A Review on Battery Thermal Management for New Energy Vehicles

: Lithium-ion batteries (LIBs) with relatively high energy density and power density are considered an important energy source for new energy vehicles (NEVs). However, LIBs are highly sensitive to temperature, which makes their thermal management challenging. Developing a high-performance battery thermal management system (BTMS) is crucial for the battery to retain high efﬁciency and security. Generally, the BTMS is divided into three categories based on the physical properties of the cooling medium, including phase change materials (PCMs), liquid, and air. This paper discusses the effect of temperature on the performance of individual batteries and battery systems, at ﬁrst. Then, a systematic survey of the state-of-the-art BTMS is presented in terms of liquid-based, PCM-based, and air-based BTMS. To further utilize the heat source of the vehicle, the BTMS integrated with the vehicle thermal management system (VTMS) is discussed. Finally, the challenges and future prospects for BTMS with the ability to cut off the thermal runaway are discussed. The primary aim of this review is to offer some guidelines for the design of safe and effective BTMS for the battery pack of NEVs.


Introduction
Nowadays, energy conservation and emission reduction drive the auto industry to abandon the internal combustion engine step by step [1,2]. New energy vehicles (NEVs), powered by renewable fuels, are applied to replace the fossil-based vehicle [3,4]. Lithiumion batteries (LIBs) are considered as the most promising energy storage equipment for NEVs, including hybrid electric vehicles (HEVs) and pure battery electric vehicles (BEVs), because of their high energy/power density, long cycle life, and eco-friendly properties [1,[5][6][7].
The fundamental challenge for NEVs to be commercialized is energy storage. Developing an applicable energy storage device to achieve high power and high energy is essential. In order to achieve the power and energy requirements of NEVs, LIBs are connected in series/parallels to fabricate module and pack [8,9]. For these demanding applications, LIBs are deemed sensitive to pressure, vibration, and temperature. Among these factors, temperature has a significant impact on the performance of LIBs [10,11].
For example, the low temperature will reduce the power and energy output of LIBs, and the high temperature will result in the complicated side reaction of battery components, which can trigger thermal runaway (TR) in extreme conditions [1,12]. For battery systems, the uneven distribution of temperature will cause different electrochemical behaviors and electrically unbalanced cells, which impacts the performance of NEVs seriously [13]. Thus, it is essential to create a reliable and efficient battery thermal management system (BTMS) that can maintain the battery temperature within a defined range for NEVs. An ideal BTMS should be capable of regulating the battery pack to an optimal temperature while adding minimal weight and cost.  [16]; (b) Energy density versus environmental temperature [23]; (c) Normalized internal resistance versus temperature [23].
The previous content introduces the effect of temperature on individual battery. For NEVs, the battery system is called the battery pack, which is composed of hundreds of LIBs connected in series or parallels to fulfill the requirement of voltage and power [1,8].  [16]; (b) Energy density versus environmental temperature [23]; (c) Normalized internal resistance versus temperature [23].

Battery Thermal Management Systems
In order to be widely adopted in industry, the battery is built with fixed shapes and geometry. In some applications, the battery system will suffer extreme operating conditions such as high-rate charge/discharge and high/low temperature, which can increase the failure probability of the battery system. To maintain suitable operating temperature, the BTMS should be applied. Heat transfer medium has a significant effect on the cost and performance of BTMS. The heat transfer medium could be air, liquid, phase change material (PCM), etc.

Air-Cooling
Generally, air-cooling with the advantages of straightforward structure, lightweight, and maintenance convenience was considered as a cost-effective and reliable scheme for BTMS of NEVs [30]. It is proposed that air-cooling can be categorized into natural convection and forced convection [16]. The natural convection cools the batteries by allowing the air to sweep the battery pack spontaneously [31]. It is reported that the air natural convection for thermal dissipation is invalid for the BTMS of NEVs [32]. As a comparison, the forced convection shows better heat exchange efficiency than natural convection does because the adoption of air pumps and the cooperation with the evaporator can circulate the cooling/heating air efficiently [31]. With the better performance in BTMS, the forced convection was applied on a large scale in the automobile field. It is reported that the forced convection could be divided into passive air convection and active air convection [33].
In general, the passive system blows the air from the atmosphere or the cabin. However, the performance of the passive system depends on the ambient temperature [34]. As a contrast, the active system takes pre-conditioned air from the heater or evaporator of a heating ventilation air condition (HVAC) system. In general, the air pre-treated by a heater or HVAC shows a better thermal control capability in extreme conditions such as cold temperature (under freezing temperature) and high temperature (exceeding 45 • C). This system will be introduced in detail in the following section.
In addition, the structure of the duct has a strong impact on the efficiency of BTMS. In general, the mainstream structure of air ducts for NEVs can be divided into parallel ventilation and series ventilation as Figure 2a,b show [14]. Pesaran et al. analyze the thermal performance of the battery pack for those two air ducts [35]. They find that the BTMS with parallel ventilation provides a lower maximum temperature and a more even temperature distribution than the BTMS with series ventilation does. In consideration of the geometric dimension of a cylindrical battery, a novel axial flow air cooling system was put forward (Figure 2c). It is reported that the axial ventilation can maximize the utilization of the battery pack's space and provide higher power density of NEVs equipped with the cylindrical battery [36,37].
To realize the best performance of the battery system, the air-cooling BTMS should be optimized [38]. Nowadays, the development of computer numerical simulation technology and the conducting experiment help the optimization of air-cooling BTMS, which concentrates on the battery arrangement, the air flow velocities, flow path, and geometrical arrangement of the batteries in the pack [10,16].  [35]; (b) parallel ventilation [14]; (c) axial air forced flow [37].
To realize the best performance of the battery system, the air-cooling BTMS should be optimized [38]. Nowadays, the development of computer numerical simulation technology and the conducting experiment help the optimization of air-cooling BTMS, which concentrates on the battery arrangement, the air flow velocities, flow path, and geometrical arrangement of the batteries in the pack [10,16].

Parallel
In general, the parallel air-cooled BTMS can be divided into nine types as Table 1 describes [14,39]. Some researchers applied numerical solutions involving the CFD approach to study the thermal performance of parallel air-cooled BTMS. Figure 2b shows the velocity contours of nine parallel air ducts, which indicate that the flow pattern is significantly influenced by the air inlet and outlet locations [14]. For BTMS III, the difference in airflow velocity distribution in each cooling channel is the greatest and the temperature difference is the largest. In contrast, the difference in airflow velocity distribution in each  [35]; (b) parallel ventilation [14]; (c) axial air forced flow [37].

Parallel
In general, the parallel air-cooled BTMS can be divided into nine types as Table 1 describes [14,39]. Some researchers applied numerical solutions involving the CFD approach to study the thermal performance of parallel air-cooled BTMS. Figure 2b shows the velocity contours of nine parallel air ducts, which indicate that the flow pattern is significantly influenced by the air inlet and outlet locations [14]. For BTMS III, the difference in airflow velocity distribution in each cooling channel is the greatest and the temperature difference is the largest. In contrast, the difference in airflow velocity distribution in each cooling channel of the BTMS VII and BTMS IX is less than other BTMSs, corresponding to smaller temperature differences, consequently [14]. The simulation conducted by Chen et al. demonstrates that the maximum temperature and maximum temperature difference of BTMS IX decreases by 4.3 and 6.0 • C, respectively, compared to Z-type BTMS (BTMS I) [39].  [14].

Serial of the BTMS Details
BTMS I (Z-type) Both the inlet and outlet are perpendicular to the cooling channels and have opposite directions (Figure 2b-I).
BTMS II (U-type) The inlet and outlet are both perpendicular to the cooling channels and have the same direction (Figure 2b-II).
BTMS III Based on Z-type air-cooling BTMS, the inlet region's direction is modified to be parallel to the cooling channels' directions ( Figure  Xie et al. improve the heat dissipation performance of U-type air-cooling BTMS by testing and optimizing the air-inlet angle, the air-outlet angle, and the gap between each battery [40]. They find that under the conditions of 2.5 • inlet angle, 2.5 • outlet angle, and equal channel gaps demonstrate the best cooling performance for their BTMS [40]. Zhao et al. study the effects of ventilation type, gap size, environmental, and entrance air temperature for series ventilation on cooling effectiveness of different battery modules [41].

Axial Air Forced Convection
The structure of axial air forced flow was shown in Figure 2c [37,42]. Based on a pseudo-2-dimensional model of the electrochemical reactions, Yang et al. developed a 3-dimensional heat and mass transfer modeling. They concentrate on the axial air forced convection and research the effects of the interval between the cylindrical battery and air flux on the thermal performance of the axial flow air-cooling system. They find that the increase of the radial interval between batteries will result in a slight increase of average temperature but benefit to the temperature uniformity of the battery system. In addition to that, the larger air flux can reduce the temperature difference within the battery pack. Lu et al. studied the capability of axial air forced convection regarding temperature uniformity and hotspot mitigation with different flow paths and airflow rates [36]. They found that the maximum temperature gradually decreases as the cooling channel size increases, and the battery system with 59 vents demonstrates better performance on decreasing the maximum temperature and the temperature difference [36].
Sometimes fins are added to the surface of batteries to enhance the thermal performance of air-cooling systems. However, according to Chen's research, fin-cooling can add up to a maximum of approximately 39% extra weight to the battery, compared to liquid-cooling methods that have the same volume [43]. In comparison, direct and indirect liquid-cooling add approximately 2.95% and 7.16% weight to the battery, respectively [43]. Despite traditional air-cooling being considered the simplest and lightest method, adding fins increases weight and negates these advantages. The reduced energy density of battery systems also limits the application of fin-cooling in NEVs' BTMS.

Liquid-Cooling
Comparing to the air medium, liquid medium with higher thermal conductivity and higher heat capacity shows better behavior in the temperature distribution of battery modules, which can meet the cooling requirements for large-scale cells discharging/charging at high C-rates [44]. However, the complexity of the liquid cooling system improves the cost of manufacturing. Generally, liquid-cooling strategies can be divided into direct and indirect cooling.

Liquid-Based Direct Cooling
Generally, direct cooling is also called the liquid immersion cooling, which has been applied in transformers successfully [45,46]. Nowadays, immersion cooling has garnered significant attention for electronic devices and NEVs.
As Figure 3 shows, the battery module is partially or completely immersed in the cooling medium, which can absorb heat produced by the battery directly and helps enhance the temperature uniformity of the battery module [47]. Furthermore, direct cooling can simplify the system design and decrease the system complexity. For the purpose of reaching high heat transfer efficiency, the cooling medium for direct cooling should have excellent chemical-physical properties, such as high thermal conductivity, low viscosity, and high heat capacity [48]. Because the immersed battery has the risk of external short circuit (ESC), the potential cooling medium should be electrically insulating. In addition, non-toxic, chemical stability, and nonflammability should be taken into account for environmental and safety requirements. Although the water/ethylene glycol-based coolants were widely applied in indirect cooling systems, the conductive characteristics of water restrict the application of it in immersion cooling systems [49]. It is reported that common mediums for immersion cooling systems are hydrocarbon oils, silicone oils, and fluorinated hydrocarbons [47].
Comparing to the air medium, liquid medium with higher thermal conductivity and higher heat capacity shows better behavior in the temperature distribution of battery modules, which can meet the cooling requirements for large-scale cells discharging/charging at high C-rates [44]. However, the complexity of the liquid cooling system improves the cost of manufacturing. Generally, liquid-cooling strategies can be divided into direct and indirect cooling.

Liquid-Based Direct Cooling
Generally, direct cooling is also called the liquid immersion cooling, which has been applied in transformers successfully [45,46]. Nowadays, immersion cooling has garnered significant attention for electronic devices and NEVs.
As Figure 3 shows, the battery module is partially or completely immersed in the cooling medium, which can absorb heat produced by the battery directly and helps enhance the temperature uniformity of the battery module [47]. Furthermore, direct cooling can simplify the system design and decrease the system complexity. For the purpose of reaching high heat transfer efficiency, the cooling medium for direct cooling should have excellent chemical-physical properties, such as high thermal conductivity, low viscosity, and high heat capacity [48]. Because the immersed battery has the risk of external short circuit (ESC), the potential cooling medium should be electrically insulating. In addition, non-toxic, chemical stability, and nonflammability should be taken into account for environmental and safety requirements. Although the water/ethylene glycol-based coolants were widely applied in indirect cooling systems, the conductive characteristics of water restrict the application of it in immersion cooling systems [49]. It is reported that common mediums for immersion cooling systems are hydrocarbon oils, silicone oils, and fluorinated hydrocarbons [47]. Hydrofluoroethers were used in the power electronics direct cooling system as the cooling medium. Nowadays, excellent performance pushes it to be extended into BTMs. 3M ltd. develops Novec-engineered fluid (Novec 7000) to meet the demands of the direct cooling system [50]. If the surface temperature of LIBs increases to the boiling point of Novec 7000, the fluid will boil and absorb large quantities of heat. To a certain extent, the noninflammability of Novec 7000 can reduce the risks of LIBs' thermal runaway. In a direct cooling system, the properties of the medium are critical in determining both the cooling efficiency and thermal stability. Hirano et al. designed a battery module with 10 cells connected in series immersed in Novec 7000 [51]. The battery module was worked in highrate charge/discharge conditions. They found that the immersion cooling with Novec 7000 Hydrofluoroethers were used in the power electronics direct cooling system as the cooling medium. Nowadays, excellent performance pushes it to be extended into BTMs. 3M ltd. develops Novec-engineered fluid (Novec 7000) to meet the demands of the direct cooling system [50]. If the surface temperature of LIBs increases to the boiling point of Novec 7000, the fluid will boil and absorb large quantities of heat. To a certain extent, the noninflammability of Novec 7000 can reduce the risks of LIBs' thermal runaway. In a direct cooling system, the properties of the medium are critical in determining both the cooling efficiency and thermal stability. Hirano et al. designed a battery module with 10 cells connected in series immersed in Novec 7000 [51]. The battery module was worked in high-rate charge/discharge conditions. They found that the immersion cooling with Novec 7000 as the medium shows an excellent thermal performance [51]. It maintains the battery temperature no more than 35 • C under 10 • C/20 • C cycling, and the temperature difference of each battery is under 1 • C [51].
Hydrocarbon-based fluids can work as the immersion cooling medium, which include mineral oils and poly-alpha-olefins (PAO), etc. Mineral oil is a distillate of petroleum, which attracts the attention for direct cooling because of the low cost, low toxicity, and adequate working temperature range, as Table 2 shows. Patil et al. designed an immersion cooling system with forced flow to cool a battery pack composed of some pouch cells [52]. They found that the maximum battery temperature can maintain 32.8, 30.8, and 30.6 • C at different flow rates of 1 L/min, 5 L/min, and 10 L/min [52]. However, the impurities in mineral oil result in poor oxidation stability. Some impurities such as sulfur-containing compounds will result in the corrosion of copper in electrical systems [47]. Compared with mineral oil, PAO has a higher concentration of saturated carbon-carbon chemical bonds, which delivers a more stable structure in turn [47]. Furthermore, the viscosity of PAO can be controlled over a wide range. It is reported that the PAO was widely used as the base oil of high-performance motor oil.
Silicon oil is another potential medium for the immersion cooling system. The viscosity of the silicone oil is determined by the length of the siloxane monomer chain, similar to hydrocarbons. Matsuoka et al. applied silicon oil as the coolant to immerse the data center [53]. They compared the performance of 20 cSt silicone oil and 50 cSt silicone oil and found that the natural convection was more noticeable in 20 cSt silicone oil [53].
The mixtures of water/glycol are widely used in the indirect cooling system because of the relatively low cost and high thermal conductivity. However, the poor electrical insulating properties restrict its practical implementation in battery immersion thermal management system. In order to solve this problem, some researchers propose coating the electronic components with a thin insulating layer. For example, parylene C was coated as thin as 1 µm, and the heat flux for the water/glycol system shows better thermal conductivity than dielectric fluids mentioned above [47]. The thermal conductivity of dielectric medium called Novec 7000 is 0.08 W/mK, while the water-glycol (50:50) is 0.4 W/mK. It means that the Novec 7000 may be suitable for the lower power devices because of its lower boiling point and latent heat during the evaporation process. The improved water/glycol system is suitable for high power devices. Compared to liquid-based direct systems (liquid immersion cooling systems), the liquid-based indirect cooling system is easier to implement. As Table 2 shows, using water and ethylene glycol blends as common coolants in liquid-based indirect cooling systems with lower viscosity result in higher flow rates with the same pumping power. Thus, the indirect-contact mode has been commonly employed by passing the liquid through discrete tubing, jackets, or cooled plates [58,59]. With the addition of relatively low cost, liquid-based indirect cooling is regarded as the most widely used BTMS for EVs.

Cold Plate
A cold plate is a flat metal plate with internal channels through which a liquid-cooling medium is pumped [10,[60][61][62]. As Figure 4a shows, the cold plate can be installed in three positions: embedded within the battery monomer (mode A), sandwiched between adjacent batteries (mode B), or attached to the sides of the battery module (mode C). For mode A, the channel size must be small enough to be integrated into the battery components and the jacket should be chemically stable to resist the electrochemical corrosion (Figure 4a) [63]. For mode B, the cold plates are arranged between adjacent batteries. To enhance the energy density of the battery system, the cold plate with low thickness should be designed (Figure 4a) [64]. For mode C, cold plates are typically in thermal contact with the side or bottom sections of the battery module (Figure 4a) [65]. To conduct the heat efficiently, the heat spreaders may be placed between batteries to enhance the heat transfer from the module to the cold plates. Because of the flat shape, the cold plates are widely used in battery module, consisting of prismatic cells instead of cylindrical cells. In general, the cold plates are expected to offer structural support for the cells and integrate into the battery pack to ensure safety and compactness in EVs. indirect-contact mode has been commonly employed by passing the liquid through discrete tubing, jackets, or cooled plates [58,59]. With the addition of relatively low cost, liquid-based indirect cooling is regarded as the most widely used BTMS for EVs.

Cold Plate
A cold plate is a flat metal plate with internal channels through which a liquid-cooling medium is pumped [10,[60][61][62]. As Figure 4a shows, the cold plate can be installed in three positions: embedded within the battery monomer (mode A), sandwiched between adjacent batteries (mode B), or attached to the sides of the battery module (mode C). For mode A, the channel size must be small enough to be integrated into the battery components and the jacket should be chemically stable to resist the electrochemical corrosion (Figure 4a) [63]. For mode B, the cold plates are arranged between adjacent batteries. To enhance the energy density of the battery system, the cold plate with low thickness should be designed (Figure 4a) [64]. For mode C, cold plates are typically in thermal contact with the side or bottom sections of the battery module (Figure 4a) [65]. To conduct the heat efficiently, the heat spreaders may be placed between batteries to enhance the heat transfer from the module to the cold plates. Because of the flat shape, the cold plates are widely used in battery module, consisting of prismatic cells instead of cylindrical cells. In general, the cold plates are expected to offer structural support for the cells and integrate into the battery pack to ensure safety and compactness in EVs.  Obviously, to enhance the performance of BTMS utilizing the cold plate technology, the arrangement of channels and the liquid flow can be optimized. In general, the channel configuration can be divided into straight design, serpentine design, U-bend design, pumpkin design, spiral design, and hexagonal design, as Figure 4b shows [66][67][68].
Huo et al. designed the cold plate with straight mini-channels [69]. To investigate the impact of channel number, flow direction, and inlet mass flow rate on temperature rise and the distribution of batteries during a high-rate discharge process, they developed a three-dimensional thermal model. According to the result, an increase in the number of channels and inlet mass flow rate can lead to a decrease in the maximum temperature of the battery module [69]. Their further work shows that five channels were enough for a cold plate to reduce the battery temperature within the desired range by increasing the mass flow rate [70]. Moreover, increasing the width of the channel from 3 mm to 6 mm can reduce the energy consumption notably, which is beneficial to energy conservation and emission reduction [70]. Furthermore, Monika et al. explored and compared the thermal performance of six distinct mini channel designs mentioned above by a three-dimensional numerical method. The result shows that the serpentine and hexagonal geometries can greatly enhance the temperature uniformity of the battery, while the pumpkin geometry maintains a lower pressure drop and pumping power [66].

Discrete Tube
Compared to the cold plate structure, the discrete tube with different configurations can transfer the heat between liquid medium and cell, too. Because of the tube structure, the indirect-contact mode with discrete tubes is suitable for battery systems composed of cylindrical cells or prismatic cells [71]. Lan et al. designed an BTMs based on aluminum mini-channel tubes to conduct the heat produced by the discharge process of an prismatic battery, and the number of tubes, the flow rates, and the flow direction were optimized [72]. Furthermore, they applied this structure to the battery module as the BTMs to validate the thermal performance of it (Figure 5a) [73]. The simulation demonstrates that the minichannel tube structure can prevent the TR propagation in the module effectively, although the TR of the battery monomer cannot be ceased [73]. Zhang et al. designed a flat tube bank arranged on the battery surface in staggered formation. It is proved that this arrangement can reduce the requirements for flow path and flow rate in comparison to the cold plate [74].
impact of channel number, flow direction, and inlet mass flow rate on temperature rise and the distribution of batteries during a high-rate discharge process, they developed a three-dimensional thermal model. According to the result, an increase in the number of channels and inlet mass flow rate can lead to a decrease in the maximum temperature of the battery module [69]. Their further work shows that five channels were enough for a cold plate to reduce the battery temperature within the desired range by increasing the mass flow rate [70]. Moreover, increasing the width of the channel from 3 mm to 6 mm can reduce the energy consumption notably, which is beneficial to energy conservation and emission reduction [70]. Furthermore, Monika et al. explored and compared the thermal performance of six distinct mini channel designs mentioned above by a three-dimensional numerical method. The result shows that the serpentine and hexagonal geometries can greatly enhance the temperature uniformity of the battery, while the pumpkin geometry maintains a lower pressure drop and pumping power [66].

Discrete Tube
Compared to the cold plate structure, the discrete tube with different configurations can transfer the heat between liquid medium and cell, too. Because of the tube structure, the indirect-contact mode with discrete tubes is suitable for battery systems composed of cylindrical cells or prismatic cells [71]. Lan et al. designed an BTMs based on aluminum mini-channel tubes to conduct the heat produced by the discharge process of an prismatic battery, and the number of tubes, the flow rates, and the flow direction were optimized [72]. Furthermore, they applied this structure to the battery module as the BTMs to validate the thermal performance of it (Figure 5a) [73]. The simulation demonstrates that the mini-channel tube structure can prevent the TR propagation in the module effectively, although the TR of the battery monomer cannot be ceased [73]. Zhang et al. designed a flat tube bank arranged on the battery surface in staggered formation. It is proved that this arrangement can reduce the requirements for flow path and flow rate in comparison to the cold plate [74].  [73]; (b) Tesla Model S battery cooling [10]; (c) the BTMs based on discrete tubes and aluminum blocks for cylindrical batteries [75]. The discrete tube was widely applied in the EVs' battery system composed of cylindrical batteries, as Figure 5b shows. The metallic tubes were set in series as the ribbon shape snaking through the battery module of Tesla model S, which is the common structure for the EVs industry [10]. Moreover, some novel discrete tube structures were proposed. For example, Basu et al. applied the aluminum elements, wrapping cylindrical batteries as the thermal connect component to transfer the heat from cells to liquid tubes placed on the side of the battery module, which can avoid the electrical connection when the liquid medium leaks [63]. Similarly, Du et al. designed a BTMs based on discrete tubes and aluminum blocks with variable thermal contact surface for the battery module composed of cylindrical batteries (Figure 5c) [75]. Although these novel designs offer some benefits for BTMs, they may reduce the energy density of the battery system, which limits their large-scale application.

PCM-Cooling
PCM is a material that stores or emits heat according to the phase change process. As an innovative solution for thermal management applications, PCM can absorb a significant amount of latent heat during its melting process while maintaining a stable temperature around the phase change temperature for an extended period (Figure 6a) [10,76]. To meet the operating temperature of LIBs, the PCMs with melting temperatures between 20 to 60 • C are commonly used [76,77]. Although PCMs can promote temperature uniformity for large-scale batteries under high-rate discharge, there are some challenges to overcome, such as low thermal conductivity, high volume change, flammability, weak structural strength, and the risk of leakage from melted PCMs [15].
To overcome these disadvantages, the shape-stabilized PCM composed of PCM as the dispersed material and other materials as additive material is proposed [78]. It is reported that adding additive or frame work with different properties can improve thermal conductivity, enhance shape retaining ability, and absorb the liquid PCMs (Figure 6b) [78]. For example, Lv et al. developed a kind of nano-silica (NS)-enhanced composite PCM with excellent anti-leakage and anti-volume-change performance for BTMS by adding a small amount of NS into the paraffin (PA) [79]. They present that NS with numerous nanoscale pores ranging from 30 to 100 nm can absorb liquid phase PA effectively, which prevents the leakage of liquid PA, increases the homogeneity, and reduces the volume change during the phase change process [79]. To improve the thermal conductivity of PCM-based BTMS, various materials have been introduced and studied, including metallic particles [80], metal foam [81], carbon fiber [82], graphene [76], and carbon nanotubes [83]. Shirazi et al. prepared different PA nanocomposite structures by adding carbon nanotubes, fullerene, and graphene, as Figure 6b shows [83]. Goli et al. developed the composite based on PA mixed with 1 wt% graphene. The conductivity of this composite was improved by 60 times compared to the traditional PA [84]. Wu et al. developed novel pyrolytic graphite sheets (PGS)-enhanced PA/expanded graphite (EG) composites, in which EG can absorb the liquid PCM without leakage and form a primary thermal conductive network (TCN). PGS attached to the battery module sides acts as the secondary TCN for the battery module to improve the thermal homogeneity [85]. Furthermore, they proposed a copper mesh (CM)enhanced PA/EG (PA/EG-CM) composite for BTMs (Figure 6c) [86]. EG with a porous structure can absorb liquid phase PA and prevent its leakage during the phase change process [86]. CM serves as a framework to further improve both the thermal conductivity and strength of the entire module [86]. This composite exhibits superior heat conduction performance and temperature uniformity compared to the PA/EG plate without CM [86]. The enhanced properties of these improved PCMs are shown in Table 3 in detail.  Figure 6. (a) Temperature characteristics of the PCM-based system [10]; (b) the structure of the PCM composite added with various additives as the supporting frame or thermal conductor [83]; (c) the structure of PA/EG-CM composite and the application in BTMS [86]. l Conduc-Additives/Thermal Con-Composites Thermal Ratio of Compo-Latent Heat of PCM without/with Addi- Figure 6. (a) Temperature characteristics of the PCM-based system [10]; (b) the structure of the PCM composite added with various additives as the supporting frame or thermal conductor [83]; (c) the structure of PA/EG-CM composite and the application in BTMS [86].  Table 4 briefly summarizes the performance of various BTMS on the bases of integration, efficiency, maintenance, energy density, and other factors. Actually, with the improvement of battery packs' integration, the traditional air-cooling mode cannot meet the functional requirement of BTMS. Generally, each of NEVs' subsystems has a different ideal operation temperature range, which means that various thermal management strategies are necessary. In addition, to utilize each subsystem, the design of BTMS should be combined with the vehicle thermal management system (VTMS).  Figure 7 shows the development tendency of BTMS. The heat ventilation air conditioning system (HVAC) is a heat pump system in essence, which can be switched between heating and cooling modes by controlling the operation of the chiller. The outstanding heating/cooling capacity makes it concerned. For air-cooling systems (Figure 7a), the HVAC can be worked as the heat exchanger to reduce or increase the intake air temperature of BTMs, which can reinforce the performance of the air-cooling system [10]. For the basic liquid-cooling system, the main pipe was connected with the HVAC system as the secondary cooling loop to improve its performance (Figure 7b) [15]. The structural diagram of the direct refrigerant cooling system was shown in Figure 7d [31]. The operating principle is similar to the air conditioning system of a vehicle. In this system, the evaporator, which is parallel to the evaporator of the passenger compartment, is placed into the battery pack. After the expansion valve, the liquid refrigerant will flow into the evaporator to cool the battery directly via a large amount of evaporative latent heat [31].

The BTMS for NEVs
Although this system appears simple, the strong performance makes it be regarded as a potential candidate to compete with the liquid-cooling system [31]. Beside the HVAC system, the heat produced by the engine could be used by connecting the liquid loop with an engine-cooling system, which is a peculiar structure for HEV/PHEV (Figure 7b,c). Additionally, the PCM can be combined with liquid-cooling to enhance the performance of BTMs [15]. In this scheme (Figure 7c), the PCMs located between cells can absorb the heat generated by cells and the liquid cooling system will remove the redundant heat for PCMs [15]. Combined with HVAC, a complicated BTMS is formed. However, the optimization of VTMs is a hard job, which should coordinate each subsystem and meet the requirements of energy-saving and high efficiency.

The Future Prospect of BTMS for NEVs
The fire hazards related to the battery system of NEVs have aroused the rising attention on battery thermal safety issues [1]. Although the BTMS based on PCM and liquid  [15]; (c) schematic of BTMs combing liquid, PCM, and HVAC [15]; (d) direct refrigerant-based cooling system [31].

The Future Prospect of BTMS for NEVs
The fire hazards related to the battery system of NEVs have aroused the rising attention on battery thermal safety issues [1]. Although the BTMS based on PCM and liquid direct cooling has superior thermal protective performance for battery packs, the cost and the weight limits their application in NEVs.
In order to ensure the safe operation of battery systems, it is essential to develop a comprehensive thermal safety management system (TSMS). This system should be designed to detect potential battery failures before they occur and to provide emergency cooling and fire extinguishing measures in the event of a thermal runaway. Additionally, a thermal barrier should be implemented to prevent the spread of heat and flames to other parts of the system or surrounding environment.
To achieve these goals, the TSMS should incorporate advanced monitoring and diagnostic technologies, which can be used to detect abnormal temperature rises, internal pressure, and voltage, etc. Some sensors, such as fiber Bragg grating (FBG) and built-in flexible thin-film sensors, have been proposed [87].
In the event of a failure, emergency cooling measures should be activated to prevent further overheating and thermal runaway. This could include the use of cooling fans, liquid cooling systems, HVAC refrigerant systems, or liquid nitrogen/liquid argon/liquid argon/liquid carbon dioxide/R134a spray systems [13].
If a fire does occur, the thermal safety management system should be equipped with fire extinguishing capabilities that can quickly suppress the flames and prevent them from spreading. This could include the use of fire suppression agents, such as water or foam, or the activation of fire suppression systems built into the battery system itself [31].
Finally, a thermal barrier should be implemented to suppress the heat transfer between adjacent batteries during the TR process. Materials such as asbestos insulation, mica plates, and other composites are being considered as candidates for thermal barriers [88,89]. While a thermal barrier can provide excellent safety performance during abnormal operating conditions, it can also disrupt the original thermal conduction path of the battery thermal management system (BTMS). Therefore, finding a balance between heat transfer and thermal safety is an important issue that needs to be addressed.
Overall, the collaborative design of the BTMS and the TSMS is of paramount importance for the future of battery systems. These two systems can ensure the safe and stable operation of the battery system, which is critical for the continued growth and success of the NEVs industry.

Conclusions
NEVs are environment-friendly and energy-efficient to meet the demands of green energy conservation with the assistance of an energy storage device called a battery system. However, the reliability, safety, and efficiency of this system are severely affected by the operating temperature. BTMS plays a crucial role in mitigating thermal effects on LIBs, which improves temperature uniformity across the battery pack, increases batteries' lifetime, and improves the safety of the battery system.
In essence, there are three primary categories of BTMS: air-based, liquid-based, or PCM-based cooling systems. This paper provides a detailed review of the state-of-the-art BTMS for each category and proposes a strategy for integrating BTMS with VTMS. The air-based system has a simple structure, lightweight design, and is energy-saving, but its cooling efficiency is low, and the temperature consistency is poor. The improvement of air-based BTMS depends strongly on the air routes and the arrangement of cells in the pack. Three representative air duct designs (series ventilation, parallel ventilation, and axial air forced flow) are discussed meticulously. Liquid-based systems were widely deployed in NEVs owing to their high heat transfer efficiency and compactness. Although the directcontact mode has merits of higher heat transfer efficiency and simpler structure over the indirect-contact mode, the demand of chemical-physical property electrically insulating property for the cooling medium restricts its application. The indirect mode is popular for NEVs. Although PCM-based systems with low costs can provide a suitable operating temperature for batteries, the leakage of liquid phase material and poor conductivity are the sticking problems. Adding additive or frame work are considered. Nowadays, the biggest shortcoming of PCM is the deadweight which will reduce the energy density of the battery pack that is unfriendly to NEVs. Additionally, the new category integrating BTMS and VTMS is described, in which the core idea is utilizing the heat source of each subsystem wisely. The BTMS, engine thermal management, and HVAC should be used in unison. Revealing the synergy mechanism of multi-thermal management subsystems has great potential for better application in the NEVs field. In future work, the TSMS should be integrated with the BTMS to cut off the TR propagation of the battery pack, with the beautiful vison to manufacture safer, cleaner, and more efficient NEVs.
Author Contributions: W.L. wrote the paper; Y.Z., H.Z. and X.T. designed the structures of the paper; H.Z. and X.T. reviewed the paper. All authors have read and agreed to the published version of the manuscript.