Research Progress on Thermal Management of Lithium-ion Batteries for Vehicles

In recent years, the concept of energy saving and emission reduction has received more and more attention. In 2020, the dual carbon target was formally proposed. Against this background, the development of new energy electric vehicles in the field of transportation will also be the general trend. Lithium-ion batteries are more and more widely used in electric vehicle battery packs due to their advantages of high power, large capacity, low discharge rate and long cycle life. As we all know, the power battery pack is greatly affected by temperature, and its suitable working temperature range is only 0-50 ℃. The temperature difference between single cells should not exceed 5 ℃. Excessive temperature will destroy the internal chemical balance of the battery, and even lead to thermal runaway in severe cases. If the temperature is too low, the internal resistance of the battery will increase, which will affect the battery power and energy output. Therefore, in order to keep electric vehicles in safe and efficient working conditions, it is necessary to design an appropriate thermal management system. At present, battery thermal management systems mainly include active, passive, and active-passive combined methods.

Firstly, this paper identifies the starting point for battery thermal management. Secondly, starting with different cooling methods, the research progress of thermal management in recent years is described and the advantages and disadvantages of different cooling methods are analyzed. Finally, the application of lithium-ion battery cooling technology is prospected to provide some help for future research on thermal management.

Battery performance, life and safety issues have always been the reasons that hinder the rapid development of electric vehicles. Most of these problems are related to the temperature of the battery. The chemical reactions inside the battery only happen at certain temperatures. Too high or too low temperature will cause the battery capacity of the lithium battery to decrease during charging and discharging and seriously affect the service life of the lithium battery. Excessively high temperature will destroy the chemical balance in the battery, increase the polarization of the electrochemical reaction, and reduce the battery rate performance. After high-temperature cycling, the chemical activity of the battery core decays, and the performance and life of the battery decrease. When the temperature is too low, the viscosity of the electrolyte increases and the electrode reaction rate decreases. This causes the reaction of the positive and negative electrodes inside the battery to slow down, increasing the internal resistance of the battery. Charging a lithium battery at a low temperature may even lead to lithium precipitation, which will not only degrade the performance of the battery, but also greatly shorten the cycle life of the battery. If the battery temperature is too high and serious, it will also cause thermal runaway of the battery module, causing the battery to ignite spontaneously, resulting in an explosion or even a fire. A large part of the cause of thermal runaway is due to the internal short circuit of the battery. When the lithium battery is externally stressed, its internal diaphragm ruptures, and the positive and negative electrodes come into contact, causing a short circuit in the battery. Generate a large amount of heat The electrochemical energy stored in the material will be further released with the generation of heat. When heat builds up to a certain level, thermal runaway occurs. The energy of thermal runaway will cause the thermal spread of the module and even the system. In severe cases, the entire vehicle may be burnt down. It can be seen that when the internal temperature of the battery rises and the heat cannot be released, in order to keep the battery temperature in an appropriate temperature range as much as possible and ensure battery performance and life, it is very necessary to prevent the battery from thermal runaway and conduct thermal management research.

At present, many scholars have done a lot of research on the thermal management of lithium-ion batteries for vehicles. The battery thermal management system mainly includes air cooling, liquid cooling, heat pipe cooling, phase change cooling and composite cooling. Air cooling and liquid cooling have achieved large-scale application because of their early research and mature technology. Although heat pipe cooling and phase change cooling have better effects, they are still in the experimental stage and have not yet been applied to products. In recent years, more and more scholars have paid attention to the possibility of coupling two or more cooling methods to perform composite cooling and thermal management on batteries. This cooling method not only has a better effect, but also has an increased scope of application and has a good development prospect.

Air cooling, also known as air cooling, mainly cools the battery through the high flow rate of external air. There are two common air cooling methods: (1) Passive air cooling, which uses the high flow rate of air to remove heat when the car is running; (2) Forced air cooling, mainly by adding a fan to increase the air flow rate and take away the extra heat inside the battery.

For an air-cooled system, the main factors that affect its cooling efficiency are: the battery arrangement, the design of the air duct, the design of the air inlet and outlet locations, and the air velocity and temperature. In terms of battery arrangement research, 3 battery arrangements were compared: in-line, dislocation and crossover. Figure 1 is a plan view of dislocation and cross arrangement. Compared with dislocation and cross-arrangement, when the battery packs are arranged in parallel, not only the maximum temperature is lower but also the temperature difference between the battery packs is smaller. In addition, the size of the cell spacing also affects the temperature uniformity. The temperature uniformity is best when the spacing is controlled at 4 mm. In terms of air duct design research, for the Z-shaped air duct, the numerical simulation method is used to optimize the air duct. The comparison found that the temperature difference of the battery pack was reduced by more than 48% under the condition of the optimized air flow channel without changing the pressure drop. In terms of air inlet and outlet location design research, 3 inlet/outlet locations were simulated to obtain the optimal solution: upper inlet and lower outlet, same-side inlet and outlet, and different-side inlet and outlet. The results showed that placing the inlet and outlet on opposite sides of the battery pack was the optimal solution. Using an additional baffle structure to prevent air from passing through the distance between the case and the battery greatly improves the performance of the lateral intake cooling strategy. In terms of air velocity research, numerical simulation methods are used to find that increasing the air velocity at the air inlet or reducing the air temperature at the air inlet can also effectively improve the heat dissipation capacity of the battery.

The air cooling system has the characteristics of small size, simple structure and high reliability. However, its low thermal conductivity and poor temperature uniformity control can only meet the thermal management requirements of some low-power battery packs. Air-cooled vehicles on the market are mainly new energy vehicles with small battery capacity, such as Wuling Hongguang MINI, Toyota Prius, Euler Black Cat, Nezha and other models. When the ambient temperature is too high or the wind speed is low, air cooling cannot achieve the cooling effect. At present, although the air cooling system still has a place in the market, as the battery pack begins to develop towards high energy density, air cooling alone cannot meet the requirements.

The working principle of liquid cooling is to put a certain cooling medium into a specific flow channel by design, so that it flows through the surface of the battery to remove heat. Liquid cooling is mainly divided into direct cooling and indirect cooling. The main difference lies in the contact method between the cooling liquid and the battery.

Improvements in the liquid cold plate and flow channel arrangement are the main ways to increase the efficiency of liquid cooling. A liquid cooling thermal management scheme based on serpentine channels is proposed and optimized. The optimized liquid cooling structure can keep the battery temperature within 20-35 ℃. A liquid-cooled plate structure with parallel non-equal-length straight channels is designed, which can control the pressure drop of the liquid-cooled plate well while ensuring the maximum temperature and temperature difference within an appropriate range. A new type of small channel cooling plate is designed. Compared with the traditional cooling plate, the combination of series and parallel channels has better temperature performance. And as the flow rate increases, the heat dissipation effect is better. However, this trend gradually fails at flow rates up to 5 g/s. Figure 2 is a schematic diagram of the series-parallel structure flow channel. The influence of the number of liquid cooling pipes and the distance between pipes on the heat dissipation effect of liquid cooling is studied. The heat dissipation effect becomes stronger as the number of pipes increases. If the pipe spacing is too large or too small, it is not conducive to heat dissipation, and the optimal pipe spacing is 65 mm.

Although liquid cooling has the disadvantages of complex structure and large quality. However, compared with air cooling, liquid cooling not only has a higher heat transfer coefficient, but also can make the temperature distribution of the battery pack more uniform. At present, most mainstream new energy vehicles on the market use liquid cooling as a thermal management method. For example, the wave-shaped liquid cold plate designed by Tesla has applied for a number of patents. The Xiaopeng P7 coolant can not only cool down but also heat up, and there are many models such as Ideal ONE, BYD Yuan EV360, and GAC Trumpchi GE3. Liquid cooling is still the first choice for most new energy electric vehicles. Changing the cooling plate structure, channel structure and liquid flow rate is currently an effective means to optimize liquid cooling efficiency.

Heat pipe cooling was mostly used in the field of nuclear cooling and aerospace in the early days. In recent years, with the development of new energy batteries, heat pipe cooling technology has also been used as an effective method for battery cooling. The heat pipe is mainly composed of evaporator, heat insulator and condenser. The medium in the tube evaporates in the evaporation section, and the vapor flows to the low-temperature condenser section through the adiabatic section. The medium in the pipe is condensed in this section to form a working cycle.

In terms of the structural design of heat pipe cooling, the geometrical dimensions of the heat pipes in a designed heat pipe-based heat dissipation module are studied. By comparison, it is found that when the ratio of the horizontal section to the vertical section in the evaporation section of the heat pipe is 1, the heat dissipation effect is the best. Research has found that adding heat conduction elements to the heat pipe can increase the contact area between the battery and the heat pipe and improve the cooling efficiency of the heat pipe. Increasing the thickness of the heat conduction element can also reduce the battery temperature, and generally control the thickness below 4 mm. The designed heat pipe-aluminum plate embedded heat dissipation structure is shown in Figure 3. At a discharge rate of 2 C, the temperature difference between single cells is effectively controlled at 3.2 °C. At the same time, using the two-factor analysis of variance method, it is compared that the increase in the thickness of the aluminum plate can control the maximum temperature of the battery more effectively than the increase in the number of heat pipes. For high-power battery modules, a heat pipe-fin-collector plate combination is designed. Through finite element calculation simulation and experiments, it is found that the temperature of the battery pack can be kept within 15 ℃ at a discharge rate of 1 C.

The research on heat pipe technology in battery cooling is currently mostly in the simulation and test stage, and has not yet met the requirements of actual vehicle applications. The heat pipe cooling technology not only has higher cooling efficiency than air cooling and liquid cooling, but also can meet the requirements of high temperature and low temperature dual working conditions. Although its cost is higher and its structure is more complicated at present, it still has a good development prospect. Future research will focus on reducing system energy consumption and light-weighting.

Phase change cooling is a passive cooling with better cooling effect. It mainly uses the phase change material to absorb heat while keeping the temperature constant during the change of the state of matter, also known as the latent heat of phase change. At present, phase change materials can be roughly divided into three categories: inorganic materials, organic materials and composite phase change materials. Composite phase change materials of paraffin and graphite are mostly used in the phase change cooling of lithium batteries.

Based on the numerous studies on paraffin and graphite composite phase change materials, a regular hexagonal battery module was designed. And it is filled with graphite-paraffin composite phase change material around its battery. Its structure is shown in Figure 4. The heat dissipation characteristics of phase change materials with different battery spacing under the same discharge rate were analyzed. The results show that the temperature rise of the modules with smaller spacing is higher than that of battery modules with larger spacing. By adding different mass fractions of expanded graphite to the paraffin phase change material, the cooling effect was studied and it was found that increasing the mass fraction of expanded graphite can improve the heat dissipation capacity of the system. Composite paraffin and expanded graphite are prepared into a composite phase change material plate with excellent thermal conductivity, and the thermal conductivity is increased by nearly 30 times compared with pure paraffin material. Using this method, the maximum temperature difference of the battery pack at a discharge rate of 5 C is only 2 °C. Combining paraffin wax RT44HC with expanded graphite, the thermal conductivity is 20-60 times higher than that of pure phase change materials, and the battery temperature can be well controlled within an appropriate temperature.

Compared with other cooling methods, phase change cooling does not require a large number of accessory equipment and has high safety. Moreover, the temperature difference between battery packs can be better controlled, and local overheating can be avoided. At present, the research on phase change materials mostly relies on organic phase change materials. Given the low thermal conductivity of organic phase change materials, future research focus may shift to inorganic materials with better thermal conductivity. With the development of electric vehicles, the battery power is getting bigger and bigger. Thermal management based on phase change materials alone can no longer meet heat dissipation requirements. Therefore, future research should focus on combining phase change materials with other cooling methods. The research on phase change materials is currently mainly focused on endothermic refrigeration. However, with the promotion of new energy vehicles to alpine regions, more research on the low-temperature heat storage capacity of phase change materials is needed in the future.

The four cooling methods introduced above are all single thermal management technologies, and each has its own advantages and disadvantages. In order to further improve battery cooling efficiency, many thermal management researches have begun to choose to combine multiple cooling methods. This overcomes the disadvantages of a single cooling method and retains its advantages to achieve better thermal management. At present, most of the composite cooling combines active cooling and passive cooling.

A thermal management system combining air cooling and phase change cooling is proposed. The study compared three different thermal management methods: natural convection, natural convection combined with phase change, and forced convection combined with phase change. By comparison, it is found that the combined thermal management method of forced convection and phase change can well control the maximum temperature difference of the battery pack at 2 °C. In order to improve the heat dissipation capacity of the battery, heat conduction fins are added on the surface of the battery composed of phase change materials and liquid cooling plates. This composite cooling method can ensure that the temperature of the battery pack is maintained within a safe range of 33-38°C. A heat dissipation structure model coupled with a phase change material and a liquid-cooled water jacket is designed. Explore the effect of different flow channels on the temperature rise of the battery. The different runner structure models are shown in Fig. 5. It was found by comparison that at a discharge rate of 3 C, the 6-channel structure can control the maximum temperature of the battery surface to 33.78 °C. It is 7.23 ℃ lower than the single phase transition cooling temperature. A series of thermal management systems based on phase change materials are designed. It was found that the thermal management method combining heat pipes with liquid cooling and phase change materials can control the maximum temperature within 50 °C at a discharge rate of 3 C. At the same time, the temperature difference decreased by 3 ℃ compared with the other two methods.

Hybrid cooling combines active cooling with passive cooling. Compared with other single cooling methods, not only the cooling efficiency is improved, but also the scope of application is further expanded. At present, the main problem of compound cooling is that the structure is relatively complex, and the mass and volume are relatively large. How to reduce its mass under the premise of ensuring its cooling efficiency is an urgent problem to be solved. The comparison of different battery thermal management effects is shown in Table 1.

With the rapid development of new energy vehicle technology, the battery thermal management system plays a vital role in ensuring battery performance and service life. The main function of battery thermal management is to monitor the battery temperature in real time, maintain the temperature consistency between batteries, effectively dissipate heat when the temperature is too high, and quickly heat up when the temperature is low. At present, most new energy vehicles on the market use air cooling and liquid cooling to cool down the battery. However, heat pipe cooling and phase change cooling are still in the experimental research stage as new passive cooling, and have not been applied in large quantities in new energy vehicles. With the increase of battery capacity and charge-discharge rate, a single battery thermal management method is no longer enough to meet the requirements of battery heat dissipation. Therefore, thermal management systems coupled in multiple ways must be the future development trend.

Lithium-ion batteries are developing towards high energy density and long cycle life. At the same time, the increase in battery heat generation rate leads to an increase in peak temperature and poor temperature consistency. Therefore, the future focus will be mainly on the research on the peak temperature of single battery and the control of temperature uniformity among battery modules.