Investigations of phase change materials in battery thermal management systems for electric vehicles: a review

Taking advantage of electric vehicles’ low pollution, the world is changing its face toward electric vehicle (EV) production. As EVs rely heavily on specialized batteries, it’s important to manage them safely and properly to prevent thermal runaway. High ambient temperatures and varied charging/discharging rates increase battery temperature. To address these challenges, Battery Thermal Management System (BTMS) come into play. This work focuses on passive cooling in BTMS, which is one of two categories of BTMS, with the other being active cooling using liquid-air systems. Passive BTMS has gained prominence in research due to its cost-effectiveness, reliability, and energy efficiency, as it avoids the need for additional components like pumps/fans. This article specifically discusses recent experimental studies regarding phase change material (PCM)-based thermal management techniques for battery packs. It explores methods for enhancing thermal conductivity in PCMs and identifies methodologies for BTMS experiments using PCMs. Also recommends the importance of optimization techniques like machine learning, temperature sensors, and state-of-charge management, to ensure accuracy and uniform temperature distribution across the pack. While paraffin wax has been a popular choice in experimental studies for its capacity to absorb and release heat during phase transitions, as a matter of its low thermal conductivity (0.2 to 0.3 Wk−1m−1) limits reaction in rapid charging/discharging of batteries. So integration with highly thermally conductive additives is recommended. Additives such as heat pipes offer superior thermal conductivity compared to expanded graphite (5 to 200 Wk−1m−1). As a result, the integration of heat pipes further reduces the temperature of battery by 28.9% in addition to the reduction of 33.6% by pure PCMs in time of high charge/discharge rates (5 C to 8 C). So high-conductivity additives correlate directly with improved thermal performance and are essential for maintaining optimal battery temperatures and overall reliability in EV battery packs.


Introduction
EV Batteries are the brains of electric vehicles, and during both discharging and charging, they generate a lot of heat and temperature.This high temperature leads the battery into thermal runaway, and an electrolyte explosion may happen [1].The energy density of batteries has significantly increased in these years due to the fast growth of new battery materials and manufacturing techniques.Nevertheless, high-energy-density materials like NMC 811 are not thermally safe.For high-energy batteries, thermal runaway is a serious concern.Temperature increases in the battery and battery material decomposition are both involved in the thermal runaway process [2].Thus, BTMS was developed to decrease the increased temperature inside the battery pack and maintain an equal distribution of temperatures in the cell to extend the life, safety, and efficiency of the batteries.Furthermore, reaching temperatures that are below the operating range of a battery would result in a loss of autonomy and capacity [3].With enticing characteristics in recent years.A BTMS that currently uses a passive thermal management system has been justified as a superior BTMS for a variety of reasons, including its lightweight, very low power, and uniform distribution of temperature through battery cells [4].As part of this investigation, a hybrid thermal management system together with a PCM was developed to enhance BTMS' cooling performance.This also discusses some of the different techniques that could be utilized to increase PCM's thermal conductivity, including cellular foams, thermally conductive particles, and encapsulation [5].A review of research studies examining the impacts of parameters like thermal conductivity, specific heat capacity, thickness, mass, and cell spacing on system performance is presented.Numerous analyses on PCM cooling are given in the final section, which contains their results and a discussion of the work they conducted under various research projects in recent years.
Nowadays, researchers have been paying more attention to PCM-oriented BTMS, as shown in (figure 1) as a matter of its simple structure, excellent temperature control performance, and also energy-free operation.The idea of PCM being applied to BTMS was originally introduced by Selman and Hallaj in 2001 [6].This result shows that, during different depths of discharge (DOD), the battery that was cooled with PCM showed a much lower heating range than that of the battery with no PCM.Since PCMs have large amounts of latent heat, they can able to absorb a more quantity of heat produced inside the battery without significantly affecting their temperature [7].There are some natural weaknesses of PCMs like paraffin (PA), like their easy leakage and low thermal conductivity, which need to be addressed [8].Leakage in BTMS may result from PCM material aging or housing damage.As PCM ages, its properties, including phase change temperature, can change, potentially causing leaks.Maintaining PCM containment integrity is crucial to prevent efficiency and battery cell damage issues.Regular inspections are vital for addressing and preventing PCM leaks in BTMS [9].Additionally, Pure PCMs have low thermal conductivity, therefore thermal management systems on the basis of pure PCMs are limited [6].A PCM selection method and temperature constraints play crucial roles in tackling this problem [10].Selecting the appropriate PCM with the right phase change temperature and thermal characteristics is crucial for the effective functioning of a battery's thermal management system.Some PCMs are better suited to specific temperature ranges and discharge rates than others.Moreover, temperature constraints can exacerbate the limitations of PCMs.PCMs undergo a phase transition, changing from a solid phase below their freezing temperature to a liquid phase above their melting temperature [11].When the ambient temperature approaches or surpasses the PCM's phase change temperature, it can impede the PCM's ability to efficiently absorb or release additional heat [10].Once the PCM is chosen based on these considerations, another aspect involves incorporating highly thermally conductive materials like copper foam, heat pipes, fins, expanded graphite, and other substances with significant thermal conductivity into paraffin wax to enhance thermal management.
In addition to providing effective temperature management of batteries in an electric vehicle, PCM can absorb considerable heat from in timing of phase changing process.The time of this period, however, would permit PCM temperature to remain above the phase change temperature.Because of its poor shape adaptability and low thermal conductivity, PCM is being studied more as composites than as pure materials.Composite-PCM is well famous and composed of better materials for thermal conductivity, those are copper foam, expanded graphite (EG), different metals, and nano-sized particles [12].Composite PCMs are chosen for battery thermal management systems because they offer a combination of efficient heat absorption and release, improved thermal conductivity, customization options, safety enhancements, and environmental benefits.These advantages make them a valuable choice for maintaining optimal battery temperatures and extending the lifespan of battery systems in electric vehicle [13].Although it shows reasonable performance under normal operating conditions, PCM continues to face the challenge of efficiently dissipating the heat it absorbs, especially in demanding conditions such as elevated discharge rates and high ambient temperatures, as highlighted in the preceding section.In such circumstances rapid heat generation can outpace the PCM's heat absorption and dissipation capabilities, leading to thermal imbalances and potential overheating.In hot ambient conditions, PCMs may reach their phase change temperatures more quickly, limiting their effectiveness.Proposed solutions include optimizing PCM formulations to have higher heat absorption rates, incorporating additional thermal management methods like hybrid (active and passive) BTMS required.Alternative approaches involve the use of advanced materials with improved thermal properties or integrating multiple PCMs with varying phase change temperatures to provide continuous cooling over a wider temperature range.Addressingthese challenges is crucial for maintaining the performance and safety of PCM-based cooling systems in demanding operationalenvironments [14].

Electric vehicle batteries
An EV batteries differ from SLI's ('Starting, Lighting, and Ignition') batteries in that they are created to provide sustained power over a long period of time and are deep-cycle batteries.For electric vehicles, smaller, lighter batteries are preferable since they reduce weight and therefore improve the vehicle's performance.Battery pack features include the power-to-weight ratio, energy density, and specific energy.All-electric vehicle range can be significantly reduced by the fact that most existing battery technologies have far lower specific energy than liquid fuels.Continuous research and development in battery technology like advanced battery chemistry, Energy-Dense Anode and Cathode Materials, Improved Electrolytes, Advanced Battery Management Systems (BMS), Battery Thermal Management, along with ongoing advancements in materials science and manufacturing processes, are key to improving the specific energy of batteries in electric vehicles.As technology evolves, it is possible to see higher specific energy batteries that offer longer driving ranges and improved overall performance for EVs [15].
Electric vehicle batteries (EVBs, also known as traction batteries) are rechargeable batteries utilized to power electric motors in BEVs or HEVs.The battery provides the power required to operate electric vehicles [16].There are different types of EV batteries now a day a listed in (table 1).A lithium-ion battery has a high electric charge (or energy) capacity that is specifically designed for this purpose.Lithium-ion EV batteries have the advantages of durability, long cycle lives, low self-discharge rates, high capacities, and low self-discharge rates making them an ideal power source for the EV.But lithium-ion batteries remain prone to thermal runaway, fire, and high temperatures [17].What causes this to happen?The first reason is their chemistry: Lithium-ion batteries use lithium compounds as a key component [18].Lithium is highly reactive, especially when it comes into contact with oxygen or moisture [19].This reactivity can lead to exothermic reactions (reactions that release heat) and potentially ignite the battery.The second reason is flammable electrolytes: Lithium-ion batteries use an electrolyte that facilitates the movement of ions between positive and negative electrodes [20].If the battery is damaged or compromised, it can lead to a short circuit, which can generate heat and potentially ignite the electrolyte.
There are several other causes of internal short circuits and thermal runaway, including manufacturing defects, rapid charging/discharging, and mechanical damages.Even so, the main issue is heating buildup and extreme temperatures on the battery [21].As a result of rapid acceleration of the vehicle or when the battery is fully discharged, these symptoms can arise [22].When an EV accelerates rapidly, it demands a high amount of power from the battery to provide the necessary energy for the electric motor.This increased power draw can generate heat within the battery, potentially causing it to overheat if not managed effectively.Furthermore, when the battery is fully discharged, it may experience a buildup of heat and elevated temperatures due to the strain of providing power until it is completely drained [23].This scenario can stress the battery and contribute to temperature-related issues.
Temperature has a profound impact on the lifespan, life cycle, and capacity retention of EV batteries.Working within the optimal temperature range, which is between 20 °C and 40 °C for LIB, is crucial.High temperatures like those above 40 °C accelerate chemical degradation processes, leading to reduced battery lifespan and capacity over time [24,25].While extremely low temperatures can temporarily diminish power output and cycle life, the highest temperature variation between the module and the battery cell must not be greater than 5 °C [26].As of 'Murali et al (2021)', it has been found that a temperature difference greater than 5 °C inside a battery cell increases thermal aging by 25% and reduces power capacity by 10% [4].Keeping batteries within their recommended temperature range is vital for maintaining their capacity, extending their cycle life, and optimizing their overall performance.While there are additional factors that can adversely affect battery affect battery life, power capacity, and overall performance, such as internal short circuits, thermal runaway, rapid charging/discharging, and environmental temperature variations, the central issue remains the accumulation of heat and the occurrence of extreme temperatures within the battery.Consequently, it is of utmost importance to consistently implement advanced and well-optimized thermal management techniques to ensure the battery's ongoing reliability [9].

Heat generation of battery cells
The balance of energy and transfer of heat within a battery should be the foundation of thermal management.A thermal representation of a battery under a working situation can be derived from the following energy balance equation: Where cell r denotes cell density, C p cell , represents the specific heat capacity of the cell, k cell presents thermal conductivity  Q heat generation in the battery,  Q ext signifies heat exchange between the exterior cooling and battery Here  Q jou indicates Irreversible Joule heat,  Q re denotes heat of the reaction,  Q sr signifies the side reaction heat,  Q mix represents the heat of the mixing process.
For the majority of small format batteries tested at constant current and without significant deterioration, the heat produced by the side reaction and process of mixing is not a significant issue.Consequently, studies dealing with BTM issues usually use heat generation equations derived from this simplified example.
Where, U open voltage, I current, V thermal voltage and U T ¶ ¶ is entropy heat coefficient Reversible entropic heat and irreversible Joule heat comprise the simplified form of heat generation.When the battery ages, however, the heat generation characteristic modifications because the battery's internal resistance increases, which results in the heat generation characteristic changing as the battery ages.

BTMS
The BTMS is among the battery management systems, which are electronic devices for controlling rechargeable batteries (battery packs or cells).For example, it prevents the battery from using its capacity beyond its safe working range.Its functionality includes keeping track of its condition, computing secondary data, reporting that data, regulating its surrounding environment, and authenticating, balancing, and authenticating the battery [27].Its main purpose is to keep a uniform temperature of the battery modules during operating processes under a variety of environmental conditions and enhance safety as well as cycling performance [28].A battery's thermal management is the most crucial factor in extending its lifespan and ensuring its output quality [29].The ideal BTMS should weigh and have a volume that's less than 40% of the battery module.[30].
Factors that depend on the battery thermal management system are cell capacity, cell longevity, cell performance, and system safety.These two sources of temperature instability are a generation of internal heat at the time of discharging and charging and environmental temperature.Internal heat generation at elevated temperatures can cause the deadly destruction of batteries due to electron movements during chemical reactions in cases of charging and discharging [31] and the effect of the surrounding environment (cold weather and hot weather).To top it all uneven temperature distribution is another problem in batteries.It typically causes excessive local temperatures, variable currents, and thermal conductivity.To guarantee that a battery operates within a certain temperature range (normally −20 to 60 degrees centigrade), a BTMS is usually required [6].
BTMS could be categorized into active and passive systems depending on the way in which they cool.These are the two most common types of BTMSs their advantage and disadvantages as mentioned below in (table 2).In an active battery thermal management system, a coolant, typically a water-based mixture with an additive like glycol, circulates through channels within the battery pack.It can absorb heat generated during battery operation, with heat absorption capacities around 4-6 times greater than air [32].As the coolant absorbs heat, it increases in temperature and then flows to an external heat exchanger, where it releases the heat into the surrounding environment.This helps maintain the battery temperature within its optimal range, typically between 20 °C and 25 °C, ensuring efficient operation and extending the battery's lifespan [33].At the moment, active cooling is widley useable BTMS on many EVs, including Tesla models, BMW i3, Nio ES6, and others [34].
In passive BTMs incorporating PCMs, heat pipes, and hydrogels, PCMs function as thermal energy storage units with latent heat ranging from 150 to 250 Jg −1 .This integrated PCM-based BTMS has demonstrated remarkable capability to lower maximum battery temperatures by 19.4%, as compared to active BTMS [35].As the battery heats up, the PCM transitions from a solid into a liquid state, absorbing heat and maintaining a stable temperature without requiring power.It is due to this characteristic that the BTMS has a greater net efficiency range [8].Howevver, As far as data is concerned, PCM-based BTMs are not typically observed in EVs [34].In addition to that, the thermal conductivity values of heat pipes, ranging from 4,000 to 100,000 Wk −1 m −1 , play a crucial role in efficiently transporting excess heat away from the battery to the regions where the PCM is located [36].As of Bernagozzi et al 2023' study this integrated approach reduces the maximum temperature by 33.6% with the PCM alone and achieves a further 28.9% reduction with the addition of heat pipes [34].Hydrogels, known for their high water retention and thermal conductivity values of approximately 0.5-1.5 Wk −1 m −1 , contribute to even heat distribution within the system.Collectively, these components provide effective passive cooling, absorbing and dissipating heat while ensuring the battery stays within its optimal temperature range, typically around 25 °C [37].However, this BTMS is still under research and development for EVs.Combination of both active and passive BTMS is widely utilized in laboratories, but their adoption in commercial EVs remains limited.Nevertheless, the promising results suggest that PCM-based BTMS, potentially combined with active cooling enhancements, could emerge as a pivotal component in the next decade's novel BTMS for EVs [38].This integrated approach could strike a balance between effective passive cooling and active temperature control, ensuring that EV batteries operate within their optimal temperature range for enhanced performance and longevity.

PCM
PCMs are substances that are capable of undergoing the solid-to-liquid transformation, also recognized as the melting-solidification cycle, at temperatures in the operational range of a particular thermal application.As a substance modifies from solid to liquid, its temperature remains constant as it absorbs energy from its surroundings [39].PCM-assisted BTMS is an effective method of controlling battery temperature [28].Passivecooling systems is the system supported by PCMs, heat sinks, and heat pipes that do not require external energy.The benefits of the PCM cooling method include no power consumption, flexible geometry, and hightemperature uniformity [40] which is also cheaper and easier to manage [41].But some challenges with PCM cooling, including voltage changes, low thermal conductivity, as well as heat accumulation when PCM melted.Many researchers suggest that organic PCMs be manufactured with metallic porous materials (such as copper, aluminum, and nickel) and carbon-based foams and plates (such as carbon nanotubes or fiber as well as graphite) to improve heat transfer [30].
Portable thermal management systems, or PCMs, have received considerable attention and have been extensively studied owing to their simple structure, high latent capacity, and no power consumption.During battery discharge, the PCM could absorb the heat produced and store it as latent heat.Charging consumes a significant portion of the heat, while the operating environments absorb a smaller amount of heat [42].

Selection criteria for the right PCM
The selection criteria for PCMs in a battery thermal management system are primarily determined by their thermal properties and compatibility with the application [8].Key factors include phase change temperature, strong capacity for heat absorption and latent heat, low super cooling intensity, low cost, simple to get, and difficult to leak, high chemical stability and chemical corrosion resistance, and good thermal conductivity.For instance, PCMs with a phase change temperature near the desired operating range of the battery, typically around 25 °C-40 °C for lithium-ion batteries, are preferred to efficiently absorb and release heat during temperature fluctuations [43].High latent heat values, typically in the range of 100-250 Jg −1 , ensure that the PCM can store and release substantial thermal energy.Additionally, a high thermal conductivity, typically greater than 1 Wk −1 m −1 , aids in the rapid transfer of heat within the system [44].However, there are some challenges in selection of PCM for BTMs, challenges in PCM selection involve finding materials that meet these criteria while considering their long-term stability, cost-effectiveness, and compatibility with the thermal management system, necessitating a thorough evaluation of these numerical parameters to ensure effective temperature control and battery safety [45].

Arctactural design of battery modules
In the architectural design of PCM for battery modules, two primary structures can be considered: a honeycomb-like module and a common module structure.The honeycomb-like module features a hexagonal grid design with interlocking cells (figure 2(A)), cells with 10 cm diameter for optimal heat transfer, encapsulating a thermally conductive paraffin wax PCM with a melting point of 60 °C [46,49].In addition, the common module structure uses a box-shaped PCM with holes to accommodate cells (4680 battery cell) (400 mm × 200 mm × 85 mm) (figure 2(B)) strategically positioned to form battery modules [47].Temperature sensors are placed throughout the module in both cases for real-time monitoring.Both designs enhance thermal management, with the honeycomb-like structure offering efficient heat transfer due to its hexagonal grid configuration, while the common module structure provides simplicity and ease of maintenance as shown in (figure 2(c)) [50].

Overview of PCMs
Phase Change Materials are substances capable of storing and releasing thermal energy during phase transitions of battery thermal management system.PCMs are classified into three main categories (figure 3) based on their phase change characteristics.Organic PCMs, such as paraffin waxes, exhibit phase changes around 25 °C-100 °C.Inorganic PCMs, like salt hydrates, have phase change temperatures ranging from −20 °C to 200 °C.Eutectic PCMs consist mixtures of organic and non organic PCMs, typically around −70 °C to 60 °C.PCMs play a crucial role in managing temperature variations in BTMs, ensuring efficient and controlled energy storage and release.
Thus, pure phase change materials have their own drawbacks.PCM cooling faces challenges due to low thermal conductivity, typically ranging from 0.2 to 2 Wk −1 m −1 , which hinders efficient heat transfer.Additionally, during PCM melting, heat accumulation occurs, causing temperature spikes [14].These issues can lead to temperature fluctuations of 5 °C-10 °C within the system.Researchers are addressing different approaches to improve these problems, by introducing additives like EG, copper foam, and nanofluides/ particles to enhance heat transfer properties, and by developing advanced PCM formulation, it can be precisely tailored to match the specific temperature profiles of the battery system, optimizing energy efficiency and overall safety [51] and ensuring efficient heat absorption and dissipation.Furthermore, combining both passive and active cooling methods can provide a comprehensive solution.Passive systems, like PCMs and heat pipes, can handle lower thermal loads and stabilize temperatures, while active cooling, such as liquid or gas circulation, can address higher heat generation.This hybrid approach offers a versatile and efficient means of managing the thermal characteristics of battery systems, ensuring optimal performance and longevity [52].These efforts aim to improve the overall effectiveness and performance of PCM cooling in battery thermal management.

Experimental investigations of PCM for BTMs
An experimental system for battery thermal management typically consists of different components as shown in (figure 4).In general the schematic diagram includes a battery module, battery testing equipment, a thermostat, and a T-type thermocouple for accurate temperature measurement [53].The T-type thermocouple is placed  within the battery module to monitor temperature changes.A temperature data logger records this data for analysis.The thermostat controls the environmental temperature during testing.Additionally, a computer setup interfaces with the temperature data logger and allows for real-time monitoring and data analysis, helps to optimize the thermal management system and ensure the battery operates within safe temperature limits.
The table (table 3) presents research on experimental studies involving PCM-Based BTMS in electric vehicles.These investigations highlight the use of various PCM materials and additives to enhance thermal conductivity under different battery conditions, including varying depths of discharge, states of charge, and environmental temperatures.Notably, the battery temperature tends to increase significantly at higher depths of discharge.This observation underscores the primary influence of charging or discharging operations and external environmental temperatures on battery temperature dynamics.
The table provides a comprehensive overview of various experimental studies focusing on PCM-Based Battery Thermal Management Systems in electric vehicles.These studies employ different PCM materials and additives to enhance thermal conductivity, aiming to optimize battery performance under varying conditions.Notable findings include the use of paraffin, expanded graphite, copper foam, and innovative formulations to improve thermal conductivity.The maximum performed temperatures range from 22 °C to 57.4 °C, demonstrating the effectiveness of PCM cooling.Additionally, the maximum discharge rates vary from 1 C to 8C, illustrating the potential for rapid energy transfer.Behi et al Achieved the best result with the lowest maximum performed temperature 22 °C while maintaining a high thermal conductivity 56 Wk −1 m −1 and a remarkable maximum discharge rate of 8C.This indicates effective thermal management with minimal temperature rises.
By comparison with the PCM in the table above, paraffin is solid choice for PCM-based battery thermal management systems as main PCM and epanded graphate for enhensment material, as demonstrated by its frequent use in the studies listed in the table.So why paraffin and expanded graphates?

Paraffin
Paraffin wax is generally composed of the straight-chain n-alkanes 'CH 3 -(CH 2 )-CH 3 '.PCMs The availability of paraffin in a large temperature range makes it an ideal heat fusion storage material.However, only technical grade paraffin could be utilized as PCMs in latent heat storage systems owing to cost considerations.The qualities of paraffin include safety, reliability, predictability, low cost, and no corrosion.Their chemical composition is inert and they are stable below 500 °C.They exhibit little volume loss on melting, and their vapor  pressure is low when in the melt form.PCMs were created by incorporating different materials with high thermal conductivity like carbon nanofibers, carbon nanotubes, expanded graphite, graphene, and metal foams to improve the heat transfer of paraffin [66].In addition to testing paraffin wax's properties, differential scanning calorimeters can also be used to test other PCM materials [6].
In recent research, expanded graphite has been found to be the most preferable material for PCM materials, the expandable graphite is not added alone with epoxy resin, and curing agent (figure 5).Since expanded graphite has high thermal conductivity, it can efficiently speed up heat transfer rates.This could enhance the thermal conductivity of the resulting composite products, and widen the variety of applications of graphite composites [46,63,67].When EG is added to pure PCM, it could improve the heat transfer of PCM performance mentioned in (table 3) but also drop the latent heat.
(Figure 6(a)) illustrate the mass fractions of EG at 0.01 m/s velocity as a function of battery temperature.Temperature increases rapidly in the 1st stage (t < 100 s).The 2nd stage (100 < t < 600 s) is when the temperature reaches CPCM's phase transition temperature, so CPCM becomes liquid, absorbing a great deal of the heat produced by the battery, reducing the rate of rising.Furthermore, the material retains the heat as latent heat since the CPCM temperature is almost unaltered.The CPCM's latent heat diminishes as the mass percentage of EG rises in the final stage (t higher than 600 s) [68,69].

Conclusion and future research
Due to the current rise of electric vehicles on the ground, many researchers are shifting the focus area to electric vehicles, electric vehicle batteries, electric motors, controllers, and other fields of study around electric vehicles in general.The objective of the current work is to enhance the safety of electric vehicle BTMs and minimize arousals caused by thermal runaway in electric vehicles.To maintain the battery temperature within the required range, active and passive cooling techniques are necessary.Active cooling employs the circulation of coolants, typically liquids or air, to extract and dissipate heat from battery packs.These coolants traverse channels or pipes within the battery assembly, directly absorbing heat from the cells.The heated coolant is then channeled to a heat exchanger where the heat is released into the environment.In contrast, passive cooling systems utilize materials like PCMs to absorb and store heat through phase transitions, heat pipes to efficiently transfer heat via vaporization and condensation, and hydrogels that absorb and release water to modulate temperature.Experimental works on PCM-based BTMS are the focus of this review on passive cooling systems.
Experimental investigations of PCM-based BTMs are discussed in (table 3).Nowadays, a passive cooling system is preferable to an active cooling system for batteries because this system does not need extra power to operate.The PCM (paraffin wax) is one of the passive cooling system materials.Many researchers use paraffin wax as the main PCM and add expanded graphite, copper foam, heat pipe, nanofluids, and others to enhance the PCM thermal conductivity but commonly researchers use expanded graphite owing to its high thermal conductivity and high ability to speed up heat transfer.The paraffin allon has low thermal conductivity.In order to increase the thermal conductivity of this paraffin, additives must be added.When adding expanded graphite to paraffin, the maximum percentage is 20%, unless it loses heat transfer efficiency when changing phases.
Additionally, careful consideration of PCM thermal conductivity and the battery's charge/discharge rate is essential for achieving effective thermal management with minimal temperature spikes.Notably, in the above table, the lowest maximum operating temperature achieved is 22 °C, with a thermal conductivity of 8 Wk −1 m −1 and a remarkable maximum discharge rate of 8 C. In summary, this review primarily highlights techniques for enhancing PCM thermal conductivity and advocates for the development of novel PCM manufacturing processes.Researchers should also explore temperature optimization methods such as machine learning, temperature sensors, monitoring, and state of charge management for PCM and BTM combinations.Passive cooling poses the challenge of optimizing the quantity of PCM in the battery, since too small can result in temperature increases, while excessive PCM can add weight and reduce efficiency.Therefore, achieving optimal utilization of PCM in BTMs is crucial for avoiding both insufficient and excessive PCM use.

Figure 2 .
Figure 2. Architectural design of PCM for battery module: A. Honeycomb-like module structure [46], B. common module structure of PCM [47], C. Scheme of battery cells with the PCM module [48].

Figure 5 .
Figure 5. Materials to prepare CPCM by using EG.

Figure 6 .
Figure 6.Variations of T with mass fractions of EG at coolant velocity in 0.01 m s −1 : (a) T Max ; (b) T difference [68].

Table 3 .
A detailed description of experimental works.