An Enhanced Phase Change Material Composite for Electrical Vehicle Thermal Management

: Lithium-ion (Li-ion) battery cells are inﬂuenced by high energy, reliability, and robustness. However, they produce a noticeable amount of heat during the charging and discharging process. This paper presents an optimal thermal management system (TMS) using a phase change material (PCM) and PCM-graphite for a cylindrical Li-ion battery module. The experimental results show that the maximum temperature of the module under natural convection, PCM, and PCM-graphite cooling methods reached 64.38, 40.4, and 39 ◦ C, respectively. It was found that the temperature of the module using PCM and PCM-graphite reduced by 38% and 40%, respectively. The temperature uniformity increased by 60% and 96% using the PCM and PCM-graphite. Moreover, some numerical simulations were solved using COMSOL Multiphysics ® for the battery module.


Introduction
The traditional combustion engine has been gradually replaced by electric vehicles (EVs) and hybrid electric vehicles due to CO 2 emissions, global warming, and fossil fuel shortages. Lithium-ion (Li-ion) batteries are selected for EV energy storage due to their tremendous features like high voltage, long cycle-life, high specific energy, and low selfdischarge [1,2]. Although Li-ion batteries are the ideal energy storage for EVs, they suffer a number of drawbacks, like degradation and heat generation during the charging and discharging process [3,4]. A battery thermal management system (TMS) is vital for EVs as almost 50% of EV cost returns to the battery pack [5]. In this context, some techniques, such as equivalent circuit modeling, are used for the parameter identification of Li-ion batteries to analyze their thermal response numerically [6,7]. Moreover, online data-driven techniques are employed for the health management of these batteries [8].
Different cooling systems, constituting active, passive, and hybrid systems, have been used by researchers to cool the battery module/pack in EVs [9][10][11]. Active cooling methods like liquid and air cooling systems use external power [12,13]. On the other hand, passive cooling methods like a heat pipe [14], phase change materials (PCM) [15], and heat sinks [16] do not use power. Hybrid cooling systems are a combination of active and passive cooling systems which need external power [17,18]. Many analyses have been presented on battery TMS to specify their effectiveness. The air cooling system is an

Experimental Test Setup
The experimental test setup in this study consisted of a cylindrical battery mo (18,650 cells) with a cell nominal capacity of 2200 mAh. The module consisted of 24 with every cell fixed by a cell holder, with a space of 2 mm. The module is arrang four columns and six rows. The test setup comprises an 18,650 battery module, a PVC a power supply, PCM, two K-type thermocouples, a data logger, and a personal comp The module is connected to a PEC battery tester with a ±0.005 accuracy in order to the module. The PICO USB TC-08 was chosen as a data logger to record the mo temperature. In this study, the cooling effect of natural convection, PCM, and P graphite was considered. Figure 1 shows the schematic and dimensions of the ba module. Cylindrical cells are structured from multiple layers of the active portion of th ion battery. Therefore, there is anisotropic thermal conductivity in radial and directions. The important physical features of the cells applied in the battery modu shown in Table 1. In order to conduct the tests, the setup was connected to the battery tester module was discharged under a 1.5C rate for 2720 s. Two K-type thermocouples linked to the cells to measure the surface temperature of the cells. The picture of th setup and the location of the thermocouples is shown in Figure 2. Cylindrical cells are structured from multiple layers of the active portion of the Li-ion battery. Therefore, there is anisotropic thermal conductivity in radial and axial directions. The important physical features of the cells applied in the battery module are shown in Table 1. In order to conduct the tests, the setup was connected to the battery tester. The module was discharged under a 1.5C rate for 2720 s. Two K-type thermocouples were linked to the cells to measure the surface temperature of the cells. The picture of the test setup and the location of the thermocouples is shown in Figure 2.

Reflection of the Natural Convection
Natural convection caused by the buoyancy effect is the first cooling method that is primarily utilized to take away heat dissipation. For this reason, the surface temperature of the cells was measured under a 1.5C discharging rate. According to the data, the convection coefficient was around 6.87 W/m 2 K, which was calculated experimentally [25]. Natural convection benefits from zero energy consumption, a simple design, and low cost. Figure 3 shows the temperature of the thermocouples of T1 and T2 under the initial temperature of 25 °C. According to the results, the temperature of T1 and T2 reaches almost 57 °C and 65 °C. As can be seen, there is an 8 °C difference between the two thermocouples, which is caused due to the location of the cells, the heat dissipation effect of other cells, and the lowered effect of natural convection in the center of the module.

Reflection of the Natural Convection
Natural convection caused by the buoyancy effect is the first cooling method that is primarily utilized to take away heat dissipation. For this reason, the surface temperature of the cells was measured under a 1.5C discharging rate. According to the data, the convection coefficient was around 6.87 W/m 2 K, which was calculated experimentally [25]. Natural convection benefits from zero energy consumption, a simple design, and low cost. Figure 3 shows the temperature of the thermocouples of T 1 and T 2 under the initial temperature of 25 • C. According to the results, the temperature of T 1 and T 2 reaches almost 57 • C and 65 • C. As can be seen, there is an 8 • C difference between the two thermocouples, which is caused due to the location of the cells, the heat dissipation effect of other cells, and the lowered effect of natural convection in the center of the module.

Reflection of the Natural Convection
Natural convection caused by the buoyancy effect is the first cooling method that is primarily utilized to take away heat dissipation. For this reason, the surface temperature of the cells was measured under a 1.5C discharging rate. According to the data, the convection coefficient was around 6.87 W/m 2 K, which was calculated experimentally [25]. Natural convection benefits from zero energy consumption, a simple design, and low cost. Figure 3 shows the temperature of the thermocouples of T1 and T2 under the initial temperature of 25 °C. According to the results, the temperature of T1 and T2 reaches almost 57 °C and 65 °C. As can be seen, there is an 8 °C difference between the two thermocouples, which is caused due to the location of the cells, the heat dissipation effect of other cells, and the lowered effect of natural convection in the center of the module.

Reflection of the PCM
PCM cooling is commonly applied as an appropriate battery thermal management method, owing to its simple design, low cost, and high performance. The PCM-based cooling system is classified as a passive cooling system, which does not need any external power source. According to the PCMs physical features, it can absorb heat at a constant temperature within the normal operating range. Therefore, based on PCM latent heat, it can keep the battery cell/module at a constant temperature for a specific time. In the current study, a paraffin PCM was used for the thermal management of the module. The PCM is classified as organic paraffin, which is produced by Shanghai Tianlan New Material Technology Co., Ltd. (Shanghai, China). The main features of the PCM are mentioned in Table 2. As can be seen in Figure 4, the module temperature was controlled perfectly by PCM cooling. Moreover, the temperature uniformity increased. The maximum temperature of the module reached 40.4 • C, which shows a 38% reduction compared with natural convection. There is only a 3.2 • C difference between the two thermocouples, which increased the temperature uniformity by 60%.

Reflection of the PCM
PCM cooling is commonly applied as an appropriate battery thermal management method, owing to its simple design, low cost, and high performance. The PCM-based cooling system is classified as a passive cooling system, which does not need any external power source. According to the PCMs physical features, it can absorb heat at a constant temperature within the normal operating range. Therefore, based on PCM latent heat, it can keep the battery cell/module at a constant temperature for a specific time. In the current study, a paraffin PCM was used for the thermal management of the module. The PCM is classified as organic paraffin, which is produced by Shanghai Tianlan New Material Technology Co., Ltd. (Shanghai, China). The main features of the PCM are mentioned in Table 2.  Figure 4, the module temperature was controlled perfectly by PCM cooling. Moreover, the temperature uniformity increased. The maximum temperature of the module reached 40.4 °C, which shows a 38% reduction compared with natural convection. There is only a 3.2 °C difference between the two thermocouples, which increased the temperature uniformity by 60%.

Reflection of the PCM Graphite
Pure PCMs generally suffer from low thermal conductivity [7]. This drawback can be compensated for by using composite materials. Graphite, as a high thermally conductive material, has superior advantages to increase the thermal conductivity of PCM. PCM-

Reflection of the PCM Graphite
Pure PCMs generally suffer from low thermal conductivity [7]. This drawback can be compensated for by using composite materials. Graphite, as a high thermally conductive Designs 2022, 6, 70 6 of 13 material, has superior advantages to increase the thermal conductivity of PCM. PCMgraphite can advance the heat conductivity of the PCM cooling system, which may increase the heat transfer efficiency. In this section, the module is in direct contact with the PCMgraphite. The main physical features of the PCM-graphite are mentioned in Table 3.  Figure 5 shows the temperature curve of the battery module and a picture of the test setup using PCM-graphite. It is obvious that the module temperature has been completely controlled by the PCM-graphite. The maximum temperature of the module reached 39 • C, which shows a 40% reduction compared with natural convection. Moreover, the temperature uniformity also increased. There is only a 0.3 • C difference between the two thermocouples, which improved the temperature uniformity by 96%.
Designs 2022, 6, x FOR PEER REVIEW 6 of 14 graphite can advance the heat conductivity of the PCM cooling system, which may increase the heat transfer efficiency. In this section, the module is in direct contact with the PCM-graphite. The main physical features of the PCM-graphite are mentioned in Table 3.  Figure 5 shows the temperature curve of the battery module and a picture of the test setup using PCM-graphite. It is obvious that the module temperature has been completely controlled by the PCM-graphite. The maximum temperature of the module reached 39 °C, which shows a 40% reduction compared with natural convection. Moreover, the temperature uniformity also increased. There is only a 0.3 °C difference between the two thermocouples, which improved the temperature uniformity by 96%.

Comparison of Experimental Results
The transient temperature of the T2 thermocouple is now selected to examine the performance of the proposed cooling systems. According to the Figure 6, the temperature of T2 reached almost 65 °C, due to the natural convection effect. This temperature is far from the acceptable Li-ion battery temperature (25-40 °C) and will definitely cause its life span to decrease. The PCM cooling systems were introduced to control the heat generation of the module. The temperature of T2 reached 40.4 °C and 39 °C for the PCM and PCMgraphite methods, respectively. It was found that the module temperature decreased by 38% and 40% compared with natural convection, respectively.

Comparison of Experimental Results
The transient temperature of the T 2 thermocouple is now selected to examine the performance of the proposed cooling systems. According to the Figure 6, the temperature of T 2 reached almost 65 • C, due to the natural convection effect. This temperature is far from the acceptable Li-ion battery temperature (25-40 • C) and will definitely cause its life span to decrease. The PCM cooling systems were introduced to control the heat generation of the module. The temperature of T 2 reached 40.4 • C and 39 • C for the PCM and PCMgraphite methods, respectively. It was found that the module temperature decreased by 38% and 40% compared with natural convection, respectively.

Battery Thermal Model
In the current study, COMSOL Multiphysics ® was selected to simulate the 3D thermal behavior of the cell and the PCM. The thermal model makes it possible to enhance TMS and use the battery cells in a module/pack. The internal and external resistances are the main cause of heat generation in the battery. Generally, heat generation is divided into reversible and irreversible heat. Reversible heat returns to the heat, which is generated in the cathodes and anodes and is called entropic heating. Entropic heating is produced by electrochemical reactions from reversible entropy change. It can be exothermic or endothermic based on the state of charge (SOC). The reversible battery heat generation can be calculated as follows [41]: where I, T, and are the current, battery temperature, and entropic heat coefficient, respectively. Irreversible heat returns to joules or ohmic heating. It can be produced due to the exchange of current within the internal resistance in the electrode, electrolyte, and current collector. The irreversible heat is always exothermic, owing to quadratic current dependence. The irreversible battery heat generation can be calculated as follows: where , , and are the open-circuit voltage, battery voltage, and the battery internal resistance, respectively. The internal resistance consists of the ohmic and polarization resistances, which depend on the SOC and battery temperature.
In the present study, using a 1.5C discharge rate, a volumetric heat generation of 48,750 W/m 3 was selected for module simulation.

Descriptive Equations for PCM
The descriptive equations of continuity, momentum, and energy for the thermal response of the PCM are comprised as follows [7]:

Battery Thermal Model
In the current study, COMSOL Multiphysics ® was selected to simulate the 3D thermal behavior of the cell and the PCM. The thermal model makes it possible to enhance TMS and use the battery cells in a module/pack. The internal and external resistances are the main cause of heat generation in the battery. Generally, heat generation is divided into reversible and irreversible heat. Reversible heat returns to the heat, which is generated in the cathodes and anodes and is called entropic heating. Entropic heating is produced by electrochemical reactions from reversible entropy change. It can be exothermic or endothermic based on the state of charge (SOC). The reversible battery heat generation can be calculated as follows [41]: where I, T, and dU OC dT are the current, battery temperature, and entropic heat coefficient, respectively. Irreversible heat returns to joules or ohmic heating. It can be produced due to the exchange of current within the internal resistance in the electrode, electrolyte, and current collector. The irreversible heat is always exothermic, owing to quadratic current dependence. The irreversible battery heat generation can be calculated as follows: where U OC , U, and R are the open-circuit voltage, battery voltage, and the battery internal resistance, respectively. The internal resistance consists of the ohmic and polarization resistances, which depend on the SOC and battery temperature.
In the present study, using a 1.5C discharge rate, a volumetric heat generation of 48,750 W/m 3 was selected for module simulation.

Descriptive Equations for PCM
The descriptive equations of continuity, momentum, and energy for the thermal response of the PCM are comprised as follows [7]: In the PCM melting process, T 1 is known as an initial temperature, T m as a melting temperature, and T 2 as a final temperature (T 2 = T m + ∆T). A mushy zone happens between the solid and liquid phases during the operation of this function within the interval of ∆T. Equation (11) shows the different phases of the PCM.
The heat capacity of the PCM is mentioned as a value of phase transition function, which depends on the PCM temperature (s: solid; t: transition; l: liquid): COMSOL, based on the defined equations, tries to simulate the heat transfer for the battery and PCM. The total amount of heat that can be received by the PCM is evaluated in the following equation: where m is the mass of the PCM. Moreover, the thermal conductivity of the PCM can be expressed as follows:

Validation of the Thermal Model for Natural Convection, PCM, and PCM-Graphite Cooling System
In order to validate the numerical modeling, the maximum temperature of the module (T 2 ) was selected to be compared with the simulation results. The simulation was conducted at a module level under the 1.5C discharging rate and at an ambient temperature of 25 • C. Figure 7 displays the validation graphs for the natural convection, PCM, and PCM-graphite cooling methods. According to the graph trends, there is an acceptable agreement between the experimental and simulation results. The root mean square error (RMSE) for natural convection, PCM and PCM-graphite cooling methods was 0.72%, 2.1%, and 1.15%, respectively.

Thermal Behavior Contour of the Module in Different Cooling Methods
The heat transfer contour at the end of the discharging process is shown in Figure 8 (under the different cooling methods). Temperature contour is a suitable way to display the heat concentration inside the module. In natural convection cooling, the heat generation is more dominant in the center of the module due to the heat gradient of the border cells and the lower effectiveness of the free cooling. The maximum temperature reached almost 65 °C, which is outside of the safe zone for batteries. PCM cooling enacted a tremendous effect on the maximum temperature and temperature uniformity of the module. The maximum module temperature reached 40.4 °C, which proved that PCM cooling is an acceptable cooling method. In the third condition, the temperature of the module was considered under the PCM-graphite effect. It can be seen the maximum temperature

Thermal Behavior Contour of the Module in Different Cooling Methods
The heat transfer contour at the end of the discharging process is shown in Figure 8 (under the different cooling methods). Temperature contour is a suitable way to display the heat concentration inside the module. In natural convection cooling, the heat generation is more dominant in the center of the module due to the heat gradient of the border cells and the lower effectiveness of the free cooling. The maximum temperature reached almost 65 • C, which is outside of the safe zone for batteries. PCM cooling enacted a tremendous effect on the maximum temperature and temperature uniformity of the module. The maximum module temperature reached 40.4 • C, which proved that PCM cooling is an acceptable cooling method. In the third condition, the temperature of the module was considered under the PCM-graphite effect. It can be seen the maximum temperature reaches 38.9 • C, with more temperature uniformity, owing to the high thermal conductivity of the graphite. reaches 38.9 °C, with more temperature uniformity, owing to the high thermal conductivity of the graphite.

Liquid Fraction Contour of Module for PCM and PCM-Graphite Cooling Methods
Figures 9 and 10 show the liquid fraction of the PCM and PCM-graphite under a 1.5C discharging process. The blue and red colors identify the solid and liquid forms of the PCM (solid = 0, liquid = 1). At the beginning of the cooling process, heat generation, along with the cells, is transferred via conduction within the PCM and PCM-graphite. In this current stage, heat is absorbed in a sensible form until it reaches the phase change temperature (mushy zone). In the mushy zone, a thin axial layer of PCM melts along the cell's surface, as shown at 2000 s. As time passes, the melting process continues, with more heat dissipation inside the PCM (2740 s). At the phase change zone, the heat has been absorbed as a latent form and at a constant temperature. This phenomenon is the main advantage of using PCM as a cooling method. It is obvious the heat is dispersed more uniformly inside the PCM-graphite, owing to the high thermal conductivity of the graphite.  . At the beginning of the cooling process, heat generation, along with the cells, is transferred via conduction within the PCM and PCM-graphite. In this current stage, heat is absorbed in a sensible form until it reaches the phase change temperature (mushy zone). In the mushy zone, a thin axial layer of PCM melts along the cell's surface, as shown at 2000 s. As time passes, the melting process continues, with more heat dissipation inside the PCM (2740 s). At the phase change zone, the heat has been absorbed as a latent form and at a constant temperature. This phenomenon is the main advantage of using PCM as a cooling method. It is obvious the heat is dispersed more uniformly inside the PCM-graphite, owing to the high thermal conductivity of the graphite. reaches 38.9 °C, with more temperature uniformity, owing to the high thermal conductivity of the graphite.

Liquid Fraction Contour of Module for PCM and PCM-Graphite Cooling Methods
Figures 9 and 10 show the liquid fraction of the PCM and PCM-graphite under a 1.5C discharging process. The blue and red colors identify the solid and liquid forms of the PCM (solid = 0, liquid = 1). At the beginning of the cooling process, heat generation, along with the cells, is transferred via conduction within the PCM and PCM-graphite. In this current stage, heat is absorbed in a sensible form until it reaches the phase change temperature (mushy zone). In the mushy zone, a thin axial layer of PCM melts along the cell's surface, as shown at 2000 s. As time passes, the melting process continues, with more heat dissipation inside the PCM (2740 s). At the phase change zone, the heat has been absorbed as a latent form and at a constant temperature. This phenomenon is the main advantage of using PCM as a cooling method. It is obvious the heat is dispersed more uniformly inside the PCM-graphite, owing to the high thermal conductivity of the graphite.   Figures 9 and 10 show the liquid fraction of the PCM and PCM-graphite under a 1.5C discharging process. The blue and red colors identify the solid and liquid forms of the PCM (solid = 0, liquid = 1). At the beginning of the cooling process, heat generation, along with the cells, is transferred via conduction within the PCM and PCM-graphite. In this current stage, heat is absorbed in a sensible form until it reaches the phase change temperature (mushy zone). In the mushy zone, a thin axial layer of PCM melts along the cell's surface, as shown at 2000 s. As time passes, the melting process continues, with more heat dissipation inside the PCM (2740 s). At the phase change zone, the heat has been absorbed as a latent form and at a constant temperature. This phenomenon is the main advantage of using PCM as a cooling method. It is obvious the heat is dispersed more uniformly inside the PCM-graphite, owing to the high thermal conductivity of the graphite.

Conclusions
The present study experimentally and numerically explored the cooling effectiveness of natural convection, PCM, and PCM-graphite methods for a cylindrical battery module under a 1.5C discharging rate. The results are concluded as follows: • The maximum temperature of the module in the presence of natural convection for the initial temperature of 25 • C at a 1.5C discharging rate was measured. According to the measured results the maximum module temperature reached 64.38 • C, which is outside of the ideal Li-ion battery temperature; • The PCM cooling system reduced the temperature of the module to 40.4 • C, which is a 38% reduction in the maximum module temperature. Moreover, the temperature uniformity of the module was increased by 60%; • Using the PCM-graphite cooling system, the maximum module temperature reached 39 • C, which is a 40% reduction. Furthermore, the temperature uniformity of the module increased by 96%; • The CFD model for different cooling strategies was validated against the experimental results and attained satisfactory agreement. The temperature contours and phase change process have been investigated at different times for PCM and PCM-graphite cooling systems.