Recent Advances in Battery Pack Polymer Composites

Abstract

The use of a polymer composite material in electric vehicles (EVs) has been extensively investigated, especially as a substitute for steel. The key objective of this manuscript is to provide an overview of the existing and emerging technologies related to the application of such a composite, especially for battery pack applications, in which its high strength-to-weight ratio, corrosion resistance, design flexibility, and durability are advantageous compared to any metal in general. This study explores the key considerations in the design and fabrication of composites, including base material selection, structural design optimization, reinforcement material, manufacturing processes, and integration with battery systems. The paper also discusses the performance characteristics of composite battery pack structures, such as mechanical properties, thermal management, safety aspects, and environmental sustainability. This study aims to contribute to sharpening the direction of future research and innovations in the area of composite battery pack technology.

1. Introduction

The rapid growth of electric vehicles (EVs), aerospace applications, and renewable energy systems has led to an increasing demand for efficient and reliable energy storage solutions. Battery box structures play a crucial role in protecting and securing the battery packs inside, ensuring their safe operation and longevity [1]. Such battery enclosures can be made of metal, non-metal, or a combination of both. In recent years, composite materials have emerged as a promising choice for constructing automobile components, including battery box structures, due to their exceptional properties and design flexibility [2,3,4].

Composite materials offer several advantages that make them ideal for battery box applications. Firstly, such composites exhibit an outstanding strength-to-weight ratio, especially if they are further reinforced by particle or fiber materials, such as carbon or glass fibers [5,6,7]. This leads to excellent structural integrity while keeping the weight of the battery box to a minimum [8]. This lightweight characteristic is particularly important in the context of electric vehicles, as it contributes to increased range and energy efficiency [9,10,11,12,13]. An Iron-to-Polymer (“ItoP”) experimental vehicle is one of the finest examples of a material substitution initiative to reduce the total weight by almost half a tonne. It is developed primarily using polymers and their composites, although its current environmental impact is not at the same level as steel yet [14]. Similarly, British rail started to use carbon-fiber composite for its two-axle bogie, which is now about 60% lighter than a conventional bogie made from steel [15]. BMW, as one of the automotive manufacturers, also applied lightweight materials to their products. As can be seen in Figure 1, almost 50% of the material used in this BMW i3 product is dominated by fiber-reinforced material. The composites in general also possess high fatigue resistance, enabling the battery box to withstand prolonged cyclic loading without compromising its structural integrity. Another key advantage of composite materials is their superior corrosion resistance [16,17]. Batteries can generate corrosive substances and release moisture, posing a significant challenge to the long-term durability of battery enclosures. However, composites exhibit excellent resistance to corrosion, ensuring the protection and longevity of the battery pack. Moreover, carbon fiber composites, in particular, offer inherent electrical conductivity, facilitating efficient grounding and electrical integration within the battery system.

Understanding the performance characteristics of composite battery enclosures is vital for their successful implementation. Mechanical properties, including strength, stiffness, and impact resistance, directly impact the ability of the battery box to withstand external forces and protect the battery pack. Thermal management is also critical, as efficient heat dissipation and insulation help maintain optimal battery operating temperatures and prolong battery life. Moreover, safety aspects such as fire resistance and crashworthiness are essential considerations to prevent potential hazards and protect the battery and surrounding components.

This paper aims to provide a comprehensive review of the polymer composite application exclusively as EV battery pack enclosures, encompassing their design, manufacturing processes, performance characteristics, and safety evaluations. By synthesizing existing literature with a focus on recent key technologies and emerging trends, this manuscript could offer valuable insights into the advancements, challenges, and future directions in the development of composite applications for EV battery packs and other energy storage systems in various industries as well. The rest of this manuscript is organized as follows: Section 2 focuses on the recent advances in the area of composite strength in order to handle constant mechanical abuse, such as from rough road surfaces and in the event of a vehicle crash. Section 3 discusses more of the thermal aspects, such as heat dissipation and thermal uniformity between the battery cells, to avoid a catastrophic thermal runaway and prolong the battery lifespan. Section 4 emphasizes the topic of fire safety, including a discussion about potential flame retardant materials, since it is another crucial safety element in EV operation. Finally, the last section presents some concluding remarks on this manuscript.

2. Structural Integrity of Polymer Composite Materials

A stand-alone polymer composite without any reinforcements is relatively rare in vehicle technology since its mechanical strength and structural integrity are its main assets. The design and optimization of composite carbon fiber battery box structures require careful consideration of various factors [18,19]. Load requirements, such as mechanical forces and vibrations experienced during vehicle operation, must be addressed to ensure structural integrity and safety. Packaging efficiency is another essential consideration, as it involves optimizing the spatial arrangement of batteries within the box to maximize storage capacity and minimize wasted space.

Manufacturing processes for composite carbon fiber battery boxes have advanced significantly in recent years [20]. Techniques such as hand layup, resin transfer molding, automated fiber placement, and filament winding have been utilized to achieve precise fiber orientation, minimize defects, and enhance manufacturing efficiency. The selection of an appropriate manufacturing process is crucial for ensuring cost-effectiveness, scalability, and consistent quality of the battery box structures.

2.1. Strength-to-Weight Ratio

Composite carbon fiber materials have an exceptional strength-to-weight ratio, providing excellent structural integrity while keeping the weight of the battery box to a minimum. Ishikawa et al. [21] studied Japan’s government-driven project at the National Composites Center (NCC) to establish carbon fiber-reinforced thermoplastic (CFRTP) composites technology for automotive applications. The study discusses the physics of fiber length distribution, proposes a theory to predict flow behavior, characterizes the viscoelastic properties of such composite materials, and establishes ultrasonic fusion bonding as the primary joining method. Another related project at the University of Tokyo involves the development of new composites and evaluation technologies for CFRTP applications in automobiles. The project successfully replaces all aluminum alloy components of a car chassis with discontinuous carbon fiber-reinforced thermoplastic composites (C-LFT-D: carbon-long fiber thermoplastic-direct), achieving a 10% weight reduction while maintaining rigidity. Bere et al. [22] conducted research on carbon fiber reinforced polymer (CFRP) usage as the engine hood of a small electric vehicle. This paper focuses on the manufacturing process and experimental and numerical analysis of two composite hoods for a small electric vehicle. Figure 2 shows the experimental setting of this hood structure. The hoods have different design concepts and material layouts. The first hood follows a black metal design, while the second one is based on a composite design concept. The paper provides detailed information about the fabrication process. The first composite hood is significantly lighter, weighing 254 times less than a similar steel hood. The second hood concept is 22% lighter than the first one. The composite hoods exhibit improved lateral stiffness, with enhancements of approximately 80% and 157% compared to a similar steel hood for the two different design concepts. Transversal stiffness is higher in both composite hoods, while torsional stiffness shows a 62% increase compared to a similar steel hood.

2.2. Design and Optimization Flexibility

Composite carbon fiber materials offer design and optimization flexibility, allowing for the creation of complex shapes and optimized structures. This flexibility enables efficient packaging of batteries within the box, maximizing storage capacity while maintaining structural integrity and ensuring proper integration with the surrounding components. This carbon fiber material is not only able to be optimized alone, but it is also able to be combined with other materials to achieve desirable properties as well. Kovács [23] studied a lightweight sandwich-like material consisting of CFRP and aluminum (Al). That study aimed to develop a new lightweight structure by leveraging the properties of carbon fiber-reinforced plastic, aluminum, sandwich structures, and cellular plates. The structure consists of CFRP face sheets and Al stiffeners, which were fabricated for experimental measurements. Calculation methods were then devised and validated to assess the deflection and stresses within the investigated sandwich-like structure using experimental data and finite element analysis. Deflection measurement was also performed for this sandwich-like structure, as can be seen in Figure 3. Additionally, a mass and cost optimization method employing the flexible tolerance optimization approach was developed. The optimization process considered seven design constraints, including deflection, buckling, stress, and eigenfrequency, as well as design variable limitations. The research contributes by providing calculation methods for deflection and stress analysis, as well as an optimization approach. The primary objective was to construct the lightest possible structure, thereby enabling its application in diverse industrial sectors such as vehicle components, transport containers, and building constructions such as floors and bridge decks. An optimization method for a better battery enclosure design was conducted by Liu et al. [24] that addresses the challenges associated with the optimization design process of carbon fiber reinforced polymer materials, focusing on the lightweight design of a CFRP battery box. The study considers the complex non-linear material behavior, uncertainty in design variables, and multi-level characteristics of the structure. The proposed method integrates reliability-based design optimization (RBDO) to effectively tackle these challenges. The RBDO method consists of three main components: uncertainty quantification and propagation, finite element analysis, and optimization. The internal geometry variability of plain woven CFRP is obtained through X-ray micro-CT images, enabling the establishment of representative volume element (RVE) models to predict the elastic and strength properties of the CFRP composites, as depicted in Figure 4. In the finite element analysis part, a constitutive model is adapted to analyze the stiffness and strength of the battery box structure. The RBDO procedure developed in this study considers design variables across two scales, incorporating a modified particle swarm optimization algorithm and surrogate modeling techniques. By implementing this multiscale optimization procedure, the CFRP battery box structure achieves a weight reduction of 22.14% while satisfying performance requirements with high reliability. Overall, the application of the proposed RBDO method demonstrates the advantages of utilizing a multiscale optimization approach in the lightweight design of CFRP battery boxes. The methodology effectively addresses the challenges posed by the complex material behavior, uncertainty, and multi-level nature of the structure, leading to significant weight reduction and meeting performance demands with a high level of reliability.

2.3. Impact Resistance

Composite carbon fiber materials offer excellent impact resistance, providing an additional layer of protection for the battery pack against external shocks and collisions. This characteristic enhances the safety of the battery box structure and minimizes the risk of damage to the battery cells. Some research has used both experimental and numerical approaches to find out how CFRP gives a notable improvement in the impact resistance performance of a part or specimen. Liu et al. [25] conducted research regarding the impact resistance improvement of a wood beam by using carbon fiber reinforcement. Drop-weight testing was conducted to evaluate the impact-resistance properties of wood beams, comparing reinforced beams with CFRP and unreinforced beams. The application of axial force was employed to enhance the beams’ impact-resistance ability. Utilizing the digital image correlation (DIC) technique, impact deformation was measured. The test results, including the impact force-time history curve, beam failure modes, and impact deformation, were analyzed to assess the impact-resistance properties. The findings demonstrated that reinforcement significantly improves the impact-resistance ability of wood beams, with the strengthening effect increasing in the sequence of 1-ply CFRP sheet, CFRP plate, and 2-ply CFRP sheet. Additionally, calculations of impact strength and energy dissipation provided further insights into the beams’ behavior under impact load. Li et al. [26] performed a study that aimed to assess the effectiveness of using fiber-reinforced polymer (FRP) strengthening to enhance the impact resistance of as-built bridge piers against vehicular collisions. A refined finite-element model of a double-column reinforced concrete bridge was created and validated through lateral impact tests on bare and CFRP-strengthened columns. The study explored various aspects of FRP strengthening, including fiber orientation, strengthened height, number of layers, and FRP type. By designing 48 vehicle–bridge collision scenarios, the collapse of bridges with and without FRP-strengthened piers was evaluated. The results indicated that external FRP wrapping effectively improved the impact resistance of as-built bridge piers by reducing damage and deformation. For typical bridge piers, the recommended strengthening scheme involved a 0° fiber orientation, a 3-m strengthened height, and four-layer CFRP wrapping. Compared to bare piers, FRP-strengthened piers enhanced the safety redundancy of as-built bridges, mitigating the risk of collapse during high-speed truck collisions. The study also formulated the dynamic shear capacity of strengthened bridge piers, considering factors such as concrete strength, reinforcement rebars, and the nonuniform strain rate distribution caused by vehicular collisions. In addition to that, according to Hosseinzadeh et al. [27], when subjected to low-velocity impacts, CFRP demonstrates exceptional performance without any negative consequences. However, it tends to fail under the influence of higher energy impacts. Evidently, glass fiber reinforced polymer (GFRP) exhibits superior impact resistance compared to CFRP due to its enhanced capacity for energy absorption. As a result, GFRP can withstand prolonged strain before reaching failure, making it a more effective reinforcement option. To address the limitations related to impact resistance and energy absorption capacity, a method was developed by combining carbon fiber (CF) and glass fiber (GF) into a composite material. This technique, commonly referred to as fiber hybrid composite, was created to overcome these challenges. According to Hosseinzadeh et al. [27], the fiber hybrid composite retains proper behavior under both low velocity and greater energy impacts because the GF and CF here compensate for each other’s intrinsic limits. This particular composite also has a high degree of tolerance and great damage resistance.

In summary, the benefits of composite carbon fiber materials, including their high strength-to-weight ratio, design and optimization flexibility, and impact resistance, make them a favorable choice for battery box structures. Leveraging these advantages enhances the safety, efficiency, and longevity of energy storage systems across various industries.

2.4. Manufacturing Processes for Composite Carbon Fiber in Automotive

Some studies focused on optimizing the manufacturing process of this CFRP material into a usable product. One of them discussed the method that could possibly be used to enhance the assembly of a sandwich of CFRP and steel material. However, the adhesive-related challenges between CFRP and steel hinder the production of multi-material products. To address this, Kim et al. [28] propose a new manufacturing process utilizing prepreg compression molding for CFRP reinforcement on a hot-stamped B-pillar in this study, as can be seen in Figure 5.

A finite element (FE) simulation is employed to predict the dimensions of the B-pillar during the hot stamping process. The feasibility of prepreg compression molding manufacturing is assessed through the thermoforming simulation of a CFRP set on a shaped B-pillar. The temperature conditions for the molding, considering heat transfer between CFRP and steel, are determined. Subsequently, the molding of the B-pillar, consisting of steel and CFRP, is performed and compared with analytical results for validation. The evaluation of the B-pillar is conducted using scanning electron microscopy (SEM) to observe the cross-section of the B-pillar and interlayer, as shown in Figure 6.

The results demonstrate that the steel/CFRP B-pillar assembly can be efficiently manufactured using the prepreg compression molding (PC molding) process without the need for an additional adhesive process. Although such a molding process is one of the most common methods for CFRP part production, it still has some drawbacks related to defects, especially microgroves and voids. To overcome this issue, Lee et al. [29] came up with the study about the development of a vacuum-assisted prepreg compression molding (VA-PC molding) process to mitigate defects. The manufacturing process of PC and VA-PC moldings is shown in Figure 7.

To assess the applicability of the VA-PC molding process, the mechanical properties of small-scale specimens produced using the VA-PC molding process were compared with those of specimens fabricated using the PC molding process. Tests were conducted to evaluate surface roughness, void content, tensile strength, and three-point bending properties, including elastic modulus, tensile strength, bending stiffness, and energy absorption. Finally, the panels manufactured using each process were subjected to a bending test to validate the VA-PC molding process. By using this method, the average microgrove depth was reduced by more than 60% compared to the conventional PCM process. Also, the ultimate tensile strength (UTS) and energy absorption of this VA-PC CFRP were improved by 5.32% and 19%, respectively, compared with the conventional CFRP specimens. Another new method developed in CFRP production is additive manufacturing. Additive manufacturing, commonly referred to as 3D printing, is an advanced technology in the field of manufacturing [30]. Unlike conventional composite manufacturing techniques that rely on machining and cutting substrates to achieve desired shapes, 3D printing eliminates the need for these preliminary steps. This is made possible through the utilization of precise computer-aided design (CAD) tools, which allow for the accurate and efficient creation of complex shapes without the requirement of machining or cutting processes. Nakagawa et al. [31] studied 3D printing, which involves manufacturing carbon fiber-reinforced plastic parts by sandwiching carbon fibers between upper and lower ABS layers created using a 3D printer with fused deposition modeling. Tensile specimens of the carbon fiber-reinforced plastic were produced, and their strength was evaluated. It was found that simply sandwiching the carbon fibers did not increase the strength; thermal bonding between the fibers and layers was necessary. Furthermore, the strength of the specimens varied depending on the diameter of the nozzle, with higher strength observed for smaller diameters. To simplify the thermal bonding process, a microwave oven was used.

Based on the data presented in

Table 1. A summary of the literature comparing carbon fiber reinforced polymer (CFRP), metal composites, and traditional metals.

Composite	Control	Manufacturing	Major Findings	Ref.
Carbon fiber reinforced polymer (CFRP)	Steel	Prepreg impregnation	better stiffness compared to conventional steel	[22]
Carbon fiber-reinforced thermoplastic composites (CFRTP)	Aluminum Alloy	Compression Molding	10% weight reduction while maintaining rigidity	[21]
Sandwich-like material of carbon fiber-reinforced polymer (CFRP) and aluminum (Al)	-	The newly constructed structure method	Developing a new light-weight sandwich-like structure for many automotive applications	[23]
Reinforced wood beams with carbon fiber-reinforced polymers	Plain wood beams	Hand Lay-up	CFRP reinforcement produces better impact-resistance properties	[25]
Glass fiber-reinforced epoxy polymer (GFRP)	Steel	-	weight reduction around 36% and 14%deformation increase	[32]
Glass fiber-reinforced polyamide	Steel	Injection molding	A 45% decrease in weight and enhanced recyclability	[33]
E–glass fiber-reinforced epoxy resin	Steel	Hand Lay-up	A safety factor improvement of 64% and increased capacity to withstand higher loads	[34]
reinforced 6061 Al matrix composites	Matrix alone	High-temperature mix and stirring method	Improved mechanical properties (hardness, Yield stress, UTS)	[35]
reinforced 2024 Al matrix composites	un-reinforced alloys	High-temperature alloying andhot extrusion	Better mechanical properties (Young modulus and UTS)	[36]

, polymer composites, particularly CFRP, exhibit numerous advantages for utilization as a battery box in vehicles. These advantages include superior mechanical properties, a lightweight nature in comparison to traditional metal materials, and the absence of high-temperature requirements during manufacturing, unlike metal composites. As a result, CFRP emerges as a promising alternative to the conventional metal-based battery box.

3. Heat Rejection Mechanism and Thermal Performance of Composites

Typical battery cells used in EVs nowadays are lithium-based due to their superior energy density. There are various types of lithium-based cells that can typically be classified into two categories: lithium-ion (Li-ion) and lithium polymer (Li-Po). Examples of Li-ion batteries based on the materials used for their electrode are lithium manganese oxide (LMO: LiMn₂O₄), lithium iron phosphate (LFP: LiFePO₄), lithium cobalt oxide (LCO: LiCoO₂), lithium nickel manganese cobalt oxide (NMC: LiNi_xMn_yCo_zO₂), and lithium nickel cobalt aluminum oxide (NCA: LiNiCoAlO₂). On the other hand, Li-Po batteries use a solid polymer as their electrolyte.

One of the most challenging parts of using lithium batteries is designing effective battery thermal management systems (BTMS) because such materials are extremely sensitive to temperature. Various studies suggested a similar best temperature range for a lithium battery, that is, around 15–35 °C [37,38] or 25–40 °C [39,40,41]. Outside these ranges, significant performance loss up to hazardous levels has been reported. In a sub-zero atmosphere, a more than 50 percent drop in mileage has been reported [42]. Without sophisticated thermal protection, especially against extreme discharge loads or abusive overcharging, an uncontrollable self-heating phenomenon due to internal chemical reactions, a so-called thermal runaway, could occur, and its impact is disastrous. One may remember what happened to a Boeing 787 jet that caught fire due to a battery explosion as a result of a thermal runaway [43]. Furthermore, thermal runaway could also be triggered by other internal or external abuses, such as battery puncture, which often happens in a vehicle collision [44,45] or a scorching environment [46]. Some other thermal-related EV incidents are tabulated and discussed in [47,48]. For non-lithium battery cells, such as lead acid, nickel metal hydride (NiMH), and nickel cadmium (NiCd), thermal runaway is also possible, but normally to a lesser extent [49]. The comparisons between common batteries are tabulated in

Table 2. Comparison of common battery cells technology [50].

	Lead Acid	NiMH	Li-ion	NiCd	Li-Po
Nominal voltage (V)	2.1	1.2	3.6–3.85	1.2	2.7–3.0
Energy density (Wh/kg)	30–40	60–120	100–265	40–60	100–265
Power density (W/kg)	180	250–1000	250–340	150	245–430
Cycle life	<1000	180–2000	400–1200	2000	500
Operation efficiency (%)	50–95	66–92	80–90	70–90	90
Self-discharge rate (%)	3–20	14–71	0.3–2.5	10	0.3
Market price (US$/Wh)	0.70	0.85	0.94	2.68	2.31

[50]. Even worse, the chain reactions leading to thermal runaway could be triggered in just a few seconds, making thermal protection from a battery enclosure a must. This section covers more about protecting the battery pack from thermal runaway and other thermal abuses at high temperatures, while a fire protection system for the composite is presented in detail in the next section.

To understand the thermal runaway concept better, one should begin with the heat generation mechanism inside a battery cell, which can be described as:

$$q=q_{ir}+q_r+q_s+q_m$$

(1)

$$q_{ir}=I\left(V-E_o\right)=I^{2}R$$

(2)

$$q_r=-IT\left(\frac{\partial E_o}{\partial T}\right)=-IT\left(\frac{∆S}{nF}\right)$$

(3)

where $q$, $q_{ir}$ and $q_r$ are the total heat, the irreversible heat generation, and the reversible heat generation, respectively [51,52]. Meanwhile, $q_s$ is the heat from any side reactions is typically due to cell aging, and $q_m$ is the heat generated by mixing processes. Therefore, these two terms are negligible as long as the battery is of relatively good quality without any record of cell abuse [53,54]. $V$ and $E_o$ are the variable battery cell voltage and open-circuit voltage, respectively. $∆S$, $F$, and $n$ are the entropy variation, Faraday constant, and the number of exchanged electrons in the electrochemical reaction, respectively [55]. Therefore, the simplified equation becomes

$$q={q_{ir}+q_r=I}^{2}R-\left(IT\frac{\partial E_o}{\partial T}\right)$$

(4)

where $I$ is the battery current during charge or discharge, $R$ stand for the total internal battery resistance, and $\left(T\frac{\partial E_o}{\partial T}\right)$ is an entropy heat coefficient, which is affected by the battery surface temperature and state of charge (SOC). The second term sign is changed for charge and discharge, resulting in an always exothermic process [56]. The equations above could also explain why there is more heat being dissipated from the battery when the charge/discharge current and the surface temperature increase. The chemical reactions of lithium batteries at the cathode, anode, and overall cell reaction (which proceeds to the right-hand side during the charging process and vice versa) are written respectively as follows:

$$\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{o}{\mathrm{O}}_2\rightleftarrows x{\mathrm{L}\mathrm{i}}^{+}+x{\mathrm{e}}^{-}+{\mathrm{L}\mathrm{i}}_{1-x}\mathrm{C}\mathrm{o}{\mathrm{O}}_2$$

(5)

$$x{\mathrm{L}\mathrm{i}}^{+}+x{\mathrm{e}}^{-}+6\mathrm{C} \rightleftarrows {\mathrm{L}\mathrm{i}}_x{\mathrm{C}}_6$$

(6)

$$\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{o}{\mathrm{O}}_2+6\mathrm{C} \rightleftarrows {\mathrm{L}\mathrm{i}}_{1-x}\mathrm{C}\mathrm{o}{\mathrm{O}}_2+{\mathrm{L}\mathrm{i}}_x{\mathrm{C}}_6$$

(7)

In a single cell, heat is often more accumulated near the cathode than near the anode [57,58]. Mevawalla et al. [59] numerically reported heat generated from the cathode more than seven times under a low discharge rate, although the margin is reduced at a higher rate.

Thermal runaway mitigation is certainly crucial in developing the battery pack enclosure. Niu et al. [60] proposed a paraffin–silica aerogel composite coated with flame retardant, which has extremely low thermal conductivity. According to their thermal runaway test, the battery pack could be kept safe only by using 3 mm of such composite between each cell. On the other hand, without any composite, the damage spread to the neighboring cells in about a minute. One should carefully note that the application of such a composite at present may be limited to a low discharge rate unless it is coupled with another cooling device to absorb the excess heat during battery operation. A low thermal conductivity material around the battery also means that the heat accumulated in a cell cannot be dissipated quickly out of the enclosure. In continuous, multiple cycles of battery operation, the temperature tends to increase steadily if the heat transfer out of the battery enclosure is poor [61,62].

Paraffin, which is mentioned above, is one of the most popular phase change materials (PCM) known for its high latent heat (heat-absorbing capacity without experiencing a temperature rise), non-toxic nature, and relatively affordable price. There are wide variations of PCM available on the market at present, which are made from organic, inorganic, or eutectic materials. Each of them offers a different phase-change temperature profile and thermal conductivity. The latent feature and its abundance boost the direct usage of PCM-filled composite in the EV battery pack, especially for the organic PCMs. Furthermore, the PCM composite could be strategically designed, such as a battery holder, in order to maintain temperature uniformity among the battery cells in a pack [63]. Since PCM melts when it absorbs heat at its melting temperature, its compartment must be flexible enough to endure a volumetric change but also have a sufficiently low thermal resistance to support the heat transfer processes. The biggest drawback of PCM is its low thermal conductivity; hence, metal foams or other materials, such as expanded graphites (EG), boron nitride (BN), or olefin block copolymers (OBC), are often used in tandem for heat transfer augmentation. Also, it has been reported that the addition of paraffin as a filler tends to reduce the mechanical strength of the polymer composite using epoxy resin [51]. Furthermore, paraffin and most organic PCM are typically flammable, so a flame retardant is required. Unfortunately, the selection is rather limited, as an additional flame retardant not only increases the total weight but may also reduce the overall heat transfer quality of the composite itself. Therefore, it is important to design the composite battery pack with careful consideration in terms of heat management and its overall mechanical properties. The maximum temperature T_max from battery thermal management systems using various composite mixtures under charging/discharging rate (C-rate) is tabulated in Table 3. It can be seen that paraffin is the most commonly used PCM. Unfortunately, the T_max for most cases is somewhat above 50 °C, which makes the battery pack less efficient, even though it is still tolerable in terms of vehicle safety. A C-rate is defined as the relative charge/discharge current I based on its nominal capacity Q, as in Equation (8)

$$\mathrm{C}\text{-}\mathrm{rate}=\frac{I}{Q}$$

(8)

As such, the 1 C rate is where the battery could be completely charged/discharged in one hour, while for 2 C, it will be done in half an hour, and so on. The higher the C-rate, the higher the battery load; hence, a higher temperature increase from a battery cell is also expected. Saw et al. measured the heat generation from a single 38120 LFP battery under constant current charging, as plotted in Figure 8 [64]. It can be seen that the increase in heat generation and its subsequent surface temperature are proportional, although they are not always linear with the C-rate. It is also worth noting that within a short period, towards the end of charging, the heat generation tends to surge [65]. Due to the fluctuating nature of the EV battery load in operation, the response time is crucial in designing the thermal management system for the battery enclosure.

Table 3. Thermal management strategy using PCM composites.

Composite	C-Rate 1	Aid 2	Tmax (°C)	Ref.
Paraffin–copper foam	4 (D)	–	47.4	[61]
	4 (3CD)	A	52.0
Paraffin–EG	2 (D)	–	42.2	[66]
	2.5 (D)	–	57.0	[67]
	5 (D)	P	50.9	[62]
	5 (3CD)	A,P	50.5
Paraffin–EG–BN	3 (D)	–	45.7	[68]
	4 (D)	–	53.4
Icosane (C20H42)–OBC-EG	5 (D)	–	44.0	[69]
	10 (D)	–	53.0
Nylon 6–BN	1 (C)	–	33.0	[70]
Paraffin–OBC	2.5 (D)	–	53.2	[71]
Paraffin–OBC–EG	2.5 (D)	–	43.4

¹ (C): charge-only rate; (D) discharge-only rate; (#CD) #-cycles of charge-discharge rate. ² (A): actively aided by auxiliary energy-consuming devices, such as fan or refrigerant pump; (P): passive cooling, such as fins/heat sinks/heat pipes; (–): none.

Table 3. Thermal management strategy using PCM composites.

Composite	C-Rate 1	Aid 2	Tmax (°C)	Ref.
Paraffin–copper foam	4 (D)	–	47.4	[61]
	4 (3CD)	A	52.0
Paraffin–EG	2 (D)	–	42.2	[66]
	2.5 (D)	–	57.0	[67]
	5 (D)	P	50.9	[62]
	5 (3CD)	A,P	50.5
Paraffin–EG–BN	3 (D)	–	45.7	[68]
	4 (D)	–	53.4
Icosane (C20H42)–OBC-EG	5 (D)	–	44.0	[69]
	10 (D)	–	53.0
Nylon 6–BN	1 (C)	–	33.0	[70]
Paraffin–OBC	2.5 (D)	–	53.2	[71]
Paraffin–OBC–EG	2.5 (D)	–	43.4

¹ (C): charge-only rate; (D) discharge-only rate; (#CD) #-cycles of charge-discharge rate. ² (A): actively aided by auxiliary energy-consuming devices, such as fan or refrigerant pump; (P): passive cooling, such as fins/heat sinks/heat pipes; (–): none.

Currently, the integration of a battery pack into a vehicle structure is one of the emerging EV battery technologies. The application of structural battery composite (SBC) could potentially offer a dramatic reduction in total weight, while microvascular composite (MVC) is introduced as the BTMS, as sketched in Figure 9 [72]. A typical MVC architecture is formed by a number of microchannels filled with cooling fluids that can be controlled in order to effectively keep the temperature low and uniform across the batteries. The cooling fluid flow can be integrated into and thus automatically controlled by the battery management system (BMS), which measures the battery state of health and surface temperature at the operation level [73]. A real-time, flexible flow rate cooling system may ensure that the battery temperature and its uniformity are kept in check. In an emergency, the BMS could activate a local or entire shutdown system to reduce thermal runaway damage.

4. Flame Retardancy Improvement of the Polymer Composite

Polymer composites are considered suitable for EVs due to their strength, customizable properties, and lightweight nature, making them a current trend in product development. However, polymer composites typically have a major drawback in that they are prone to easy combustion. The polymer chains used in composite resins can easily decompose and transform into easily combustible materials when exposed to fire, producing heat, smoke, ash, and toxic volatile compounds [74]. When polymer composites are exposed to heat in an open environment, pyrolysis occurs, releasing combustible gases, forming a mixture with oxygen, and igniting the substrate if an ignition source is present [75]. This indicates an undesirable condition that should not occur in a vehicle. However, external factors can potentially cause fire incidents around the battery enclosure. In the event of an accident between an electric vehicle and a conventional vehicle on the road, conventional vehicles may spill fuel onto the road. If the road temperature is sufficiently high and heats up the spilled fuel, a spark from an electrical system failure can potentially ignite the fuel, leading to a fire. The fire can spread to the battery storage compartment or even the battery enclosure itself, which can be referred to as exposure to fire on the surface of the battery enclosure. If the battery enclosure is made of polymer composites, there is a possibility of decomposition and loss of its primary functions as a structure and cover. The risk of catastrophic damage increases if the fire breaches the battery enclosure and directly affects the battery cells, resulting in thermal runaway from external abuse. A battery pack consisting of battery cells and electronic components enclosed within the battery compartment must meet the ISO 12405 standard, which requires no physical damage to be shown. Finally, the summary of flame retardancy, mechanical properties, and manufacturability is tabulated in Table 4 at the end of Section 4.

4.1. Flame Retardants

In general, battery enclosure products made from polymer composites are called glass fiber reinforced polymer (GFRP). However, polymer composite products with other types of reinforcement are also being developed, such as carbon fiber, Kevlar, and even natural fibers. GFRP has a good reputation for ease of manufacturing and cost-effectiveness. GFRP consists of two main components: glass fiber as the reinforcement and resin as the binder. The resin component is a concern when exposed to fire. Commercial general-purpose resins are highly flammable, producing smoke and toxic gases. Engineering efforts are required to improve the flame retardancy of the resin. One practical approach is to add flame-retardant materials to the resin. These flame retardants are typically in the form of powders with particle sizes ranging from micrometers to nanometers. The composition of flame retardants varies depending on the type of resin and the desired target material properties. Although natural reinforcement can have good strength, it still requires fire protection when applied in automotive applications [76]. The indication of improved flame retardancy in the resin can be determined through material characterization and fire reaction tests, such as higher thermal stability, a higher limiting oxygen index (LOI), a lower burning rate, a peak heat release rate (PHRR), or other assessments that signify better flame retardancy.

There are numerous types of flame retardants that can enhance the flame retardancy of resins. Aluminum tri-hydroxide (ATH: Al(OH)₃) can improve the flame retardancy of unsaturated polyester resin (UPR) and even make the polymer composite a self-extinguishing material [77]. Self-extinguishing is a phenomenon where the material exposed to fire ignites, but the burning process is not sustained. The flame gradually diminishes and eventually extinguishes on its own. The decomposition of resin containing ATH is illustrated in a scheme in Figure 10. ATH absorbs heat generated from resin decomposition (an endothermic process). As the temperature increases, ATH releases moisture content from its hygroscopic nature. Then, at a certain temperature, ATH decomposes into water vapor and alumina. ATH decomposes at temperatures between 220 and 350 °C [78]. Water vapor from the decomposition of ATH dilutes the gases produced from resin decomposition, reducing its flammability. Decomposing resin produces gases during the pyrolysis phase, including carbonyl compounds, hydrocarbons, and aromatic compounds [79]. These gases are cooled down by water vapor, hindering the free radical chain reaction process [80]. Alumina forms a layer on the resin surface, acting as a barrier between flammable gases and oxygen from the air. Alumina acts as an inert material and exhibits high temperature resistance [81,82]. An almost balanced weight percentage of ATH to UPR can produce a composite that meets the UL-94 V0 criteria (vertically-positioned specimens would be self-extinguished or the afterflame time for each individual specimen is less than ten seconds), albeit with the consequence of degraded mechanical properties [83]. Even with only 30% of the total composite weight being ATH, it can still meet the V1 criteria (afterflame time is less than 30 s) [84]. In epoxy (EP) resin, the fire resistance improves with the addition of ATH [85]. This improvement is indicated by a decrease in PHRR and smoke emissions. A lower PHRR value suggests a reduction in the material’s activation energy. Meanwhile, the reduction in smoke emissions is evidence that the release of toxic gases such as carbon monoxide (CO) can be minimized. The addition of ATH flame retardant to ultraviolet-curable resin results in a composite with a low PHRR, prevents substrate dripping, and reduces smoke emission [86].

Magnesium hydroxide (MH: Mg(OH)₂) is another potential material for flame retardancy in GFRP [87]. It has the ability to reduce the internal temperature of the battery pack when exposed to external fire. Lower temperatures make the fire less reactive. Practically, MH is capable of extending the flame propagation time during the resin decomposition process [88]. MH can also increase the limiting oxygen index (LOI) of EP resin and prevent EP substrate dripping when it burns [89]. Melting and dripping of resin, carrying embers and fire, pose a higher risk of fire spreading. Modifying MH particles into microwhisker structures can improve the flame retardancy of EP resin [90]. The decomposition mechanism of MH is similar to that of ATH. MH decomposes thermally to form magnesium oxide (MgO) and water vapor [91]. The decomposition temperature of MH is higher than that of ATH, making MH more efficient as a flame retardant [92]. However, the cost of the material should be taken into consideration, as MH is more expensive than ATH.

Ammonium polyphosphate (APP) is a flame retardant that is also effective in enhancing flame retardancy. The LOI value of UPR composite with APP is higher compared to neat UPR [93,94]. A higher LOI value indicates that a material requires a higher concentration of oxygen to sustain stable combustion. If the oxygen content in the open environment is 21%, materials with LOI higher than this percentage are difficult to sustain stable flame propagation when ignited (self-extinguish). The decomposition temperature range of APP is quite wide, from 200 to 700 °C [95]. Water vapor and ammonia are released during the early stages of decomposition, followed by the formation of phosphoric acid, and finally leaving behind phosphorus oxides as residue. At a certain concentration in the air, ammonia has autoignition characteristics at a temperature of 650 °C [96]. Although this material has a lower burning velocity compared to carbon-based flammable gases [97], its lighter density than air allows for easy dispersion of ammonia. Therefore, concerns regarding fire hazards caused by ammonia are reduced. Studies have shown that modifying the surface of commercial APP particles to enhance crosslinking with polyurethane (PU) can reduce the PHRR [98]. An interesting aspect of APP decomposition is the formation of intumescent char residues [99]. It involves the swelling of a layer triggered by temperature increases, forming a cellular foam-like structure. This structure acts as a physical barrier that protects the resin from heat propagation [100].

High-strength flame retardant Silica has a significantly higher decomposition temperature compared to resin. Silica can inhibit the resin’s decomposition by trapping the heat released during resin oxidation [101]. This results in less reactive flame propagation. Alumina and graphene as flame retardants have been studied and shown promising results. EP added with alumina and graphene flame retardants can produce a self-extinguishing composite of natural fiber-EP [102].

To achieve optimal flame retardancy, two or more flame retardant materials can be combined. ATH and Silica Aerogel (SA) demonstrate higher thermal stability and sufficient burning rates in UPR [103]. SA synergistically interacts with ATH by widening the temperature range of ATH decomposition. The combination of flame retardants ATH and melamine can elevate the LOI value, decrease PHRR, and enhance the thermal stability of EP resin [104]. The hybridization of zinc borate (ZB) and APP can enhance the flame retardancy of rubber composites, as indicated by lower PHRR compared to those without flame retardants [105]. The synergistic flame retardancy between MH and carbon black (CB) can prevent decomposition and even improve the thermal stability of Ethylene Vinyl Acetate (EVA) composites [106]. Combining two fillers can improve flame retardant efficiency compared to using only one filler. Further studies on the combination of three flame retardants in EP resin resulted in improved flame retardancy compared to using one or two fillers. ATH, MH, and organophosphorus compounds (Roflam F5) in EP exhibited the lowest PHRR values [107]. Flame-retardant materials can be derived from natural raw materials. Clay is one natural material used in composites to enhance strength. Montmorillonite (MMT), a processed clay product, is used as an additional filler to improve resin strength. A study on the hybrid combination of ATH and MMT in UPR showed increased flame retardancy in GFRP [108]. The improvement in flame retardancy is indicated by a decrease in the burning rate. Activated nanopumice produced using the sol–gel precipitation method can enhance the flame retardancy of UPR [109]. Activated nanopumice inhibits the spread of fire by prolonging the ignition time and reducing the burning rate.

4.2. Impact of Flame Retardant to the Polymer Composites Mechanical Properties

The addition of fillers to resin results in changes in mechanical properties. Just like a polymer composite that consists of several components, each with different mechanical properties, when combined into a composite, it acquires specific characteristics without altering its original mechanical properties. Flame retardants have different characteristics compared to resin. The addition of flame retardants to certain compositions can enhance substrate strength, but it can also decrease it. The addition of ATH concentration to GFRP tends to cause a decrease in flexural strength [110]. ATH particles tend to agglomerate, so at high concentrations, they can reduce substrate strength. Observations of flame retardant agglomeration indicate a decrease in composite quality due to increased porosity [103]. Another aspect of substrate elasticity is reflected in its toughness. The addition of ATH to EP resin results in more than half the toughness degradation compared to the neat substrate [111]. Increasing the concentration of MH flame retardant shows a decrease in the energy absorption of the composite with natural fibers [112]. This is because MH is a rigid particle, and with increased composition, the substrate becomes more rigid as well. Therefore, the composition of flame retardants in the resin requires special attention as it affects the mechanical properties of the composite.

Only when the concentration of flame retardant in the resin is very low do the mechanical properties of the composite remain unchanged or even improve, at least if the flame retardant is not modified. The flexural and impact strengths of UPR show improvement with the addition of 3 wt% ATH [113]. The addition of 1 wt% APP processed with a material to enhance dispersion successfully increases the tensile and flexural strength of EP composite [114]. Furthermore, using flame retardants with smaller particle sizes can have a positive effect on the mechanical properties of the composite. UPR using ATH with a particle size of 1.5 μm shows an increase in tensile strength without significantly affecting the curing characteristics [115]. A study on modified flame retardants proves that UPR with microencapsulated APP demonstrates better mechanical properties compared to UPR with regular APP [116]. This is due to the improved adhesion of APP to UPR as a result of the microcapsule material being compatible with the resin.

4.3. The Influence of Flame Retardants on the Manufacturing Process of Composites

The evaluation of the manufacturing process of composites can be determined by the physical performance of the flame retardant in the resin. The addition of flame retardant increases the viscosity of the resin. The addition of 15 wt% APP to UPR increases the viscosity by more than two times [93]. This poses new challenges in the manufacturing process of polymer composites. Resin with a high viscosity leads to reduced interfacial bonding between the resin and reinforcement. That means the resin and fibers become more difficult to bond together. However, the negative effects of adding flame retardant to the manufacturing process of composites can be minimized. Although it does not have a significant impact on the viscosity of the resin. The mixing process of ATH and resin, which is hard and time-consuming, can prevent the agglomeration of flame retardant and ensure good dispersion [117]. This is done to minimize the degradation of mechanical properties, even though the manufacturing process is a manual hand-lay-up. In the manufacturing of polymer composites using the vacuum-assisted resin fusion technique, the viscosity of the resin determines the success of the process. Increasing the filler composition can raise the viscosity of the resin, thereby reducing its ability to flow into the fiber material [118]. The distribution of flame retardant particles becomes uneven, resulting in non-uniform flame retardancy across different cross-sections [119]. Monomer diluents can be used to reduce resin viscosity. However, an evaluation of the mechanical properties after the addition of diluents must still be carried out. Styrene can be used to reduce the viscosity of UPR [120]. The negative effect of a high styrene composition in the resin is a longer curing process. Another manufacturing technique that has been developed is the addition of flame retardant to resin gel coatings. The flame retardancy of a polymer composite may fall short of the desired target when trying to maintain optimal mechanical properties. However, the flame retardancy of the composite can still be enhanced by incorporating a fire-resistant gel coat. A gel coat containing APP can improve flame retardancy while ensuring the quality of the composite surface remains intact [121]. Ultimately, a compromise between the selection of flame retardant and suitable manufacturing techniques is required to achieve a composite with both optimal mechanical properties and flame retardancy.

Table 4. Effects of adding flame retardants on the flame retardancy, mechanical properties, and manufacturability of the polymer composites.

Flame Retardant	Flame Retardancy	Mechanical Properties	Manufacturability
ATH	Increased ATH loading: Self-extinguished and meets UL 94 V-0 at loading higher than 40 wt% in UP Decrease PHRR Increase time to ignition (TTI) Decrease peak mass loss rate Increase residual mass Decrease burning rate Decrease smoke yield	Increased ATH loading: Decrease flexural strength Increase tensile strength Increase porosity Decrease toughness/increase brittleness	Increased ATH loading: Increase matrix viscosity Decrease reinforcement wettability
MH	Increased MH loading: Comply with UL 94 V-0 at loading of 60 wt% in EVA Increase LOI Decrease PHRR Decrease smoke yield	Increased MH loading: Increase tensile strength	Increased MH loading: Increase matrix viscosity
APP	Increased APP loading: Increase LOI Decrease PHRR	Increase tensile and flexural strength when the APP load is low	Better wettability than ATH and MH
Silica, Alumina, Graphene	Increase decomposition temperature Self-extinguished in certain composition	Increase in fillers loading: Decrease flexural strength Increase tensile strength
MMT, Pumice	Decrease mass loss rate Decrease rate of burning Increase time to ignition	Increase flexural strength if using modified MMT Decrease surface hardness if MMT loading increase Increase flexural and impact strength along with Pumice loading

Table 4. Effects of adding flame retardants on the flame retardancy, mechanical properties, and manufacturability of the polymer composites.

Flame Retardant	Flame Retardancy	Mechanical Properties	Manufacturability
ATH	Increased ATH loading: Self-extinguished and meets UL 94 V-0 at loading higher than 40 wt% in UP Decrease PHRR Increase time to ignition (TTI) Decrease peak mass loss rate Increase residual mass Decrease burning rate Decrease smoke yield	Increased ATH loading: Decrease flexural strength Increase tensile strength Increase porosity Decrease toughness/increase brittleness	Increased ATH loading: Increase matrix viscosity Decrease reinforcement wettability
MH	Increased MH loading: Comply with UL 94 V-0 at loading of 60 wt% in EVA Increase LOI Decrease PHRR Decrease smoke yield	Increased MH loading: Increase tensile strength	Increased MH loading: Increase matrix viscosity
APP	Increased APP loading: Increase LOI Decrease PHRR	Increase tensile and flexural strength when the APP load is low	Better wettability than ATH and MH
Silica, Alumina, Graphene	Increase decomposition temperature Self-extinguished in certain composition	Increase in fillers loading: Decrease flexural strength Increase tensile strength
MMT, Pumice	Decrease mass loss rate Decrease rate of burning Increase time to ignition	Increase flexural strength if using modified MMT Decrease surface hardness if MMT loading increase Increase flexural and impact strength along with Pumice loading

5. Challenges and Future Directions

Implementing polymer composites as battery pack materials presents several challenges and future directions for development, with various things to consider carefully, as presented in Figure 11. Challenges include the relatively high cost of additional reinforced materials, such as carbon fiber, compared to traditional materials, requiring the exploration of cost-effective production methods. The specialized manufacturing techniques for any reinforced composite, such as autoclave curing and resin infusion, call for the development of simplified and scalable manufacturing processes. Furthermore, optimizing the design of battery box structures with reinforced composites requires considerations of weight reduction, structural integrity, and thermal management, necessitating further research and development.

Future directions involve advancements in reinforced polymer composites through ongoing research to enhance performance and reduce costs. This includes exploring new types of fibers or other reinforcements, matrix materials, and composite architectures. Additionally, innovation in manufacturing techniques is crucial to streamline production, reduce cycle times, and increase scalability. Integration of additional functionalities into composite battery boxes, such as sensors for structural health monitoring, energy harvesting capabilities, or thermal management features, represents another avenue of exploration. Addressing the environmental impact of polymer composites through recycling methods and sustainable manufacturing practices, including the use of bio-based or recycled fibers, is also important. Lastly, establishing industry standards and regulations specific to reinforced composite battery box structures ensures consistent quality, safety, and performance.

Future research to enhance the flame retardancy of polymer composites involves combining several flame retardants. This is considered effective in achieving a compromise between optimal mechanical properties, heat-absorbing quality, and flame retardancy. Combinations within polymer composites can be observed between hybrid flame retardants, both inorganic with organic and among inorganics [122]. In studies aiming to improve the effectiveness of a flame retardant, complex material preparations may be required. Modifying APP through microencapsulation techniques using a bio-based EP results in optimal thermal, flammability, and mechanical properties of bio-composites [123]. This technique can also be applied to other single flame retardants as well as hybrid flame retardant combinations, which are subsequently processed through microencapsulation. Microencapsulated ATH and APP flame retardants perform better compared to untreated APP [124].

The effectiveness of flame retardant fillers can be enhanced by using smaller particles, such as nanoparticles (NPs). NPs have better dispersion capabilities within the resin and a higher surface-to-volume ratio compared to micrometer-sized particles [125]. ATH in NP form can prolong the time to ignition, withstand heat release, and provide better protection against heat exposure [126]. These advantages are not limited to flame retardancy; NPs can also improve heat management and mechanical properties up to a certain concentration. NPs can be used to augment the heat transfer of PCM-filled composites, making them efficient heat absorbers. But the typical challenge with NPs addition lies in their different density than the PCM itself; they may settle when the PCM melts [41]. Tensile strength and elongation at break in composites with ATH NPs are higher than those without fillers [127].

Material engineering innovation serves as another option to discover the best and most effective flame retardants for enhancing the flame retardancy of polymer composites. Various flame retardancy mechanisms can be tailored through combinations of multiple fillers or processes. However, achieving the target of obtaining the best flame retardancy becomes increasingly challenging but worthwhile. Polymeric glycerol-based (PCH₃PG) is a flame retardant for UPR resin that works in the gas phase and provides thermal barrier properties [128]. The next flame retardant, phosphorus-containing acrylate (ODOPB-AC), acts as a gas-phase flame inhibitor and produces char in the condensed phase [129]. Indeed, flame inhibition through the production of inert gas or water vapor and a tough thermal barrier (char) is an appropriate mechanism for enhancing flame retardancy. Modeling and simulation can also be used as tools to predict the fire behavior of a polymer composite. Flame retardancy can be predicted using AI-based models for thermal propagation, surface temperature, or mass loss rate of the composite [130]. However, this method requires accurate datasets obtained from experimental results.

Nevertheless, the challenge in developing polymer composites for battery packs lies in ensuring that the representation of material characterization, namely flame retardancy, thermal performance, and mechanical properties, can reflect real-world conditions. However, this is often insufficient. Characterization and fire testing of polymer composite specimens are only suitable for understanding the material’s reaction to fire. Different results may apply to more complex systems within an entire vehicle. Indeed, the proper approach is to perform fire testing on a complete-sized battery pack. However, this would require costly operations and facilities. Nonetheless, it can be concluded that a polymer composite for an EV battery pack still requires a very fine lining to select, design, and fabricate. It is important that the end product meet certain standards and regulations so that sufficient protection against mechanical, electrical, thermal, and fire abuse can be provided.