to improve the high/low temperature resistance of the battery and strengthen the battery's tolerance. For example, the latest solid-state batteries are better than liquid batteries in terms of energy density, safety, and charging speed. On the other hand, starting from the outside of the battery, the temperature of the battery is controlled within the optimal working range by means of air forced convection, liquid medium flow, and phase change material covering. In order to pursue better heat dissipation efficiency, a liquid cooling system with a complex structure is designed. In terms of structure, as many flow channels as possible are arranged to contact each battery unit. Optimize the flow path structure, choose the best inlet and outlet positions, and design the appropriate flow path length to reduce energy loss, etc. The development of new materials in recent years combines materials with cooling or heat preservation effects with thermal management systems to enhance heat dissipation or heat preservation performance. The development of technologies such as new heat pipes, cold plates, and direct cooling also provides new research ideas for battery thermal management. This paper analyzes the main heat transfer methods from the heat generated by power batteries, and leads to the current mainstream battery thermal management technologies-air cooling technology, liquid cooling technology, PCM cooling technology, heat pipe cooling technology, and battery heating technology in low temperature environments. Its characteristics, research status, advantages and disadvantages are reviewed, and its future development trend is summarized and proposed.

1. Heat conduction analysis of power battery

To design a battery thermal management system with good performance, one should first understand the heat generation and heat transfer methods of power batteries. During the charging and discharging process of the power battery, different electrochemical behaviors occur inside the battery. Complex chemical reactions are mostly accompanied by heat generation, and the presence of battery internal resistance will also generate Joule heat.

Heat transfer includes three basic ways of heat conduction, convective heat transfer and radiation heat transfer. The heat generated by the battery can be transferred in a certain way or combined with each other.

Convection is the main way of heat transfer in liquids and gases. It is a process in which a fluid (gas or liquid) realizes heat transfer through the macroscopic flow of its various parts, and is often accompanied by heat conduction. According to the flow state, there are laminar heat transfer and turbulent heat transfer. According to the cause of the flow, it is divided into natural convection and forced convection. Forced convection is better than natural convection.

Since liquid cooling is mainly used for heat dissipation in vehicles at present, convective heat transfer is the main one. Radiation heat transfer is by radiating and absorbing radiation energy to surrounding objects and converting it into heat energy.

In summary, the heat generated by the battery is mainly transferred by convection heat transfer and heat conduction. The current heat dissipation technology also mainly utilizes these two heat transfer methods.

2. Thermal management technology

Electric vehicles initially generally adopted a simple-structured air-cooling system. Use the suction effect of the blower to suck the external air into the power battery assembly. Air flows around the battery module, and finally the heat generated by the power battery is discharged from the air outlet along with the air to achieve the effect of cooling the battery. Air cooling can be divided into natural convection and forced convection due to different ventilation methods. Natural convection is the use of external cold air to flow through the surface of each battery cell for heat exchange to achieve cooling purposes. Forced convection cooling is based on the addition of mechanical devices, which need to consume part of the energy of the battery to drive. Forced convection is more reliable and easier to maintain than natural convection. Therefore, forced convection is more common on different models. However, the temperature non-uniformity between cells is a big problem that forced convection needs to solve. According to different ventilation modes, air cooling has two ventilation modes, serial and parallel, as shown in Figure 1. During serial ventilation, air enters the ventilation line and flows over the surface of each battery cell in turn. During the air flow, the temperature of the air gradually increases, and the temperature difference between the batteries continues to shrink. Because the temperature and flow velocity on both sides of the battery module are different, the side where the air flows first has a lower battery temperature and a higher air flow velocity. The efficiency of heat transfer decreases when the airflow reaches the other side. At this time, the surface temperature of the battery does not change much, resulting in uneven temperature between the battery packs on both sides. During parallel ventilation, the air flows over different battery surfaces at the same time, and the flow rate is relatively consistent. The heat exchange of each battery is almost the same. This improves module temperature equalization. Therefore, parallel ventilation is widely used. The way the batteries are arranged also affects the air cooling effect. Although the flow resistance of the cooling airflow of the battery modules arranged in a row is small, the contact area of the battery cells is small. This convection effect and cooling efficiency are poor, usually not used. The cross arrangement increases the airflow disturbance flowing between the batteries and improves the heat dissipation effect, but the loss of flow resistance is relatively large. The heat transfer coefficient can be improved by adopting trapezoidal arrangement. Balancing the heat dissipation effect at both ends of the battery keeps the overall temperature of the battery pack at a relatively stable level.

In order to increase the driving range of the vehicle, it is necessary to arrange as many batteries as possible to provide energy. More cells will generate a lot of heat. If not discharged in time, this can form thermal runaway. Enhance the cooling capacity by changing the structure of the air inlet and outlet. Through alternate ventilation, the air is allowed to pass intermittently from the left and right sides of the battery, avoiding the phenomenon of excessive temperature on one side.

The active air cooling and passive air cooling configurations are shown in Figure 2. The passive air-cooled heat dissipation structure is relatively simple, and the air in the surrounding environment is directly used. The active cooling structure uses preconditioned air from the air conditioner to flow through the battery. Passive air cooling is less efficient than active air cooling.

Air cooling is only suitable for low-density batteries due to the characteristics of low air heat capacity and low thermal conductivity. Large battery packs require large flow channels, which makes the system bulky. Active air cooling systems, which use fans to increase heat transfer, add cost, produce a lot of noise and affect ride comfort. To improve air cooling performance, relevant measures can be taken, such as increasing air volume, flow rate, channel size, and optimizing unit location without compromising space utilization.

Air cooling has low heat dissipation efficiency and is generally used in models with relatively low motor power. Such as the early Nissan Leaf, Kia Soul EV and so on. Prioritizing the use of air-cooled heat dissipation helps reduce the cost of the vehicle. However, when there are many battery modules and the required heat dissipation effect cannot be achieved, other thermal management methods need to be considered.

When air cooling cannot meet the heat dissipation requirements, liquid cooling is introduced. During the liquid cooling process, the heat transfer fluid absorbs heat from the battery, and transfers the heat to the outside air in a timely manner through continuous circulation, reducing the temperature of the battery pack. Compared with air cooling, the heat dissipation efficiency is higher and the cooling speed is faster.

There are active and passive approaches to liquid cooling systems. In active liquid cooling, the heat exchange between the thermal fluid and the outside world is mainly done by the combination of the engine refrigerant or the air conditioning system, which is less affected by the ambient temperature. However, its complex structure increases the cost of manufacturing and maintenance. Energy-consuming components also cause secondary loss of battery energy. In passive liquid cooling, a liquid medium flows through the battery to absorb heat. Thermal fluid is pumped to the heat exchanger, which dissipates heat to the external environment to cool the battery. The medium (coolant) can be reused. The structure is simple and the cost is low. Since passive liquid cooling mainly relies on external ambient air for heat exchange, effective heat dissipation cannot be achieved when the external ambient temperature is high. The heat dissipation effect of the passive liquid cooling system is inferior to that of the active liquid cooling. The principle of active and passive liquid cooling is shown in Figure 3.

According to the contact mode between the liquid medium and the battery, it can be divided into direct contact and indirect contact liquid cooling. When the battery is in direct contact with the liquid medium, the medium can be water, ethanol, and refrigerant. The medium is usually an electrically insulating liquid (oil) with a high thermal conductivity to solve the problem of module temperature equalization. These media have high viscosity and low flow rates, which consume more energy and reduce cooling efficiency. Therefore, it can be improved by changing the thermal conductivity, flow velocity, viscosity, density and other parameters of the medium to increase the heat exchange rate. In the indirect liquid cooling system, the liquid flows in the pipeline or integrated channel in contact with the battery, taking away the heat generated by the battery to achieve the purpose of heat dissipation. Typically low viscosity fluids (water, glycol, etc.) are used in this system to transfer heat. Therefore, it requires less power consumption and is not limited by the flow rate, but its temperature uniformity is poor. Although direct liquid cooling is more efficient than indirect liquid cooling, indirect liquid cooling systems are commonly used in electric vehicles due to their practicality, stability and reliability.

PCMs are capable of absorbing or releasing a large amount of latent heat as the state of matter changes and keep the temperature constant over time. PCM cooling technology is to take advantage of this characteristic. The battery is in direct contact with the PCM, and heat is transferred from the battery to the PCM. Store and release heat in the process of changing the state of matter to achieve the effect of low-temperature heating and high-temperature heat dissipation for the power battery. PCM includes three types: organic, inorganic and composite PCM. Organic PCM has the characteristics of low price, good stability, low toxicity, non-corrosion, no supercooling and phase separation, but has the disadvantages of poor thermal conductivity and flammability. In order to solve the above problems, researchers tried to add high thermal conductivity materials and flame retardant materials to organic PCM. This is a hot issue in the field of battery thermal management. Due to the limitation of phase transition temperature, most available inorganic PCMs are hydrated salts, whose thermophysical properties are unstable. Inorganic PCM is completely non-flammable and costs much less than organic PCM. Inorganic PCMs suffer from poor thermal conductivity and stability due to phase separation, dehydration, or supercooling, hindering their widespread application. In order to improve these shortcomings of organic PCM and inorganic PCM, composite PCM has been developed by combining the advantages of the above two, which has better thermal conductivity and latent heat of phase change.

Compared with traditional thermal management methods, PCM does not need to consume energy, has low cost and good temperature uniformity, and is often used in combination with other methods.

Heat pipe technology is a new technology that has developed rapidly in recent years. It mainly uses the phase change characteristics of substances to eliminate the shortcomings of PCM heat management technology. An alternative system called heat pipe is proposed, which is an upgraded version based on PCM. A traditional heat pipe is mainly composed of three components: a tube shell, a liquid-absorbing core and an end cap. According to the heat transfer conditions, the heat pipe can usually be divided into three parts: the hot end, the adiabatic part, and the cold end. The combination of heat pipe and battery is shown in Figure 4. The heat pipe is filled with an appropriate amount of refrigerant after the closed metal shell is pumped into a negative pressure. When one end of the heat pipe absorbs the heat generated by the battery, the refrigerant evaporates. The gaseous refrigerant flows to the other end under the action of the pressure gradient to release heat and re-condenses into a liquid. The liquid flows back to the evaporator under the action of capillary force, and the above continuously circulates to achieve the effect of heat dissipation.

High production and maintenance costs and difficulty in controlling the amount of heat exchange medium are the main reasons hindering the application of heat pipes. In the heat dissipation system of the heat pipe, the power battery can not only maintain the normal operating temperature range, but also maintain the temperature uniformity among the battery cells, with reversible heat flow direction. This is an effect that other cooling systems cannot achieve. The heat pipe cooling method is also easy to couple with the above technologies to improve heat dissipation performance and has broad development prospects.

The direct cooling system is also a relatively advanced thermal management system at present, which absorbs the advantages of the liquid cooling system and PCM. Two-stage cooling is achieved by using liquid and PCM. Liquid coolant flows within a cooling plate attached to the battery pack. Heat exchange with the battery pack and cooling plate by conduction and convection, and the coolant is directly cooled by the battery system using the evaporation phase change process. Its adaptability to the air conditioning system can couple the cooling of the battery system and the cabin in one system, improving the cooling efficiency. Compared with the liquid cooling system that uses the sensible heat of the coolant, the direct cooling system uses the latent heat of vaporization of the coolant to meet the heat demand of the battery, and the system cooling efficiency is increased by nearly 5 times. Additionally, fewer components are required. Savings in equipment contribute to weight reduction, reducing the cost of electric vehicles. The direct cooling plate is used as the evaporator of the battery module in the direct cooling system, and its performance directly affects the heat exchange effect between the battery and the refrigerant. At present, the most used ones are "G" shaped cold plates and honeycomb single-sided inflated aluminum plates.

When the battery energy density and fast charging rate requirements are getting higher and higher, the battery direct cooling solution has the advantages of small size, light weight, fast cooling speed, and good performance, and is considered to be one of the potential alternatives for the next-generation battery thermal management system . Considering future innovative applications, a digital twin-based approach can provide guidance for the design of future direct cooling systems. This approach is based on the potential of digital technologies and cloud-based control platforms to better realize thermal management.

In a low temperature environment (such as −20°C or lower), the capacity, power and discharge efficiency of the battery will decrease significantly. The service life is shortened, making it difficult to charge and discharge, and may lead to thermal runaway in severe cases. Therefore, it is necessary to heat or keep warm the battery in cold regions to keep the temperature of the battery from being too low and ensure the normal driving of the car. At present, the heating system is divided into internal heating and external heating. Internal heating includes self-heating, high and low frequency AC heating and pulse current heating. External heating includes PCM heating, air heating, hydronic heating, etc.

The way of internal heating is to heat the battery mainly by using the internal resistance of the battery and the heat generated by the internal chemical reaction. This method has high efficiency but low energy efficiency, which is likely to cause battery performance degradation and poor consistency. External heating is to heat the battery by generating heat through an additional heating element. This method is simple to heat and has high safety, but the efficiency is low.

The colder the battery temperature, the smaller the battery capacity and the less it will discharge. This will not only affect the driving range of the vehicle, but also affect the power performance and energy recovery of the vehicle, thereby limiting the promotion of electric vehicles in cold regions. Battery heating technology has great development prospects, which can be explored from many aspects. In terms of battery shape, the battery is designed in a hexagonal shape, and the batteries are arranged to form a hexagonal close-packed, which is conducive to space optimization and heat preservation between batteries. In terms of battery and electrolyte materials, graphene and superconducting materials are used to improve the conductivity of batteries in low temperature environments. In terms of battery module structure, some shapes in nature can be considered, such as designing flow channels with the help of leaf veins, ant caves and other bionic structures to heat and keep the battery warm.

3. Summary

There will be more fast-charging vehicles in the future, and fast-charging will generate more heat. Taking into account the battery discharge characteristics, heat dissipation effect, system energy consumption and light weight and other indicators, proposing an efficient heat dissipation scheme for the thermal management system is the focus of future research on enhanced battery heat dissipation. In order to keep the operating temperature of the battery within the optimal temperature range, it is necessary to improve the heat dissipation performance of the battery thermal management system. A single cooling system can no longer meet the needs, and air cooling, liquid cooling, phase change cooling, heat pipe cooling, and direct cooling can be combined for comprehensive cooling. Improve thermal performance by optimizing each cooling method. Research on variable width air duct structure for air cooling. Consider the reciprocating and bi-directional flow of liquid for liquid cooling. Use inertia to reduce energy dissipation, change liquid properties to improve thermal conductivity, optimize liquid channel structure, control flow velocity and flow, etc. The phase change cooling is optimized by controlling the melting temperature of PCM, the thermal conductivity of PCM, the quality of PCM, the distance between cells, the thickness of PCM and other parameters. Add specific substances to PCM to develop composite PCM with excellent performance to obtain better cooling performance. Coupling the heat pipe with air, water or refrigerant cooling has good heat transfer efficiency. Batteries are the heart of electric vehicles. Starting from the battery itself, improving its ability to adapt to the external environment temperature is also a research hot topic.

In the application of thermal management by car companies, Tesla's comprehensive cooling system includes a cooling cycle and a heating cycle. At the same time, multi-sensor fusion technology can also be used to monitor battery temperature more accurately. Through machine learning with a large amount of data, a thermal management system capable of energy distribution and adaptive temperature adjustment has been developed. Huawei's intelligent and integrated thermal management system through integrated design and development achieves optimal overall energy consumption.

Therefore, in order to improve the cooling and heating efficiency of the power battery, under the premise of ensuring safety and increasing driving range, optimize its performance, and strengthen research on intelligence, integration and adaptability to various working conditions. For example, the intelligent monitoring temperature cloud control platform can set high and low temperature trigger thresholds. It can be combined with a supercapacitor to form a hybrid power to improve the power and battery life of electric vehicles. Perform multi-objective design optimization to obtain a more cost-effective structure. According to intelligent algorithms, implement thermal management control strategies that can be optimized in real time. Optimize the fast charging protocol based on the model to reduce heat generation during charging. It can also intersect with other disciplines, such as bionics, materials science, computer science, etc. Utilize the characteristics and advantages of these disciplines to develop a self-regulating safe, efficient and integrated battery temperature ecosystem.