Review of the Use of the Carbon-Based Phase Change Material Composites in Battery Thermal Management for Electric Vehicles

Abstract

With increasing attention to the environment issues, the replacement of traditional energy vehicles with new energy vehicles has gained support from more countries. Lithium battery is an energy storage component of an electric vehicle and hybrid vehicle. Due to the nature that lithium batteries are very sensitive to operating temperature, a battery thermal management system is required to improve its efficiency and life. This chapter discusses the technology of phase change materials in battery thermal management systems and reviews the performance characteristics and thermal properties of various types of lithium-ion batteries. Furthermore, it summarizes the breakthroughs and bottlenecks of carbon-based phase change material composites used in the lithium battery heat dissipation systems.

1. Introduction

With the rapid development of the global economy, there is an increasing environmental problem caused by traditional fossil energy. Not only do the carbon dioxide and other harmful gasses produced by burning fossil fuels lead to the damage of the environment and the rise of global temperature, but they result in long-term risks to human health [1]. Thus, after the 2015 United Nations Climate Change Conference, most countries have reached a consensus to replace traditional energy with new one, reducing carbon emission. According to bp Statistical Review of World Energy 2021, the consumption of primary energy decreased by 4.5% worldly and the carbon emissions reduced by 6.3%, which was the largest drop since 1945 [2]. Due to the fact that transportation is one of the major causes of high carbon emissions, many countries have made a plan for a total ban on the sale of traditional fuel vehicles [3]. For example, the UK attempts to achieve zero carbon emissions of all new vehicles by 1940; France will implement a policy regarding a ban of traditional fuel and hybrid vehicle sales; India plans to inhibit the sales of petroleum fuel vehicles in 2030. Besides these countries, Spain, Norway, and the Netherlands have already set up a plan for banning the sale of traditional vehicles. Because the sales of traditional fuel vehicles are limited, the recent focus has shifted to new energy vehicles. Based on the Electric Vehicles Outlook 2021 published by BloombergNEF, passenger electric vehicle (EV) sales reached 3.1 million in 2020 and the sales of EV will continue to rise increasingly in the future, it is likely to reach 4 million in 2025, for instance [4].

At present, the development of new energy vehicles mainly focuses on electric vehicles. Based on the power source, electric vehicles are divided into three categories, battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and fuel cell vehicles (FCEVs) [5]. BEV technology uses a battery to drive an electric motor to generate power. Because there is no intervention of the internal combustion engine, it creates an environment where harmful gasses have zero emission during the working process. The increasingly popular kinetic energy recovery system (KERS) can use the brakes to charge the battery during driving, thereby expanding the drive range. HEVs rely on the work of both internal combustion engines and electric motors. HEVs have smaller engines than conventional cars, so it provides better fuel economy, effectively reducing the emissions of harmful gasses. FCEVs have not started in most countries due to some problems such as their high cost and immature technology. In the U.S., for example, the total sales of hydrogen FCEVs declined 12% to 2089 in 2019 [6].

Previous research and investigation showed that the increase in EVs, HEVs, and necessary power auxiliary facilities established on the basis of related policies can effectively reduce the emissions of harmful gasses and greenhouse gasses [7, 8]. However, the battery’s limited temperature range and endurance has hindered the development of EVs and HEVs, which can only be realized by effectively improving the performance of battery pack and a large part of the improvement of the battery performance depends on thermal management.

2. Lithium batteries

Electric energy storage system (storage battery) is the energy storage component of EVs and HEVs, which determines the driving range of a vehicle. While using, storage batteries experience thousands of times of charging and discharging. Different types of batteries have various performances in energy density, battery life, and heat release.

EVs and HEVs have many types of batteries to choose from, such as nickel cadmium batteries, lead-acid batteries, and lithium batteries. Among them, lithium battery has higher energy density and stronger power than the others, which is why it has become the best choice of EVs and HEVs energy storage spare parts [9].

In respect of the low cost, cruising range, and safety requirements of EVs, lithium batteries are constantly innovating and developing. Current lithium battery cathode materials mainly contain lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel cobalt manganese oxide (LiNiMnCoO2 or NMC), lithium iron phosphate (LiFePO4), etc. LiCoO2 is gradually being replaced by LiMn2O4 because of its short life, low thermal stability, and limited load. LiMn2O4 battery has a three-dimensional spinel structure, so it has the advantages of high stability and high safety. However, the disadvantage is that the charge and discharge cycle life is limited. Therefore, LiCoO2 and LiMn2O4 batteries are no longer used in the mainstream electric vehicle field. NMC batteries are composed of three active metals, nickel, cobalt, and manganese, mixed in different proportions to achieve large capacity, high load, and long life. It also has good overall performance and the lowest self-heating rate in the current battery technology, making it suitable as an energy storage unit for EVs. LiFePO4 battery has been used as a substitute for lead-acid battery for a long time and has become the energy storage unit of EVs. LiFePO4 has good electrochemical performance and low resistance. Its advantages include good thermal stability and safety, and long cycle of charge discharge under high rated current. Lithium titanate battery replaces the graphite in the anode of a typical lithium-ion battery and forms a spinel structure. The cathode of the battery can be LiMn2O4 or NMC. Lithium titanate batteries have outstanding performance in terms of safety, low temperature and longevity, but the disadvantage is the high cost and low capacitance. The properties of lithium batteries with different cathode materials are illustrated in Table 1 [10, 11]. The types of anode materials are mainly divided into two categories, carbon-based and non-carbon-based. Carbon-based materials include graphite, carbon fiber, carbon nanotubes, graphene, etc. and non-carbon-based materials primarily include nitrides, tin-based materials, silicon-based materials, etc. Silicon-based and carbon-based materials are now considered to be the most promising anode materials.

Battery types	Specific energy (electric capacity) (Wh/Kg)	Charge rate (C)	Discharge rate (C)	Cycle life	Thermal runaway (°C)
LiCoO2	150–200	0.7–1	1	500–1000	150
LiMn2O4	100–150	0.7–1	1	300–700	250
LiNiMnCoO2	150–220	0.7–1	1	1000–2000	210
LiFePO4	90–120	1	1	1000–2000	270
LiNiCoAlO2	200–260	0.7	1	500	150
Li4Ti5O12	50–80	1–5	10	3000–7000	—

Table 1.

Properties of lithium batteries with different cathode materials [10, 11].

When the actual vehicle is running, the operating temperature range of the battery pack is much wider than the optimal temperature range. The charging and discharging of the battery generates a lot of heat [12]. The temperature being too high or too low will affect the efficiency and life of lithium battery charging and discharging cycles and cause safety issues [13]. The battery pack is composed of several lithium batteries in series and parallel. Due to the uneven heat dissipation inside, the temperature of a certain area will be higher than the others, resulting in differences in the work efficiency and lifespan of individual lithium batteries inside the battery pack. Therefore, the battery thermal management system is an important part of the whole battery power system. It uses air cooling, liquid cooling, phase change materials, and other technologies to control the internal temperature of the battery pack under the strategy of the electronic control unit, so as to improve the working efficiency of the battery pack and prolong its operation.

3. PCM composites

The preparation of carbon-based PCM composite is divided into four steps, shown as the diagram in Figure 1 [14]. To start with, myristic acid (MA) is heated above 70°C and melted completely for 10minutes [14]. Subsequently, carbon-based nanoparticles are added to MA while magnetic stirring for 30 minutes. Following this, the material needs ultrasonic vibration for 1 hour. After cooling down to room temperature, carbon-based PCM composite is prepared.

Figure 1.

Semantic diagram of sample preparation process [14].

Reviewing the characteristics of the material mainly requires morphology analysis and chemical interaction examination. Specifically, the Scanning Electron Machine (SEM) is used to observe the microstructures of carbon-based PCM composite and Fourier transform infrared (FTIR) spectroscopy is employed to investigate the interaction between carbon additives and PCM. Thermophysical properties of the composite such as thermal conductivity is measured by thermal conductivity meter TC 300E. The relationship between enthalpy and temperature is explored by Differential Scanning Calorimetry (DSC) and the relationship between materials mass and temperature is examined via thermo gravimetric analysis.

Compared with the traditional air cooling system and liquid cooling system, the phase change material has the advantages such as simple structure, uniform temperature control, and no additional energy consumption. Its principle is to use the phase change process of the material at a specific temperature to absorb the heat energy generated by the battery in order to control the temperature. During the phase transition, though the temperature is constant or the temperature change range is small, it can absorb or release a large amount of latent heat [15]. Because the melting point of PCM is almost equal to the operating temperature, how to choose PCMs depends on the operating temperature of the lithium battery placed in the battery pack. The phase change temperature of PCMs that are generally used for battery thermal management is mainly concentrated at 30–50°C, and the temperature above 65°C or below 0°C has obvious adverse effects on its performance [16]. As the power of the battery pack increases, the volume and calorific value of the battery also rise, so the phase change material also needs to possess higher thermal conductivity and higher latent heat per unit mass to improve the optimal temperature control time. Moreover, PCMs should have other properties, such as the stable thermal cycle life, low density, safe, and non-toxic and inert chemical components, etc. PCMs can be roughly divided into two categories, organic materials and inorganic materials. Table 2 shows the advantages and disadvantages of the two types of PCMs [17].

	Pros	Cons
Organic PCMs	Wide distribution of phase transition temperatures	Low latent heat of phase per unit volume
	No supercooling and phase separation	Low thermal conductivity in solid state
	Chemically and thermally stable	Volume changes during phase transition
	Good compatibility with traditional materials
Inorganic PCMs	High latent heat of phase per unit volume	Severe supercooling problems and prone to phase separation
	High thermal conductivity	Corrosive and thermally unstable
	No volume change during phase transition

Table 2.

The advantages and disadvantages of the two types of PCMs [17].

El Idi et al. focused their study on the topic concerning Li-ion cell thermal behavior and the optimization of a passive thermal management system utilizing a PCM-Metal Foam composite [18]. The material they used involved an aluminum foam/paraffin RT27 composite and Li-ion 18,650 cell, which was kept below 27°C. To test the efficiency of the passive thermal management system, they developed a two-dimensional numerical model, building on the enthalpy-porosity model and non-equilibrium equation. According to the results, they concluded that several factors including current rate, duration charge and discharge cycle, and ambient temperature influence the temperature of the cell surface. Although in the solid–liquid phase change stage, the PCM absorbed the heat produced by the cell in latent form, its efficiency was restricted by PCM’s low thermal conductivity. To increase the efficiency of the thermal management of a Li-ion cell, an aluminum foam can be added. They also pointed out that the temperature of the cell surface was not significantly affected by an excessive amount of PCM added.

Heyhat et al. compared different composites used in the battery thermal management system, including PCM and porous metal foam, PCM and nanoparticles, and PCM and fin [19]. The PCM used was n-eicosane and its properties are shown in Table 3. From the table, it could be said that the melting temperature of n-eicosane was 309.55 K. The initial temperature of the battery was 298.15 K was reasonably assumed. The metal foam, nanoparticles, and fins were copper. In 4.6 W and 9.2 W heat generation rates, adding nanoparticles to PCM insignificantly affected the performance of the system. Contrasting nanoparticles with fins somehow improved the performance with three fins performing better than 5 fins. Among these three composites, the combination of metal foam and PCM performed more efficiently, resulting in the decline of battery mean temperature by 4 to 6 K.

	Density (g cm^−3)	Latent heat of fusion (kJ/kg)	Melting point (K)	Molecular weight (g/mol)	Specific heat capacity (J/(kg∙K))	Thermal conductivity (W m^−1 K^−1)	Thermal expansion coefficient (1/K)
Solid n-eicosane (298.15 K)	910	248	309.55	282.54	1926	0.423	—
Liquid n-eicosane (at 323.15 K)	769	—	—	282.54	2400	0.146	8.161 × 10–4

Table 3.

Properties of solid and liquid n-eicosane [19].

Karimi et al. studied the performance of thermal battery management systems after adding metal matrix and metal nanoparticles including Cu, Ag, and Fe3O4 to paraffin composite PCM [20]. The results showed that the addition of both metal matrix and metal nanoparticles improved thermal conductivity. Among these metal nanoparticles, Ag nanoparticles had the best performance. Particularly, it had the lowest body temperature, while metal matrix-PCM composite achieved the least temperature difference.

Jilte et al. used nano-enhanced PCM arranged in trans-radial and trans-axial multi-layer to enhance heat transfer [21]. The trans-radial configuration was a cell in two coaxial cylindrical containers, where PCM1 and PCM2 were filled in the containers adjacent to each cell respectively, whereas equal volume PCM1 and PCM2 were arranged in trans-axial configuration. To accommodate the same volume of PCM1 and PCM2, the trans-axial structure was preferred for battery modules. Two different arrangements were employed in this study, a two-layer 7 × 7 × 1 arrangement consisted of seven battery cells, seven nePCM1 containers, and one nePCM2 container, and a two-layer 7 × 1 × 1 arrangement comprised of seven cells, one nePCM1 container, and one nePCM2 container, as shown in Figure 2. In order to improve thermal conductivity, nanoparticles Al2O3 were doped with PCM1 Na2SO4∙H2O and PCM2 eicosane. Their thermophysical properties are illustrated in Table 4. As shown in the table, under the same melting temperature, Na2SO4∙H2O and eicosane had high thermal conductivity, which were preferred in the experiment. Based on the experiment conducted under the condition where the battery was at a 3C discharge rate and the ambient temperature was 30°C, they found that the 7 × 7 × 1 configuration was more efficient compared to 7 × 1 × 1 arrangement for improved battery cooling and the container next to the battery surfaces should be filled with nePCMs with a smaller value of melting temperature.

Figure 2.

7 × 7 × 1 and 7 × 1 × 1 arrangement [21].

Base PCM	Nanoparticles	Composite PCM	melting temperature (°C)	Latent heat (J/kg)	Specific heat (J/kgK)	Thermal conductivity (W/mK)	Density	Nanoparticles diameter (nm)
Disodium sulfate decahydrate(Na2SO4∙H2O)			36.4	254	1930	0.544	1485
Eicosane			36.4	241	1900	0.27	778
	Aluminum oxide (Al2O3)				765	36	3600	59

Table 4.

Thermophysical properties of PCM and nanoparticles [21].

Lv et al. developed a novel material, a nanosilica (NS)-enhanced composite PCM (CPCM-NS), to overcome the obstacles such as PCM leakage and volume changes presented by many PCM cooling technologies [22]. This kind of composite is anti-leakage and anti-volume-change as NS has nanoscale pores that absorb liquid paraffin, which prevents PCM leakage and diminish volume change. CPCM-NS was prepared through melting paraffin in an oil bath at 85°C for an hour and adding different amounts of NS, 7 wt% EG, and 30 wt% low density polyethylene to the paraffin respectively. The entire preparation process is shown in the form of a diagram in Figure 3. It could be concluded from the results that CPCM-NS helped the battery achieve better cooling efficiency and durability. At the first, second, fourth, sixth, and eighth charge and discharge cycle, the maximum temperatures of CPCM-NS with 5.5 wt% of NS reached were 1.6, 2.4, 4.5, 5.3, and 5.9 lower than CPCM without NS. For the remaining cycles, the temperature difference was 6.22 ± 0.05°C.

Figure 3.

Schematic diagram for the preparation of NS-enhanced CPCMs [22].

Wei et al. chose cellulose nanocrystals (CNCs), graphene nanoplatelets (GNPs), and polyethylene glycol (PEG) as their PCM composite for GNPs offered thermally conductive path and CNCs restricted the leakage of PEG in the phase change process [23]. 8 wt% CNC was added to the composite, while four different types of GNP were added, 0.5 wt%, 1 wt%, 2 wt%, and 4 wt%. When GNPs were at 4 wt%, the enthalpy of the composite was 145.4 J/g and the thermal conductivity was 2.018 + −0.067 W/m K, proving that CNC enhanced the composite’s enthalpy even when the thermal conductivity remained high. Additionally, this type of composite displayed outstanding light to heat and electricity to heat energy conversion abilities based on the fact that the composite had high latent heat and retention rate.

Zhang et al. utilized kaolin, EG, and paraffin as PCM composite [24]. Among these three materials, kaolin and EG had different percentages. Their properties are shown in Table 5. The latent heat of these materials did not differentiate greatly from one to another considering the invariance of the mass fraction of paraffin. It was found that the bigger amount of EG is needed for a higher thermal conductivity of the composite and K20 and K15E were not desirable materials for they showed paraffin leakage. Despite the same thermal conductivity it had as K10E10, K8E12 was not selected due to the high cost and inadequate availability. Thus, K10E10 was used as the suitable material. The results indicated that the maximum temperature decreased by 13.4% at 2C discharge rate, 20.76% at 3C, and 27.74% at 4C under 26–28°C. The temperature difference was maintained at 4.04°C at 4C discharge rate, lower than the temperate difference when PCM was not used.

	Melting onset (°C)	Peak temperature (°C)	Latent heat (J/g)
K20 (20 wt% kaolin)	39.08	42.26	158.14
K15E5 (15 wt% kaolin, 5 wt% EG)	37.66	42.15	160.57
K12E8 (12 wt% kaolin, 8 wt% EG)	37.72	41.81	162.66
K10E10 (10 wt% kaolin, 10 wt% EG)	37.87	41.64	165.21
K8E12 (8 wt% kaolin, 12 wt% EG)	38.04	41.51	169.55

Table 5.

Properties of kaolin, EG, and paraffin composite [24].

Lv et al. created serpentine CPCM (S-CPCM) plates to enhance secondary heat dissipation capability [25]. In doing so, the S-CPCM plates provide a larger surface area and more air flow channels. The S-CPCM plates utilized 5 wt% of expanded graphite and low-density polyethylene (LDPE) of differentiated wt%, ranging from 0 to 25wt%, which was added to the resultant CPCM. The properties including, the onset melting temperature, latent heat, and bending strength are indicated in Table 6. Based on the properties, S-CPCM-25% was eventually chosen for constructing the S-CPCM module for that S-CPCM-25% had suitable onset melting temperature, thermal conductivity, latent heat, and bending strength compared to S-CPCM of other types. The results from the experiment demonstrated that the S-CPCM structure effectively reduced the weight of the CPCM module by approximately 70% and provided larger heat exchange surface and more air convection channels, implying that the S-CPCM module performed better in heat dissipation than the traditional CPCM module. The positive aspect of the S-CPCM module provided a potential PCM cooling structure for lightweight battery modules.

Base PCM	Nanoparticles	Composite PCM	Melting temperature (°C)	Latent heat (J/kg)
The PCM composed by hexadecane stearic acid and paraffin(11:1)			49.6	187.6
		S-CPCM+0% (low-density polyethylene)	49.6	172
		S-CPCM+3.5% (low-density polyethylene+3.5%Expanded graphite)	49.8	157.1
		S-CPCM+10.5% (low-density polyethylene+10.5%Expanded graphite)	50.2	142.2
		S-CPCM+17.5% (low-density polyethylene+17.5%Expanded graphite)	50.1	130.1
		S-CPCM+25% (low-density polyethylene+25%Expanded graphite)	50.3	119.4

Table 6.

Properties of S-CPCM of different composite [25].

Ling et al. applied a 60 wt% RT44HC/expanded graphite (EG) composite and a 60 wt% RT44HC/fumed silica composite to investigate the performance of these two materials at 5 and −10°C [26]. RT44HC is an organic PCM produced by Rubitherm Technologies GmbH (Rubitherm, n.d.). Its properties and the two composite materials’ properties are included in Table 7, which showed similar phase change temperature and phase change enthalpy of the composites, but different thermal conductivity of them. The battery pack is composed of 20 cells and discharged at 0.5C, 1C, 1,5C, and 2C over 20 charge–discharge cycles. It was found that PCMs were effective in keeping the battery pack from cooling too fast. The RT44HC/fumed silica composite was superior in extending the cooling period to the RT44HC/EG composite. However, it was not suitable for a multi-cell battery pack for its low thermal conductivity contributed to a higher than 12°C temperature difference and a high voltage difference between battery cells. The RT44HC/EG composite was also successful in inhibiting battery overheat. They found the maximum temperature difference was 6°C. Furthermore, the RT44HC/EG composite improved temperature uniformity, further reducing the voltage differences.

Base PCM	Composite PCMs	Phase change temperature (°C)	Specific heat capacity (J/g/°C)	Thermal conductivity (W/(m K))	Phase change enthalpy (J/g)
RT44HC		43.1	2.5	0.24	232.0
	60 wt% RT44HC/EG	42.8	2.4	9.57	134.3
	60 wt% RT44HC/fumed silica	41.5	0.8	0.18	133.4

Table 7.

Properties of RT44HC, 60 wt% RT44HC/EG composite, and 60 wt% RT44HC/fumed silica composite [26].

Hussain et al. combined paraffin, chosen as their PCM, with graphene coated nickel (GcN) foam to study thermal management systems [27]. Four other materials were also compared including nickel foam, paraffin wax, GcN foam, and nickel foam saturated with paraffin. Among these materials, under 1.7A discharge current, GcN foam decreased the temperature rise of battery surface 17% compared to nickel foam. This novel material also improved the thermal conductivity. The results showed that the thermal conductivity of paraffin was improved by 23 times whereas that was improved by 6 times when using the nickel foam. Figure 4 evidently showcases these results. Moreover, compared to pure paraffin, GcN foam’s latent heat declined by 30% and its specific heat declined by 34%.

Figure 4.

Thermal conductivity (at 25°C) and latent heat paraffin composites [27].

Wu et al. developed a battery thermal management system using thermally induced flexible composite PCM (FCPCM) [28]. FCPCM is composed of paraffin, olefin block copolymer (OBC), and EG. OBC and EG were mixed with paraffin correspondingly and their properties are illustrated in Table 8. Their properties suggested a good compatibility of these three materials as their enthalpy was close to the theoretical values. Since the storage modulus decreased from 1081 to 63.9 MPa when temperature rose from 25–60°C, FCPCM has good compatibility and flexibility. When the battery is discharged to 0%, the temperature of FCPCM reaches 43.4°C at 2.5C, 28.8°C lower compared to PCM were not used. This being said, the battery thermal management system with FCPCM performed well at thermal control, which was a result of low thermal contact resistance between the battery and FCPCM. Additionally, the results also suggested that FCPCM had lower temperature difference within the acceptable range and long-time function of latent heat.

	Phase change temperature (°C)	Thermal conductivity (W m^−1 K^−1)	Phase change enthalpy (J kg^−1)
PA and OBC	39.82	0.45	200.6
PA, OBC, and EG	39.50	2.34	185.4

Table 8.

Properties of PA and OBC composite and PA, OBC, and EG composite [28].

Miers and Marconnet designed PCM heat sinks with three kinds of PCM composites for enhanced passive thermal management [29]. The PCMs were chosen based on their thermophysical properties such as melting temperature, thermal conductivity, and heat capacity. Eventually, PT42 supplied by PureTemp, PT 68 supplied by PureTemp, and S70 supplied by PlusICE were selected by PCM Products Ltd., n.d.; PureTemp LLC, n.d.). These materials’ properties are shown in Table 9. In the table, an important property called figure of merit (FoMq) was included for it indicated the storage potential and the easiness of heat to be added or removed when selecting the PCM. It can be seen that S70 had the highest FoMq, suggesting it might be a more suitable material for the PCM. Based on the results, it was concluded that the isokite package design and S70 performed the best among other composites. In particular, at 7.5 W cm⁻² heat flux, this design extended the time spent on reaching 95°C by 36.2% compared to S70 with a solid package. Furthermore, the weight of this design was 17.3% less than the solid aluminum package.

	Melting temperature (°C)	Figure of merit (×103W^2∙s/m4∙K)	Latent heat of fusion (kJ/kg)	Density (kg/m3)	Specific heat (liquid: kJ/kg; solid: kJ/kg∙K)	Thermal conductivity (W/m∙K)	Sensible heat storage per unit mass (∆Esens, 20 → 90°C, kJ/kg)
PT42 (organic)	42	27.80	218	Liquid: 850; Solid: 940	Liquid: 2.12; Solid: 1.98	Liquid: 0.15; Solid: 0.25	155.92
PT68 (organic)	68	28.71	220	Liquid: 870; Solid: 960	Liquid: 2.38; Solid: 2.04	Liquid: 0.15; Solid: 0.25	162.18
S70 (inorganic)	70	105.34	110	Liquid: 1680; Solid: –	Liquid: 2.10; Solid: –	Liquid: 0.57; Solid: –	157.50

Table 9.

Properties of PT42, PT68, and S70 [29].

Zhang et al. developed an innovative flame-retarded composite comprised of paraffin, EG, ammonium polyphosphate (APP), red phosphorus (RP), and epoxy resin (ER) [30]. At a 3C discharge rate under 25°C, the composite reduced the peak temperature by 44.7% and 30.1% and controlled the maximum temperature difference within 1.36°C. Under high temperature of 45°C, the temperature uniformity was maintained within 5 °C. These results suggested that this novel composite PCM had outstanding thermophysical properties, improving the effectiveness of the thermal management system.

Huang et al. utilized Styrene butadiene styrene (SBS), paraffin, and aluminum nitride (AlN) as PCM composite to study the battery thermal management system [31]. Five types of composites with different ratios of paraffin, SBS, and AlN were compared. Based on the properties of different materials shown in Table 10, CPCM#3 was selected as it had better flexibility, smaller temperature difference and higher thermal conductivity, enhancing the temperature uniformity. Out of the three materials of the PCM composite, SBS has a unique structure to support the composite, AlN contributes to the stability and thermal conductivity of the composite, and paraffin has high latent heat. At 3C discharge rate, the maximum temperature reached was 42.4°C and the temperature difference was 9.2°C. Compared to other types of the composites, CPCM#3 helped the maximum temperature stay at 48.4°C at 3C discharge rate after nine cycles, and the temperate difference was 8.7°C, suggesting it had better heat dissipation performance and temperature uniformity.

	Thermal conductivity (W/m∙K)	Phase transition temperature (Tp) (°C)	Phase change enthalpy (∆H) (J/g)
CPCM#0 (50% paraffin, 50% SBS, and 0% AlN)	0.26	47.58	78.07
CPCM#1 (45% paraffin, 50% SBS, and 5% AlN)	0.38	48.32	76.83
CPCM#2 (40% paraffin, 50% SBS, and 10% AlN)	0.47	48.47	73.98
CPCM#3 (35% paraffin, 50% SBS, and 15% AlN)	0.51	46.82	57.06
CPCM#4 (30% paraffin, 50% SBS, and 20% AlN)	0.53	45.88	39.11

Table 10.

Properties of CPCM#0, CPCM#1, CPCM#2, CPCM#3, CPCM#4 [31].

Yan et al. focused specifically on the performance of paraffin and EG PCM composite [32]. Three types of paraffin wax of different phase change temperatures were used in the experiment and their properties are illustrated in Table 11. Their different properties gave rise to their different thermal performances. The results indicated that the PCM system had better cooling performance than the natural convection system especially when the cycling rate was high since the PCM composite absorbed a substantial amount of heat throughout the phase change period. The researchers also found out that RT45 had better performance in the dynamic cycling, so the optimal phase change temperature of PCM composite was 45°C. The cooling performance of the PCM composite could be further improved by increasing the laying-aside time.

	Phase change temperature (°C)	Thermal conductivity (W/m∙K)	Latent heat (kJ/kg)
RT36	36	0.37	21.4
RT45	45	0.22	111.3
RT58	58	0.16	163.3

Table 11.

Properties of RT36, RT45, and RT58 [32].

4. The combination of air cooling and liquid cooling

Malik el al. conducted a study to compare a thermal management system using phase change composite material with no cooling and liquid cooling system [33]. Graphene was combined with PCM to increase its thermal conductivity. The battery back consisted of LiFePO4 prismatic cells that were charged at 1C and discharged at 1C, 2C, 3C, and 4C. The results showed that the temperature gradient was significantly lower when the thermal management system contained phase change composite material compared to no cooling and liquid cooling. Specifically, the temperature declined by 20°C at a 4C discharge rate when 6 mm thick phase change composite plates were used. At a 1C discharge rate, the temperature decreased about 4°C, from 33.5°C to 29.1°C. These results implied that the thermal management system using phase change composite material could maintain the battery temperature in a certain range.

Yang et al. proposed a thermal model to evaluate the performance of the PCM/liquid combined cooling system with start-stop control [34]. To be specific, they calculated the battery heat generation by a semi-empirical equation and compared the cooling effects in different cooling schemes. Three factors including the PCM thickness, the channel width, and the coolant flow rate influenced the cooling effects. With respect to the thickness of PCM, due to the performance of huge latent heat and approximately isothermal phase change process, a reasonably thick PCM layer could improve the heat dissipation capacity of the cooling system and the temperature uniformity within the battery pack. The thickness of the PCM layer they chose was 1.5 mm for that not only could it enhance the cooling effects, but it could meet the premise of the energy density. The PCM/liquid coupled system they proposed was superior to the traditional liquid cooling system in terms of the start-stop frequency, the parasitic energy consumption, and the temperature control reliability. It reduced the start-stop frequency less than half, saving the power dissipation and reducing the pumping power consumption by at 60%. Additionally, the PCM layer had the early warning function, which allowed the PCM/liquid system to control the battery temperature better than the traditional liquid cooling scheme.

Akabarzadeh et al. proposed a new concept of an innovative liquid cooling plate for thermal management of Li-ion batteries by incorporating a phase change material inside [35]. The geometrical model of the plate is illustrated in Figure 5. The cooling plate the researchers utilized was hybrid as it offered active and passive cooling methods. In addition, the hybrid liquid cooling plate was 36% lighter than a traditional aluminum liquid cooling plate of equivalent volume and had the capability of slowing the temperature loss during the cold stop via heating solution. Through testing the prototype of the hybrid liquid cooling plate, it was found that compared to an aluminum plate, the hybrid liquid cooling plate lowered up to 30% of energy consumption of the pump for circulating the coolant. Not only did the hybrid liquid cooling plate increase the temperature uniformity, it could effectively reduce a fast temperature drop of the cooling plate, which further decreased the energy required for the active heating process after short-term parking. Therefore, the hybrid liquid cooling plate with light-weight structure can be a promising candidate for electric vehicle battery packs.

Figure 5.

Geometrical model of the liquid cooling plate [35].

Chen et al. developed a hybrid-PCM-liquid cooling system to study a Li-ion battery module under fast charging [36]. The experiment was conducted under 25°C and under 2C, 2.5C, and 3C fast charging rates, pure PCM, pure liquid, and hybrid-PCM-liquid cooling performance were investigated. Except for this, how the thickness of PCM and liquid coolant flow influenced the cooling effect was also analyzed. The results showed that temperature rise or distribution did not help improve each increment of PCM thickness and thick PCM affected the effectiveness of natural convection between the battery cell and the environment. Moreover, the attempt to achieve the optimization of thermal performance cannot be completed through the highest coolant flow rate and the thickest PCM. In contrast to pure PCM cooling systems and pure liquid cooling systems, hybrid-PCM-liquid cooling systems are capable of reaching temperature rise and distribution of acceptable range. Particularly, under 3C fast charging, 0.65 mm PCM and 54 mL/min coolant flow rate could enable the hybrid cooling system to achieve optimization and control the energy consumption within a suitable range.

Cao et al. combined PCM with delayed liquid cooling technology to achieve high temperature uniformity during high discharge rate [37]. The results showed that the delayed liquid cooling strategy decreased the temperature and temperature difference. It could even save energy consumption by shortening the period of liquid cooling. At 4C discharge rate with a cooling flow rate of 40 L/h, this design performed better than traditional strategy with continued liquid cooling.

Zhang et al. created a battery thermal management system with the combination of PCM and liquid cooling, taking advantage of the heat dissipation function this hybrid system presented [38]. They found that the PCM prevented thermal runaway diffusion under extreme conditions and the liquid cooling system effectively transmitted the heat PCM absorbed.

Wang et al. used different wt% OP28E nano-emulsions for a liquid cooling thermal management system. The results showed that the 10% OP28E nano-emulsion performed better than water and the increased mass fraction of OP28E resulted in decreased maximum temperature and maximum temperature difference [39]. Moreover, these two features were also related to the increase in coolant flow rate.

Fan et al. studied air cooling systems by adding metal fins to the PCM systems to enhance thermal control of the battery [40]. They found that the application of fins increased the working time of the battery by 98.4% compared to the battery-PCM system. Even under circumstances where the ambient temperature was high, the safe operation time of the system improved by 1.48 times for 20°C, 1.49 times for 30°C, and 1.81 times for 40°C.

Lv et al. employed EG, paraffin, and low density polyethylene (LDPE) coupled with low fins to improve the issues of traditional battery thermal management regarding PCM leakage and low surface heat transfer capability [41]. The LDPE-enhanced composite PCM (L-CPCM) was prepared via melting paraffin in an oil bath at 60°C for half an hour and adding EG, LDPE to the paraffin. Their properties are shown in Table 12. It was found that the thermal conductivity of L-CPCM is much higher than that of paraffin and its latent heat decreased greatly with the existence of EG and LDPE. Not only does L-CPCM have better bending strength, impact strength, and Shore hardness, but it performs well at heat dissipation. At 3.5C discharge rate, the maximum temperature of L-CPCM coupled with fins could remain below 50°C and the temperature difference was 5°C, while that L-CPCM without fins stayed up to 52.6°C and the temperature difference was 5.7°C. Thus, L-CPCM with fins could enhance the surface heat transfer capability.

	Density (g cm^−3)	Thermal conductivity (W m^−1 K^−1)	Specific heat capacity (J g^−1 K^−1)	Latent heat (J g^−1)	Phase change temperature (°C)
Paraffin	0.910	0.16	2.68	182	44.0–50.2
EG	0.230	8.0	0.71	—	—
LDPE	0.925	0.30	2.30	—	—
L-CPCM	0.856	1.38	2.48	87.4	44.5–50.2

Table 12.

Properties of paraffin, EG, LDPE, and L-CPCM [41].

Jiang et al. designed a battery pack with EG and paraffin composite as PCM coupled with a forced air cooling system, which consisted of aluminum tubes, baffles and a shell [42]. The results indicated that the EG and paraffin composite reduced the temperature rising at 5C discharge rate and maintained the maximum temperature difference within 1 to 2 °C. The baffles applied improved the efficiency of heat transfer and the interaction of fluid by changing the air fluid flow direction.

Wu et al. studied the performance of PCM coupled with an air cooling system by using a heat pipe [43]. The forced art convection allowed the highest temperature to reach below 50°C at 5C discharge rate and the temperature difference to vary less. Moreover, the addition of a heat pipe significantly reduced the temperature compared to the usage of PCM only. Even after the first cycle, this module was able to maintain a stable stage with the same temperature profile as the first one.

Chen et al. investigated the influence of heat pipes and the thickness of PCM on the battery thermal management system [44]. They found that increased thickness of PCM and latent heat decreased the maximum temperature but increased the temperature difference. The heat dissipation could be improved via increasing the equivalent thermal conductivity of the heat pipe and reducing the temperature at start. The optimization of PCM’s thickness could effectively improve the performance of the system without expanding the system volume.

Huang et al. investigated the performance of three battery thermal management system modules, including pure PCM, PCM coupled with air cooling, and PCM coupled with liquid cooling [45]. The results showed better performance of PCM coupled with a liquid cooling system than the other two modules. It was especially able to maintain the highest temperature at 50°C at 3C discharge rate and a lower temperature difference, which indicated it had outstanding temperature control and balance.

5. Conclusion

This paper discusses the classification, advantages, and disadvantages of lithium battery technology for electric vehicles, as well as the current status and application prospects of BTMS technology for composite phase change materials. The following conclusions have been drawn:

• At present, liFePO4 batteries are currently the most suitable batteries for electric vehicles. Lithium titanate batteries have excellent performance in terms of safety, low temperature, service life, etc. If there is a breakthrough in the improvement of its permittivity in the future, it will become the most promising lithium-ion battery.

• The use of composite PCM is currently the most potential thermal management technology for batteries. The fusion of carbon nanotubes and other composite materials can solve the problem of low thermal conductivity and the combined cooling technology can solve the issues of latent heat and thermal conductivity. The technology regarding the fusion of nanoparticles, such as silica, has also brought progress in properties including anti-leakage and reduced volume changes in phase transitions. However, there is still room for improvement in the compatibility of composite materials and stability in cyclic testing.

• To improve the thermal conductivity of PCMs, carbon materials could be added. However, carbon-based PCM composites tend to have relatively low latent heat compared to PCMs. Therefore, factors such as the pore size and mass fraction should be taken into consideration when balancing the thermal conductivity and latent heat of carbon-based PCM composites.

• There are various ways to enhance the performance of the battery thermal management systems, for example, the use of PCM and heat pipe, and air or liquid cooling techniques. PCM, heat pipe, and liquid cooling techniques are more efficient than air-forced systems. On the other hand, liquid cooling is difficult to maintain and has a short life of only 3 to 5 years. In terms of the cost, the application of a heat pipe costs more than that of PCMs. That being said, PCM is a better choice to promote the performance of the battery management systems.

Abbreviations

AIN	Aluminum nitride
APP	Ammonium polyphosphate
BEVs	Battery electric vehicles
BTMS	Battery thermal management system
CNCs	Cellulose nanocrystals
EG	Expanded graphite
EVs	Electric vehicles
ER	Epoxy resin
FCEVs	Fuel cell vehicles
FCPCM	Flexible composite PCM
GNPs	Graphene nanoplatelets
HEVs	Hybrid electric vehicles
KERS	Kinetic energy recovery system
LDPE	Low density polyethylene
NS	Nanosilica
OBC	Olefin block copolymer
PCM	Phase change material
PEG	Polyethylene glycol
RP	Red phosphorus
SBS	Styrene butadiene styrene