Interest in electric-driven vehicles like hybrid electric vehicles, plug-in hybrid electric vehicles and battery electric vehicles has undoubtedly increased in recent years, following the intensification of environmental regulations on greenhouse gas emissions ^[1][2][3][1,2,3]. The present lithium-ion (Li-ion) batteries, with their large specific energy, high specific power, low self-discharge rate, high voltage, relatively long life and good recyclability, have been widely used in electric vehicles (EVs) and are regarded as the most suitable energy storage device for electric-driven vehicles ^[4][5][4,5]. However, operating and even storage temperature will affect the performance of Li-ion batteries [6]. Studies have shown that temperature is the main factor influencing battery aging with a negative impact on capacity and internal resistance as well [7]. Additionally, an uneven temperature distribution inside a battery pack can lead to electrically unbalanced cells, and an electric imbalance leads to a capacity loss of the entire battery pack, and to the overcharge of the affected cells during charging, resulting in power losses and increased temperatures ^[8][9][8,9].

Most of the temperature effects are related to chemical reactions occurring in the batteries and also materials used in the batteries [10]. The operational temperature range of EV batteries system commonly varies from −40 °C to 60 °C ^[11][12][11,12]. However, it is generally accepted that to obtain the best performances, the preferred working temperature of EV batteries ranges from 15 °C to 35 °C ^{[6][13][14][15]}[6,13,14,15].

Low temperatures typically heavily reduce the power and energy capacity of the batteries and increase their internal impedance ^[16][18]. It has been measured that at −20 °C the increase of internal resistance causes a 60% drop in capacity ^[17][19]. Thus, a significant challenge is the charge and discharge rate in subzero operating temperatures.

High temperatures increase the reaction rate, which results in the delivery of higher power and in an improved capacity, but in the same time leads to even higher temperatures and an increase in thermal load ^[18][23]. Moreover, an additional problem caused by high temperatures may be the capacity fading because of the electrolyte decomposition and the non-uniformity of the passivation layer ^[19][24]. If heat is not dissipated at least at the same rate at which it is generated in the batteries, temperatures may increase uncontrollably, resulting in the disintegration of materials and components ^[20][25] or even in the thermal runaway of the batteries ^[21][26]. Thermal runaway is an incident that leads to a sudden temperature rise, gas generation and even explosion of the battery ^[22][27], putting to risk the safety of the vehicle and its passengers ^[23][28].

A non-uniform temperature distribution inside a single cell leads to a variable rate of electrochemical reactions in different zones of the cell and therefore to a partial energy utilization and a reduced battery lifetime ^[24][29]. Studies show that a temperature difference of more than 5 °C inside a battery cell results in a 25% increase in thermal aging and a 10% reduction in power capacity ^[25][26][27][30,31,32]. At a battery pack level, it is common and normal for differences between cells to develop, regarding their capacity, voltage and internal resistance ^[28][33]. These variations result in different thermal behaviors and therefore in a thermal gradient across the battery pack ^[29][34]. A 5 °C temperature difference can cause a capacity reduction of 1.5%–2% of the battery pack ^[30][35], as well as a power capability reduction of 10% ^[31][36]. Therefore, the design of efficient battery thermal management systems (BTMS) is necessary to maintain the battery temperatures in the desired range and to reduce as much as possible the temperature non-uniformity inside the battery pack ^[32][37].

23. Battery Thermal Management Systems

2.1. Air-Based BTMS

3.1. Air-Based BTMS

The advantages of these systems are the low cost, simplicity, light extra weight ^[33][34][56,57], and despite their low-conductivity and low-efficiency ^[35][58], they are still studied by researchers and used in electric-driven vehicles. A first classification of air-based systems can be made considering whether it is natural or forced convection. Despite its simple structure, the natural method creates large thermal gradients in the battery pack and is highly dependent on ambient conditions ^[36][59]. Therefore, a forced-air system is necessary for most applications, despite its higher costs and additional power consumption. A second classification of air-based systems is based on the source of the air, which can be external, cabin pre-conditioned or pre-conditioned by an auxiliary heat exchanger [6].

2.2. Liquid-Based BTMS

3.2. Liquid-Based BTMS

Liquid-cooled systems are the most used in practical applications ^[37][39], due to their high efficiency and compactness. Liquid possesses good heat capacity and heat transfer coefficient, representing a suitable solution for the BTM of EVs ^[38][68]. Compared to an air-cooling system, 2–3 times less energy is needed to maintain the same average temperature of the batteries ^[39][69]. The disadvantages are the increased complexity, added weight, extra costs, rigidity of the structure ^[40][41][55,70]. Moreover, a significant additional power is required to circulate the fluid at the desired flow rate, increasing the overall energy consumption of the vehicle ^[42][71]. Liquid-cooling is easily applicable for prismatic batteries due their regular shape, allowing a compact arrangement of the BTMS, while cylindrical batteries require special structures to integrate the cooling channels, increasing the system’s complexity and mass. The most common solution for prismatic and pouch batteries is the implementation of cooling plates. General investigations analyze the temperature and velocity distribution inside the cooling plates ^[43][72], and several studies ^[44][45][46][73,74,75] investigate by means of orthogonal test the influence of different parameters, such as liquid inlet temperature, cooling plate width, mass flow rate or number of channels. Shang et al. ^[44][73] obtained an improvement of 12.61% compared to the maximum temperature without the orthogonal optimization. Ye et al. ^[45][74] achieved a lowering of maximum temperature by 5.24% and of the pressure drop by 16.88%. The sensitivity analysis of the orthogonal test also showed that the greatest influence on the pressure loss is caused by the center channel distance and the size of the inlet plenum. The numerical calculations conducted by Monika et al. ^[46][75] showed that for cold plates placed between the batteries, a parallel flow arrangement with the inlet placed near the tab region represents the best solution. The authors found that a 5-channeled cold plate of 4 mm width represents the best trade-off between heat transfer and pressure drop. Patil et al. ^[47][76] conducted a parametric study to evaluate the thermal performance of different U-turn type microchannel cold plates. The results suggest that a surface area coverage ratio of 0.75 and a hydraulic diameter of the channels of 1.54 mm offer the best cooling capacity. The numerical simulations also show that alternating single inlet and outlet channels decrease the maximum temperature and improve temperature uniformity.

2.3. PCM-Based BTMS

3.3. PCM-Based BTMS

Phase change materials represent an interesting alternative to the conventional forced-air and liquid cooling systems, because it could allow the implementation of a passive BTMS. It is applied especially for cylindrical batteries, because it allows the filling of the unused spaces between the batteries. PCMs have high latent heat capacity and are able to maintain the temperature of the batteries in a small range, close to the melting temperature of the PCM ^[48][89]. The melting point is in the 5.5–76 °C temperature range and the phase change process is usually not isotherm [9]. In case of continuous charge and discharge cycles at high rates, there is the risk of the PCM melting completely, leading to thermal instability and leakage of the PCM. Moreover, the melting of the PCM automatically increases its volume, which must be considered when designing a PCM-based BTMS. Another disadvantage of PCMs is their low thermal conductivity, resulting in a difficult heat dissipation. Numerous studies addressed the problem of low thermal conductivity. Azizi et al. ^[49][90] constructed a BTMS made of PCM and aluminum wire mesh plates as a thermal conductivity enhancer. The wire mesh plates with high voidage values are preferred over the aluminum foams for a better filling of the pores during the phase change of the PCM. Zhang et al. ^[50][91] study the influence of different concentrations of aluminum nitrite (AlN)-based CPCMs. The measurements showed that increasing the AlN mass fraction improved thermal conductivity from 1.48 W/mK to 4.331 W/mK at 50 °C, as well as the tensile strength, bending strength and impact strength. Zhang et al. ^[51][92] designed and experimentally investigated a BTMS using copper metal foam saturated PCM. For a 5 C discharge rate the maximum temperature was lowered to 47.86 °C compared to 54.115 °C in the case of pure PCM. The temperature non-uniformity increasing rate also showed smaller values.

2.4. Heat Pipe-Based BTMS

3.4. Heat Pipe-Based BTMS

Heat pipes are heat transfer devices with extremely high thermal conductivity, 90 times higher than copper ^[52][111], ensuring better heat dissipation and temperature uniformity. They represent a passive, compact, lightweight BTMS, with no need of maintenance and long life cycle ^[53][112]. The working principle consists in the phase change of the internal working fluid, which absorbs heat at the evaporation section and releases it at the condensation section. As long as there is a temperature gradient between the two sections, the transfer of heat goes on, without any power consumption. Despite its very high thermal conductivity however, in most applications heat pipe is coupled with forced air or liquid cooling for a more effective heat dissipation at the condensation section. Heat pipe-based BTMSs are studied mostly for prismatic and pouch batteries due to the adaptability of the shapes and the large available contact areas, while in case of cylindrical batteries special attention needs to addressed to the contact surface between heat pipe and batteries. Zhang et al. ^[54][113] proposed a passive heat pipe-based BTMS for prismatic batteries, using flat heat pipes with fins, as presented in Figure 11a. Compared to natural convection and aluminum plates, the maximum temperature was 73.7% lower and the maximum temperature difference 50.1% lower. Behi et al. ^[53][112] designed a smart heat pipe-based BTMS, using the least number of heat pipes based on the multizone analysis of temperature distribution determined by thermal imaging. The high discharge investigation revealed that the most critical zone of a prismatic LTO cell is the center top region. The thermal analysis, illustrated in Figure 11c, showed that a single heat pipe with an optimal placement is sufficient. Kleiner et al. ^[55][114] introduced a concept where heat pipes are attached to the terminals of prismatic batteries, as presented in Figure 11b. This reduces the heat flow path and thermal resistance from the heat generation sources, located in the busbar and inside the battery, to the cooling system at the bottom of prismatic batteries. The results show that without any increase in charging time, temperature at the terminals and in the jelly roll are 19 °C, respectively 2 °C lower. Temperature distribution is also improved, due to conduction of 27% of the generated heat.