Recent Development of Thermal Insulating Materials for Li-Ion Batteries

Abstract

As one of the core components of electric vehicles, Li-ion batteries (LIBs) have attracted intensive attention due to their high energy density and good long-term cycling stability. However, some abuse conditions inevitably occur during battery operation, resulting in safety accidents such as the thermal runaway (TR) of LIBs. Therefore, the efficient and appropriate thermal insulation material design is crucial for LIB packs to effectively reduce or even inhibit the spread of TR. Based on it, in this review, we present the principle and influences of TR to provide the necessity of battery thermal management and thermal insulating materials. Then, we deeply discuss and compare the two kinds of representative thermal insulating materials: phase change thermal insulating materials and barrier-type thermal insulating materials. Their properties, synthesis methods, and modification means are investigated to provide some guidance for the future application of high-performance thermal insulating materials in the field of LIBs.

1. Introduction

The imperative significance of energy conservation has precipitated a fervent scholarly pursuit on a global scale. The relentless advancement of science and technology has fueled an escalating quest towards enhancing energy efficiency, curbing non-renewable resource depletion, and pioneering novel energy modalities [1]. The inherent cyclicality of renewable energy poses a principal obstacle, necessitating the implementation of suitable energy storage remedies to mitigate and surmount these challenges [2]. The manifestations of the greenhouse effect and resource depletion have caused serious damage to the natural ecosystem, prompting a profound recognition of the imperative to cultivate novel energy reservoirs with elevated energy efficiencies [3]. In recent years, the paradigm of energy selection has undergone a transition progressively from chemical-based fuels towards electrochemical energy storage devices, with a gradual migration from conventional fuel vehicles to electric counterparts [4,5,6]. Indeed, the battery pack serves as the fulcrum of these electric vehicles, directly dictating their energy utilization and consequent driving range. LIBs have widespread adoption within the automotive sector owing to their exalted attributes: elevated energy densities, potent power outputs, minimal self-discharge tendencies, and enduring life cycles. Commercial lithium-ion battery configurations feature a structural demarcation between the cathode and anode, mediated by the electrolyte comprising an organic solvent, a lithium-based electrolyte salt (e.g., lithium hexafluorophosphate LiPF₆), and assorted additives. Nevertheless, the volatile and inflammable nature of organic solvents usually leads to thermal runaway (TR) accidents in Li-ion batteries (LIBs) when the internal temperature is too high [7,8,9]. Therefore, the exploration of effective and suitable ways to inhibit the TR spread is necessary.

One of the effective ways to suppress or reduce TR spread is the modification of battery components, such as the protective coatings for electrodes. The coating layer can effectively mitigate the undesirable side effects that occur at the active material/ electrolyte interface in the cathode mixture, further increasing the safety of LIBs. For example, Wei et al. [10] coated a layer of LiNbO₃ on the surface of the LiCoO₂ electrode and investigated the degradation mechanism of the bare and LiNbO₃ coated LiCoO₂ electrodes in all-solid-state batteries when cycled at different cut-off voltages. The existence of the coating layer enabled the battery to have better cycle stability and performance. The LiNbO₃ coating did not change the phase transition process of the cathode material but improved the structural stability of the electrode material and electrolyte, effectively enhancing the safety of LIBs.

Besides component modifications, the addition of thermal insulating materials in battery packs is another suitable way to suppress or reduce the TR spread of LIBs. Thermal insulating materials for LIBs can usually be divided into two categories, including phase change materials and barrier-type insulation materials. Initially, conventional insulating materials, such as polyurethane foam, fiberglass, ceramics, and others, were employed for LIBs. To a certain extent, these materials can insulate a significant amount of heat, but their high weight and cost restrict further applications.

With the continuous advancement of science and technology, certain thermoplastic materials have emerged, such as polystyrene (EPS) and polypropylene (PP), which have started to be utilized in LIBs. While they possess lighter weight and lower cost compared to traditional insulating materials, they still exhibit a certain degree of thermal insulation performance degradation and limitations to meet the requirements of actual applications. In recent years, researchers have initiated the design and fabrication of high-performance thermal insulating materials, resulting in the emergence of materials such as aerogel, porous materials, and other novel substances with outstanding insulation properties and lightweight characteristics [11,12,13]. Simultaneously, the potential of phase change materials (PCMs) in thermal insulation has started to capture researchers’ attention due to ongoing investigations into energy storage techniques. It is noteworthy that PCM can provide phase transitions under specific conditions, thereby altering their physical properties. Based on the phase change process and the performance, PCM has been widely used in Li-ion battery packs [14,15,16]. With the advancement of technology, PCMs are currently being commercially utilized in battery systems, and certain electric vehicle manufacturers are starting to integrate them as a crucial component of battery insulation systems.

Given the rising demand for electric vehicles and energy storage in the foreseeable future, the utilization of PCM in the battery sector is anticipated to undergo further expansion [17,18,19,20]. Unlike phase change types, barrier-type thermal insulating materials rely on their low thermal conductivity and high thermal resistance to achieve TR suppression. In the early period, the research on barrier-type thermal insulating materials focused on the exploration of the insulation mechanism and the suitable synthesis methods of the materials. Later, due to the frequent occurrence of the TR problem of LIBs, the research on barrier-type thermal insulating materials with excellent insulation performance has become an important means to improve the safety of batteries. Furthermore, due to the demand for practical applications, the design and characterization of new materials have become hot topics of discussion at the current time. With the increasing demand for LIBs in the new energy market, the requirement for studying novel barrier-type thermal insulating materials for the thermal management of batteries will be further increased.

With the above-mentioned factors in mind, this review will comprehensively examine the battery system from the standpoint of insulating materials. It will primarily summarize the various thermal insulating materials presently employed in batteries and delineate their specific roles in enhancing the safety and reliability of battery systems. Some of the most common types of thermal insulating materials are categorized and listed in Figure 1. The objective of this review is to investigate and compare the properties, synthesis, and modification of various materials used for lithium-ion battery insulation. Section 2 will discuss the possible reasons for the TR of LIBs. The necessity of suppressing TR will be presented. Afterward, two types of thermal insulating materials will be discussed, including the phase change and barrier-type thermal insulating materials. In Section 3, the properties, synthesis methods and modification methods of organic and inorganic PCM will be summarized, and the advantages and disadvantages, as well as the conditions of application of different PCM materials, will also be compared. Similarly, the properties, synthesis and modification methods of organic and inorganic barrier-type thermal insulating materials will be summarized in detail in Section 4, and the advantages and disadvantages of both will be discussed. Finally, Section 5 will present the inhibition performance of TR by phase change and barrier-type thermal insulating materials.

2. Battery Thermal Behavior

Due to their chemical properties, the components of LIBs are prone to decompose at high temperatures, resulting in the TR of LIBs. For example, the initial decomposition of the solid–electrolyte interphase occurs around 80 °C [21,22,23]. When the temperature continuously rises, the decomposition of conventional electrolytes takes place, resulting in the production of a large number of flammable gases, such as hydrogen, methane, ethane, etc., which are easily ignited at high temperatures, leading to further heat generation [24,25,26]. Therefore, appropriate thermal management can not only allow the battery to last longer but also reduce the probability of the TR.

2.1. Battery Heat Generation

Typically, the primary sources of heat leading to TR in batteries include the following factors [27,28,29,30].

Chemical reaction: The chemical reaction between the electrolyte and the positive and negative electrode materials inside the battery is the basis of the operation of LIBs. During the charging and discharging process, the oxidation–reduction reaction takes place at the positive material (such as lithium cobalt oxide) and the negative material (such as graphite), which always generates heat. However, in some cases, such as battery damage, overcharge, over-discharge, or a high-temperature environment, these reactions may be out of control, resulting in a large amount of heat release [31,32,33].

Resistance heating: There is a certain resistance inside the battery. When the ions migrate inside the battery, the resistance will cause the electric energy to be converted into heat energy. Especially in the case of high-rate charge/discharge or excessive use of high-power equipment, the internal resistance of the battery will increase, leading to the increase of heat release accordingly [30,34,35].

Overcharge/discharge: The overcharge/discharge usually causes the electrolyte to overheat and thereby generate exclusive heat. Besides, the chemical reaction inside the battery will be out of control, further increasing the release of heat [36,37,38,39,40].

External ambient temperature: the increase in ambient temperature can also cause the TR of LIBs. In high-temperature environments, the chemical reaction rate inside the battery is accelerated, resulting in increased heat generation [41].

2.2. TR in LIBs

LIBs are mainly composed of a positive electrode, a negative electrode, the electrolyte and a separator. The negative electrode of LIBs is mainly made of carbon materials and noncarbon materials. Carbon materials are divided into graphite, amorphous carbon, and silicon nanomaterials. Noncarbon materials mainly include titanium, tin, silicon, and nitride materials. The positive electrode mainly contains lithium-containing transition metal oxides and other compounds. Complex chemical reactions occur during the operation of LIBs, resulting in a large amount of heat generation, including polarization heat, Joule heat, chemical reaction heat, chemical side reaction heat, etc. The total calorific value of the battery is expressed as follows [27,42,43]:

$$Q=Q_r+Q_J+Q_P+Q_{sr}$$

(1)

In the formula, Q is the total heat of the battery, Q_r is the chemical reaction heat, Q_J is the Joule heat, Q_p is the polarization heat, and Q_sr is the chemical side reaction heat.

Battery TR is mainly caused by exothermic reactions during improper overcharge/discharge or the short circuit from the tearing of the separator inside the battery. These can result in a series of chemical reactions that further produce a great amount of heat. Figure 2a enumerates several abusive behaviors that are susceptible to causing TR. Based on the above reasons, the TR of LIBs can be divided into the following periods [18,44,45,46]. The first is the decomposition of the solid–electrolyte interface (SEI). SEI contains metastable components, which decompose exothermically when the temperature is around 90–125 °C. The decomposition of the SEI can lead to the direct contact of the negative electrode and the electrolyte, which promotes the reaction between them and generates large amounts of gas. The second is the decomposition of the negative electrode. At around 125–180 °C, the internal gas release and temperature rise of the battery are accelerated, causing the acceleration of the gas production and the decomposition of the negative electrode materials. The third is the occurrence of TR. Above 180 °C, the exothermic reaction and electrolyte decomposition reaction rate increased sharply, accompanied by the fast increase of the internal temperature, leading to the occurrence of TR.

In fact, when the heat generated during the heat release is not released to the outside in time, the temperature inside the battery will rise within a certain period of time, leading to the generation of uncontrollable heat. However, due to the requirements of the actual operation, multiple cells are usually connected to battery modules, and multiple battery modules are connected to battery packs to achieve high power of batteries. When a single battery encounters TR, the TR can easily spread to the whole battery module, resulting in disastrous consequences. Therefore, effective thermal insulating materials for the battery are imperative to prevent TR from spreading.

2.3. The Necessity of Thermal Management

As the high temperature will affect the internal resistance, cycle life, and charging/discharging functions of LIBs, irreversible damage of LIBs may occur at high temperatures. The increase in the overall temperature in LIBs will aggravate the uneven temperature between modules and reduce the life and safety of the batteries. Therefore, it is necessary to study the thermal management systems of LIBs. For example, Chen et al. [47] investigated the all-solid-state lithium metal batteries using the accelerating rate calorimeter (ARC) to quantify the thermal stability of four prevalent oxide solid-state electrolytes (SEs) with metallic Li. It was found that the oxygen produced by SE decomposition at elevated temperatures was the main cause of TR. The results showed that the highly reactive metal lithium and oxygen in SE brought potential safety problems to LIBs at elevated temperatures [48].

Sun et al. [49] built a thermal diffusion inhibition experiment system for LIB modules and achieved the goal of TR and zero diffusion between LIBs in the modules through the thermal insulation layer. They studied the effects of six different thermal insulation layer materials on the thermal diffusion process of lithium-ion battery modules. The results showed that the thermal insulation layers can effectively inhibit the heat spread in the battery module. Liu et al. [50] used different thermal insulating materials to alleviate an overcharge LIB TR propagation. It was discovered that the fiber-based material has a temperature drop efficiency of 71.83%, while the aerogel materials are at least 13% more efficient in temperature reduction than fibrous materials. Feng et al. [43] comprehensively addressed the safety concerns of LIBs in electric vehicles by meticulously outlining the mechanisms of TR, summarizing the abusive conditions that precipitate TR, and ultimately proposing an innovative concept to mitigate the risk of TR. In Figure 2b, Zhang et al. [51] elucidated the internal thermal dynamics of LIBs through detailed TR modeling and assessed the critical characteristics of TR under various abusive conditions, with particular emphasis on the magnitude of the trigger energy. In Figure 2c, Zhao et al. [52] identified a correlation between internal TR and external combustion in LiFePO₄ batteries through controlled heating treatment, confirming the interplay between internal thermochemical reactions and external combustion during TR, thereby providing a theoretical foundation for future battery design. In Figure 2d, Chen et al. [53] successfully synthesized a quasi-solid electrolyte (QSE) using an in-situ polymerization method. Compared to the flammable non-aqueous organic liquid electrolyte (LE), the enhanced stability of the QSE indicates its suitability for ensuring battery reliability under various abusive conditions. Moreover, the unique solid electrolyte interface and reduced reactivity with Li of the QSE could significantly enhance the safety profile of LIBs.

Sun et al. [54] developed an innovative hybrid battery thermal management system incorporating active liquid and passive cooling mechanisms. This system utilized copper foam and novel foamed graphite–paraffin composites for passive cooling, effectively mitigating the risk of TR propagation. The thermal performance of the battery module equipped with this hybrid thermal management system was evaluated under 3C discharge conditions and various drive cycles. The study’s findings demonstrated that this newly designed thermal management system effectively prevented TR propagation. Integrating active and passive cooling strategies in battery thermal management systems lays a robust theoretical foundation for future advancements in this field. Because the temperature of the battery was crucial to its performance and service life, improving the characteristics of the battery at elevated temperatures was the main barrier to prohibiting TR.

Figure 2. (a) Lithium-ion battery accidents and some of the potential abuse conditions causing TR; reproduced with permissions from ref. [43], copyright 2018, Elsevier. (b) LiFePO₄ internal TR mechanism and four-stage internal temperature distribution; reproduced with permissions from ref. [51], copyright 2024, Elsevier. (c) Relating internal TR to external burning in lithium batteries; reproduced with permissions from ref. [52], copyright 2024, Elsevier. (d) TR charts for LE and QSE lithium batteries. Reproduced with permissions from ref. [53], copyright 2024, Elsevier.

Figure 2. (a) Lithium-ion battery accidents and some of the potential abuse conditions causing TR; reproduced with permissions from ref. [43], copyright 2018, Elsevier. (b) LiFePO₄ internal TR mechanism and four-stage internal temperature distribution; reproduced with permissions from ref. [51], copyright 2024, Elsevier. (c) Relating internal TR to external burning in lithium batteries; reproduced with permissions from ref. [52], copyright 2024, Elsevier. (d) TR charts for LE and QSE lithium batteries. Reproduced with permissions from ref. [53], copyright 2024, Elsevier.

3. Phase Change Materials

PCMs have the ability to change their physical state within a certain temperature range. Taking the solid–liquid phase transition as an example, when heated to the melting temperature, the phase transition from solid state to liquid state occurs. During the melting process, the PCM absorbs and stores a large amount of latent heat. When the PCM is cooled, the stored heat will be emitted to the environment within a certain temperature range, and the reverse phase transition from liquid to solid will be carried out. The energy stored or released in these two-phase transitions is called the latent heat of phase transition [55,56,57,58,59]. When the physical state changes, the temperature of the material itself remains almost unchanged before the phase transition is completed, forming a wide temperature platform. Although the temperature remains unchanged, the latent heat absorbed or released is large. PCM can be divided into three categories: the organic PCM, the inorganic PCM, and the composite PCM.

3.1. The Organic PCM

Common organic PCM mainly includes advanced fatty hydrocarbons, aromatic hydrocarbons, alcohols, and carboxylic acids [60,61,62].

The advantages of organic PCM can be reflected in the following aspect: Low undercooling. Supercooling is not prone to occur during phase transition, i.e., high latent heat of phase change. In the process of phase transition, it is capable of absorbing or releasing a large amount of heat, which is conducive to improving the utilization rate of energy and providing excellent thermal stability. Even at elevated temperatures, it is able to maintain good performance and is not easy to decompose or deteriorate. They are non-toxic and non-corrosive.

3.1.1. Paraffin

Paraffin is one of the most commonly used organic PCM. The general formula of paraffin wax is C_nH_2n+2, and its enthalpy phase transition is about 200 kJ/kg. Paraffin is a white, odorless, waxy solid in the standard state. Its density is slightly lower than that of water. It is easily soluble in non-polar liquids such as gasoline, ether, benzene, and carbon tetrachloride and insoluble in polar liquids such as water. Pure paraffin has very excellent properties, and its good insulation properties make it have lower resistivity than most materials. Because its components are mainly mixtures of alkanes, the mixture also shows different physical properties with the change in the number of carbon atoms in alkanes. In the standard state, when the number of carbon atoms is less than 5, alkanes mainly exist in the form of gas. When the number of carbon atoms is between 5 and 16, alkanes mainly exist in the form of liquid. When the number of carbon atoms is greater than 16, alkanes mainly exist as solids. Their fusion temperature and latent heat of fusion increase with the length of the carbon chain, but their thermal conductivity and flammability are difficult to inhibit. Sarra et al. [63] analyzed literature data and found that when the content of paraffin in PCM exceeded 50%, the overall heat transfer ability and the material’s thermal stability were improved to a certain extent. In general, the fusion temperature range of commercial paraffin wax is between 45 °C and 60 °C, making it safe, reliable, and non-expensive.

Table 1. Thermal properties of paraffin waxes with the number of C atoms [63].

Paraffin Wax	Molecular Formula	Melting Temperature (°C)	Crystallization Temperature (°C)	ΔHfus (kJ·kg−1)
n-Dodecane	CH3(CH2)10CH3	−10	−16	216
n-Tridecane	CH3(CH2)11CH3	−5	−9	160
n-Tetradecane	CH3(CH2)12CH3	5–6	0	227
n-Pentadecane	CH3(CH2)13CH3	10	5	205
n-Hexadecane	CH3(CH2)14CH3	18–19	17	237
n-Heptadecane	CH3(CH2)15CH3	22	22	171
n-Octadecane	CH3(CH2)16CH3	28	25	242
n-Nonadecane	CH3(CH2)17CH3	32–33	27	222
n-Eicosane	CH3(CH2)18CH3	36–37	31	247
n-Heneicosane	CH3(CH2)19CH3	39–41	32	201
n-Docosane	CH3(CH2)20CH3	42–45	43	157
n-Tricosane	CH3(CH2)21CH3	48.9	51	142
n-Tetracosane	CH3(CH2)22CH3	50–51	48–49	160
n-Pentacosane	CH3(CH2)23CH3	54	47	164
n-Hexacosane	CH3(CH2)24CH3	56	53–54	255
n-Heptacosane	CH3(CH2)25CH3	59	53	159
n-Octacosane	CH3(CH2)26CH3	61	54	202

lists the thermal properties of paraffin with different C atoms.

• Properties

The phase transition temperature, a critical parameter indicating the occurrence of phase transitions in materials, dictates the thermal conditions under which the material can absorb or release energy [64]. For paraffin, as a mixture of various alkanes, the phase transition temperature is highly sensitive to the source and processing methods of these alkanes, resulting in a narrower range. The phase transition temperature significantly influences the properties of paraffin materials. When the ambient temperature exceeds the phase transition temperature, paraffin transforms from solid to liquid, absorbing heat and reducing thermal transfer to enhance the insulation effect. Conversely, paraffin solidifies if the ambient temperature falls below the phase transition temperature, releasing the stored energy. Therefore, selecting the appropriate phase transition temperature is crucial to ensure the efficacy of paraffin materials in practical applications. An excessively high phase change temperature or one that is too low will prevent the material from absorbing or releasing energy as required by LIBs.

Thermal conductivity is another crucial factor affecting the thermal insulation properties of PCM. High thermal conductivity contrasts sharply with low thermal conductivity: a material with high thermal conductivity conducts heat rapidly, facilitating heat transfer through the layers of paraffin wax PCM, thereby diminishing its insulation effectiveness. Conversely, a material with low thermal conductivity has limited heat transfer, allowing the paraffin PCM to absorb or release heat during the phase change process for a longer time, thereby enhancing the insulation effect. During the phase change process, the material itself absorbs or releases a substantial amount of heat. If the material’s thermal conductivity is too high, the heat will rapidly transfer through the material layer to the other side, reducing the efficiency of heat storage and release. Lower thermal conductivity, however, helps to prolong the phase change process, enabling the material to store and release heat more efficiently.

Additionally, paraffin, a mixture of several alkanes, melts into a liquid state at high temperatures, making it easy to leak. The leakage condition leads to a loss of storage material and adversely affects heat storage capacity, potentially causing the failure of the entire battery system. Therefore, selecting appropriate encapsulation materials is essential to prevent paraffin leakage and reduce the risk of battery failure. Moreover, paraffin wax’s oily and viscous nature makes it difficult to grind into powders at room temperature. Its density differs significantly from other materials (e.g., graphite). Thus, preparing composite materials to achieve the improved phase transition temperature and thermal conductivity properties is challenging.

• Synthesis and modification

There are numerous synthesis methods for paraffin PCM. In this review, several typical synthesis methods will be intensively discussed [65,66,67].

Physical mixing, chemical copolymerization and melt impregnation are commonly used to prepare paraffin PCM.

Physical mixing is the simplest method to prepare PCMs, with which the contents can be easily controlled. Li et al. [68] prepared graphite additive/paraffin composite PCM by physical mixing method. During the experiment, it was observed that the composite PCM was prone to deposit caused by inhomogeneity, which seriously affected the temperature rise rate of the composite PCM. Paraffin PCM synthesized by chemical copolymerization has better compatibility and better material properties. Dong et al. [69] prepared a phase change polymer (PCP) with intrinsic leak resistance, reprocess ability and heat dissipation by copolymerization of octadecyl acrylate (OA) and methyl methacrylate (MMA). The copolymerized PCM had more stable phase transition properties than the traditional composite PCM. OA provided 100.5 J/g latent heat for copolymerized PCM, which improved the phase transition ability. The melt impregnation method is suitable for large-scale production because of its simplicity and high production efficiency. Xia et al. [70] successfully prepared EG/paraffin composite PCM by melt impregnation method. Due to the difference in density between the two, bubbles and voids appeared in the composite PCM. With the increasing EG content, the existence of pores led to the actual density of the material being less than the theoretical density, resulting in a decrease in performance.

Although the above-mentioned methods have certain advantages, they are no longer the mainstream methods for synthesizing paraffin PCM due to the fact that products synthesized with these methods usually have low thermal stability, differential thermal conductivity and easy leakage properties. Research scholars have developed a new preparation method, which is called microencapsulation technology. The purpose of protecting the core material is achieved by selecting a suitable shell layer material. It not only improves the stability of the core material but also controls the release rate of the core material by adjusting the nature of the shell layer material, realizing timed or quantitative release. On the one hand, due to the protective effect of the shell layer material, the defects of paraffin PCM, which is easy to leak and has poor thermal stability, can be well solved. On the other hand, the microcapsules can effectively avoid the problem of separating paraffin from the base material. The shell layer material forms a physical barrier on the surface of the paraffin, which not only wraps the paraffin but also effectively fixes it inside the microcapsule. Even if the material undergoes several phase change cycles, the paraffin remains bound within the shell layer, achieving physical isolation. The alleviation of the shortcomings of paraffin phase separation enables the material to maintain good performance after multiple uses, greatly extending its service life and greatly meeting the needs of special scenarios.

Table 2. The advantages and disadvantages of different synthesis methods.

Synthesis Method	Advantages	Disadvantages
Physical mixing	simple operation	poor uniformity
Copolymerization	performance improvement	strict process
Melt impregnation	high production efficiency	obvious internal voids
Microencapsulation	high material stability	high cost

is a summary of the advantages and disadvantages of the above-mentioned synthesis methods.

In order to meet more applications’ requirements, it is imperative to explore synthetic modification methods to enhance the performance of paraffin PCM. The most common modification methods include acrylic acid graft modification and composite modification. Acrylic acid graft modification is a significant method for modifying paraffin materials utilizing benzoyl peroxide (BPO) as an initiator. The acrylic acid molecule contains both carbon-carbon double bonds and carboxylic acid functional groups, facilitating grafting reactions and enabling linkage with other polymer chains. Zhao et al. [71] utilized the polar monomer acrylic acid to modify paraffin wax. They analyzed the composition and sequence structure of the grafted products through various characterization techniques and investigated the thermal properties of paraffin wax materials post-graft modification. The graft modification significantly increased the latent heat capacity compared to pure paraffin wax. Composite material modification is also a widely used method. This approach leverages the excellent properties of different materials, combining the advantages not only to meet the production requirements but also to optimize the defects of the original material, resulting in comprehensive excellent properties suitable for practical applications. Due to the inherent defects in paraffin wax, researchers have employed various materials for composite modifications.

In Figure 3a, Shen et al. [72] employed composite materials to address the issues of facile leakage and inadequate thermal conductivity of PCM. Initially, cellulose nano-fiber (CNF) foam was synthesized as a hollow support matrix for paraffin wax to mitigate leakage. Subsequently, carbon nanotubes (CNTs) were incorporated into the CNF foam to enhance thermal conductivity. The incorporation of methyltrimethylsilane-treated CNF significantly improved the compatibility between the foam matrix and paraffin wax, contributing to the absorption capacity as high as 90%. The synthesized novel paraffin material exhibited negligible leakage, and the inclusion of carbon nanotubes markedly enhanced the thermal conductivity of the paraffin-based PCM. In Figure 3b, Zhou et al. [73] integrated phase-change microcapsules with SiO₂ shells and polydimethylsiloxane (PDMS) to develop a novel composite material, effectively mitigating paraffin wax leakage during the phase-change process. The study demonstrated excellent compatibility between the paraffin@SiO₂ microcapsule structure and PDMS. The phase change temperature of the paraffin@SiO₂ microcapsule/PDMS composite was elevated compared to the microcapsule alone. The application of chip thermal management showed a temperature reduction of 15 °C. These findings indicate that the composite material exhibits superior thermal insulation properties, offering substantial potential for future applications in battery thermal management. Despite the advancements in composite modification of paraffin materials, identifying suitable composites that align with the intrinsic properties of paraffin remains a critical consideration.

3.1.2. Non-Paraffin

In high-heat storage scenarios, paraffin-based PCMs are distinctly non-competitive. Therefore, another kind of PCM is needed to be explored. Non-paraffinic PCM addresses these shortcomings, with common examples including polyols, fatty acids, and their derivatives.

• Properties

The phase transition temperatures of polyols span from approximately 10 °C to 200 °C, rendering them highly versatile and applicable across an extensive array of scenarios [74,75,76]. Furthermore, alcohol-based PCM generally exhibits a high latent heat of transition, signifying their capacity to store and release substantial quantities of heat, thereby enhancing energy utilization efficiency.

Physical state and stability: Unlike paraffin-based PCM, most alcohol-based PCM undergo phase transitions by altering their crystal structure to absorb and release energy. Consequently, they experience minimal volume change during phase transitions, reducing the risk of container or encapsulation material deformation or rupture, thereby enhancing operational safety and reliability. Notably, alcohol PCMs do not experience overcooling during their phase transitions, requiring no additional energy for melting. Their high-density, viscous liquid state reduces the likelihood of leakage during phase change processes. Regarding stability, alcohol-based materials retain their phase change properties and physical integrity effectively after prolonged use and storage, ensuring reliability in long-term applications. Their inherent thermal and chemical stability makes them highly suitable for phase change energy storage materials.

Polyethylene glycol (PEG) is the most widely used polyol, and its properties vary according to the molecular weight. When the molecular weight is below 600, it mainly exists in liquid form. When the molecular weight is greater than 600, it exists in semi-solid or solid form [63,77,78]. Due to its excellent physical and chemical properties, such as good moisture absorption, lubricity, non-toxic, non-irritating and good stability under heat, acid and alkali conditions, it has been widely used in the biological and pharmaceutical industries. Some properties of PEG are summarized in

Table 3. Thermal properties of PEG with different molar mass [63].

Average Molar Mass (g mol−1)	Melting Temperature (°C)	ΔHfus (kJ·kg−1)	Crystallization Temperature (°C)	ΔHcryst (kJ·kg−1)
400	3.2	91.4	−24	85–86
600	22.2	108.4	−7	116
1000	32.0	149.5	28	140
1500	46.5	176.3	39–40	169
2000	51.0	181.4	35	168
3400	56.6	174.1	29	159
4000	59.7	189.7	22	167
6000	64.8	189.0	33	161
10,000	66.0	189.6	38	167
20,000	68.7	187.8	38	161

. It can be observed from the table that with the continuous increase of molecular weight, the melting temperature and heat release of PEG increase gradually. The crystallization temperature and crystallization enthalpy change are also in positive proportion to the molecular weight. Therefore, controlling the melting temperature and exothermic energy is feasible by adjusting the proportion of PEG with different molecular weights.

In addition to polyols, fatty acids and their derivatives have also been considered as the promising choice, which is mainly due to their low commercial value, high renewability, non-tox and good biodegradability. At the same time, its thermal stability is not inferior to that of traditional paraffin. It is found in the PCM of fatty acids that mixing two or more fatty acids can regenerate the eutectic mixture with a lower phase transition temperature.

Table 4. Thermal properties of eutectic mixture of fatty acids [63].

Eutectic Mixture of Fatty Acids	# of C Atoms in Fatty Acids	Composition by Mass	Melting Temperature (°C)	ΔHfus (kJ·kg−1)
Lauric-palmitic	12C:16C	66:34	33–37	169
Lauric-myristic	12C:14C	63:37	31–37	170
Lauric-stearic	12C:18C	76:24	37	171
Myristic-stearic	14C:18C	50:50	35−52	189
Myristic-palmitic	14C:16C	66:34	44	181
Palmitic-stearic	16C:18C	65:35	51	179
Capric-lauric	10C:12C	65:35	13–14	117
Capric-palmitic	10C:16C	75:25	26–33	142
Capric-myristic	10C:14C	74:26	23	155
Capric-stearic	10C:18C	87:13	27	160

lists the thermal properties of several fatty acid eutectic mixtures. Generally speaking, the single fatty acid, due to the high phase transition temperature, always has a certain odor and is easy to volatilize when heated. Considering fatty acids’ easy esterification with alcohols, some researchers have synthesized some ester derivatives. In Figure 3d, Mika et al. [79] designed a size-controllable fatty acid hybrid nano-capsule and confirmed the feasibility and applicability of the PCM under dry and wet conditions. AR et al. [80] took palm fatty acid distillate (PFAD) as the research object, assembled it with a LIB module, and tested it under three different electrical loads of 50 W, 100 W and 150 W, respectively. The results showed that PFAD-based PCM could effectively reduce the temperature of lithium-ion batteries, which decreased by 9.8%, 19.5% and 12.4%, respectively, under three loads. It has improved LIB’s electrical power. Nevertheless, given the pervasive issue of organic PCM, the poor thermal conductivity of fatty acid materials is also a significant concern.

• Synthesis and modification.

To preserve the intrinsic properties of PCM, including polyols, fatty acids, and their derivatives, the selection of an appropriate synthesis method is paramount. This section delineates several prevalent synthesis techniques.

The direct melting method is suitable not only for the synthesis of polyols but also for fatty acids and their derivative PCM. During the synthesis procedure, pentaerythritol, butyltetradecanol, and xylitol are heated in specific proportions until the powders are fully melted, yielding a transparent liquid that eventually solidifies into a white polyol composite PCM. Similarly, fatty acids or their derivatives are typically melted together in specific ratios, heated until liquefied, then mixed and cooled to produce PCM. The direct melting method represents the simplest approach for producing these materials, albeit requiring precise control of heating and cooling conditions to maintain the desired properties of the PCM [74,81].

Graft modification, primarily applicable to the synthesis of polyol PCM, involves using polyols as a backbone and chemically grafting other molecular chain segments onto this backbone (e.g., via amidation). Notably, the grafting process necessitates stringent control over temperature, pressure, and catalyst selection.

Microencapsulation is a principal method for synthesizing PCM from fatty acids and their derivatives. In this process, the fatty acids or their derivatives are the core material and are encapsulated in microcapsules via physical or chemical methods. The microcapsule comprises two primary components, including the core material and the shell. The shell is primarily used to regulate the timing, mode, and rate of release of the fatty acids or their derivatives, enabling the release rate to be adjusted either slowed or accelerated according to the synthesis requirements. This method is effective in enhancing the stability and processability of PCM.

Additionally, aside from the aforementioned synthesis methods, esterification reactions and other chemical synthesis techniques have also been investigated to some extent. However, these are not currently among the most mainstream methods. When selecting a synthesis method, it is crucial to consider not only the intrinsic characteristics of the PCM and the synthesized performance but also the complexity of the process. Furthermore, the material’s application scenarios can significantly influence the choice of synthesis method.

Research on the modification of polyols, fatty acids, and their derivatives shares similarities with that on paraffin-based PCM, primarily aiming to enhance key properties such as phase change temperature, latent heat, thermal conductivity, and thermal stability. The main distinction lies in the differing modification techniques employed. Current research on polyol-based materials concentrates on incorporating specific additives or altering the synthesis process to optimize phase change temperature, thermal conductivity, and thermal stability. Yuan et al. [82] successfully developed a novel PCM with high morphological stability, suitable for medium-temperature storage, using the impregnation-pressing-sintering method. The synthetic pathways and methodologies are illustrated in Figure 3c. The resultant composite PCM, comprising erythritol and expanded graphite (EG), exhibited enhanced performance. It was observed that increasing the EG content led to an increase in the thermal conductivity of the composite material, albeit with a concomitant decrease in melting point and latent heat. Additionally, the study revealed that at 10% EG content, the composite material’s overall performance reached its optimum, providing a theoretical foundation for future high-temperature PCM research. Moreover, the subcooling degree and the latent heat of phase change of PEG were improved to a certain extent.

Yan et al. [83] successfully prepared EG/PEG composite phase change materials based on the physical adsorption method to study the changes in the phase change properties of the composites with different mass fractions of EG. The skeletal structure of EG resulted in a significant increase in the thermal conductivity of the composite phase change materials. Meanwhile, the leakage rate of PEG was negatively correlated with the mass fraction of EG, and the optimum mass fraction of EG was determined to be 7 wt%. Qiu et al. [84] investigated the effects of different nano-additives to solve the drawbacks of PEG/AIN composite phase change materials, such as low thermal conductivity and easy leakage. By comparing the different nano-additives of alumina, silica, titanium dioxide, and boron nitride, it was found that 4 wt% alumina had the most obvious performance enhancement for the composite phase change material compared to the others, with a thermal conductivity as high as 20.41 W/m·K, which was 65 times higher than that of pure PEG. The heat storage rate and enthalpy of phase change were significantly improved, and the excellent properties made it have great potential for application in the field of high heat dissipation requirements. Teppei et al. [85] synthesized a composite material using porous nickel and erythritol, finding through characterization studies that its thermal conductivity was two orders of magnitude higher than that of pure erythritol PCM. This developed material boasts a high heat transfer rate and cost-effectiveness, suggesting its potential applicability in various industrial scenarios. Research on modifying fatty acids and their derivatives, which is more extensive than that on polyols, primarily addresses compatibility issues. Thus, efforts are concentrated on adding compatibilizers and adjusting ratios to enhance compatibility with other materials while preserving the performance of PCM. Similar methods are employed for fatty acids, their derivatives, and polyols to modify other properties. However, the organic PCM has some disadvantages, including poor thermal conductivity, low density, flammability, and volatility. The low thermal conductivity property leads to a slower rate during the charging and discharging processes, while the leakage of the material itself is also a difficult problem to overcome due to its material properties. In addition, low density, flammable characteristics, and instability are also among its main drawbacks.

Figure 3. (a) SCNF foam and foam-based PCM production process; FE-SEM of SCNF/carbon nanotube foam, (i) and (ii) SCNF, (iii) and (iv) SCNF/CNT₂₀, and (v) and (vi) SCNF/CNT₅₀; reproduced with permissions from ref. [72], copyright 2021, Elsevier. (b) Synthesis of paraffin@SiO₂ microencapsulated composites; Infrared thermal images of paraffin@SiO₂ microcapsules/PDMS composites during heating; (i) and (ii) SEM images of paraffin@SiO₂ microcapsules; reproduced with permissions from ref. [73], copyright 2022, Elsevier. (c) Process for the preparation of an erythritol/expanded graphite composite PCM; SEM image of (i) pure erythritol and (ii) EG; reproduced with permissions from ref. [82], copyright 2019, Elsevier. (d) Preparation of fatty acid/lignin hybrid nano-capsules [79].

Figure 3. (a) SCNF foam and foam-based PCM production process; FE-SEM of SCNF/carbon nanotube foam, (i) and (ii) SCNF, (iii) and (iv) SCNF/CNT₂₀, and (v) and (vi) SCNF/CNT₅₀; reproduced with permissions from ref. [72], copyright 2021, Elsevier. (b) Synthesis of paraffin@SiO₂ microencapsulated composites; Infrared thermal images of paraffin@SiO₂ microcapsules/PDMS composites during heating; (i) and (ii) SEM images of paraffin@SiO₂ microcapsules; reproduced with permissions from ref. [73], copyright 2022, Elsevier. (c) Process for the preparation of an erythritol/expanded graphite composite PCM; SEM image of (i) pure erythritol and (ii) EG; reproduced with permissions from ref. [82], copyright 2019, Elsevier. (d) Preparation of fatty acid/lignin hybrid nano-capsules [79].

3.2. The Inorganic PCM

From the above discussions about organic PCM, it is evident that low thermal conductivity, poor thermal stability, and other shortcomings are prevalent issues. Consequently, alongside modifying organic materials, researchers have begun exploring alternative materials to replace organic PCM, leading to the development of inorganic PCM. In contrast to organic PCM, inorganic PCM offers high latent heat, superior thermal conductivity, and low cost. Common inorganic PCM primarily includes salt hydrates and metals, as well as their alloys, which will be briefly introduced below.

3.2.1. Salt and Salt Hydrates

Salt hydrates constitute a significant class of inorganic PCM, typically comprising two components: the salt part and the crystalline water part. The salt part consists of metal ions and acid ions, while the crystalline water part consists of water molecules bound to these ions. The general formula of a salt hydrate is expressed as AB·nH₂O. These materials exhibit a fixed melting point, high phase change enthalpy, thermal conductivity, and volumetric heat storage density. Due to their advantageous properties, such as low cost, non-toxicity, and non-combustibility, they have promising application prospects. The heat storage mechanism of salts and salt hydrates primarily relies on their ability to crystallize water. When the external temperature increases, the crystalline water in the hydrate is released and melts to absorb heat. Conversely, when the external temperature decreases, the water re-crystallizes to release heat, thereby regulating the overall temperature [86,87].

• Properties

Several crucial factors affect the properties of salt hydrates during phase transition, including subcooling, phase separation, and salt solubility in water.

Subcooling is a phenomenon where the actual onset of crystallization occurs at a temperature lower than the theoretical crystallization temperature of the salt hydrate, leading to reduced efficiency in energy storage. The extent of subcooling primarily depends on the cooling rate and the presence of additives. A faster cooling rate results in a lower actual crystallization temperature and a higher degree of subcooling, whereas a slower cooling rate leads to a lesser degree of subcooling. For salt hydrates, subcooling does not trigger an immediate phase transition. Instead, the hydrate must wait until specific critical conditions (e.g., nucleation) are met, resulting in a delayed phase transition process and consequently affecting the energy storage and release efficiency. Additionally, supercooling enhances the intermolecular forces within the substance, effectively increasing the molecular resistance and slowing down the phase transition process. The slow phase transition rate significantly impacts the rate of energy storage and release.

Phase separation is another critical factor influencing the phase changeability of salt hydra. Ideally, during a phase change, a salt hydrate’s solid and liquid states are interconvertible, meaning the phase change is reversible. However, phase segregation causes the salt to dissolve in the crystallization water until saturation, with undissolved salt depositing at the bottom due to its higher density. This results in a layered structure: the middle layer of water and salt, the upper layer of saturated salt solution, and the bottom layer of solid salt. With repeated melting-solidification cycles, the solid salt at the bottom accumulates progressively. When this deposition reaches a critical level, the salt hydrate PCM loses its heat storage capability and fails.

The solubility of salt in water significantly affects the phase change capability of salt hydrates. When the salt hydrate transitions from solid to liquid, the crystalline water is released from the lattice, becoming a solvent for diluting salt ions. Therefore, in salt complexes, the solubility of the salt in water, the interaction between water molecules and salt, and the salt concentration in water critically influence the thermodynamic performance of the PCM.

Table 5. Thermo-physical properties of salt hydrates [88].

Compound	Latent Heat (J/g)	Thermal Conductivity (W/m·K)	Supercooling Degree (°C)	Drawback
LiClO3·3H2O	253	NR *	8.00	high price
CaCl2·6H2O	174	0.550	47.6	high supercooling
LiNO3·3H2O	256	NR	10.1	high price
Na2CO3·10H2O	247	0.600	13.7	high supercooling
Na2HPO4·12H2O	280	0.514	13.0	high supercooling
FeCl3·6H2O	223	NR	NR	NR
Ca(NO3)2·4H2O	153	0.570	87.0	high supercooling
Mg(NO3)2·2H2O	142	NR	NR	NR
Fe(NO3)2·9H2O	155	NR	NR	NR
MgSO4·7H2O	202	NR	NR	NR
Ca(NO3)2·3H2O	104	NR	NR	NR
Zn(NO3)2·2H2O	68.0	NR	NR	NR
FeCl3·2H2O	90.0	NR	NR	NR

* NR: no report.

enumerates several properties of salt hydrates alongside their primary drawbacks, such as subcooling, which influences the phase change behavior of salt hydrate-based PCM [88]. Addressing this drawback could significantly enhance the suitability of salt hydrates for battery thermal management, owing to their latent heat of phase change, superior thermal conductivity, and other advantageous properties.

• Synthesis and modification

Currently, there are two primary methods for synthesizing salt hydrate PCM, including the direct mixing and eutectic methods [89].

The direct mixing method, which is relatively straightforward to operate, involves directly combining inorganic salts and water with additives. This method requires specific selection criteria for additives to produce the PCM by heating and stirring. The eutectic method involves mixing two or more types of salts and water in specific ratios to exploit the eutectic effect, thereby preparing materials with a designated phase change temperature. Currently, these two methods are the most widely used. However, relying solely on these synthesis techniques for salt hydrates is insufficient to address some issues, such as supercooling and phase separation.

Consequently, researchers have begun to explore various modification methods to enhance the performance of salt hydrates. In Figure 4a, Hamir et al. [90] enhanced the hydrophilicity of both EG and TiO₂ by modifying their surfaces with hydrophilic SiO₂. The nanoparticles were synthesized using the sol–gel technique and subsequently combined with EG to produce the modified MEG. Furthermore, by adjusting the mass ratio of SiO₂@TiO₂ in EG, the composite exhibited a notable reduction in subcooling and an enhancement in thermal conductivity, indicating its potential as a future candidate for energy storage applications. In Figure 4b, Deng et al. [91] addressed the limitations of aqueous salt composites, including facile supercooling, low thermal conductivity, and unstable morphology, by integrating alumina and expanded vermiculite (EVM) with Na₂HPO₄·12H₂O. The resultant composite material, exhibiting excellent wettability, ensured morphological stability. The incorporation of alumina markedly enhanced the thermal conductivity of the composite PCM. Characterization revealed that the thermal energy storage capacity of the material was significantly increased, demonstrating substantial thermal storage capability. The melting process (97–151 J/g) and solidification process (60–89 J/g) demonstrated its considerable application potential. In Figure 4c, Lu et al. [92] have designed and successfully synthesized an innovative MXene/Na₂HPO₄·12H₂O@SiO₂ composite phase change microcapsule material, which demonstrated excellent thermal storage and release capabilities, with a remarkable latent heat of 169.62 J/g. Notably, the SiO₂-coated microcapsules significantly mitigated the supercooling effect of the hydrated salt PCM, thereby facilitating rapid energy storage and release.

Adding nucleating agents, stabilizers, and thickeners to mitigate supercooling and phase separation is a common modification approach. Adding nucleating agents effectively can reduce supercooling, enabling phase transition at lower temperatures. Stabilizers ensure material stability during phase transition, while thickeners increase system viscosity, reducing salt deposition and delamination, thus preventing phase separation. Additionally, other modification methods include adding nano-modifiers and incorporating new materials. Although these methods do not significantly mitigate supercooling and phase separation, they can improve the latent heat of phase change and thermal conductivity of salt hydrates. Future research is anticipated to develop various strategies to enhance the properties of salt hydrates comprehensively.

3.2.2. Metals and Alloys

As discussed previously, salt hydrates as PCM exhibit inherent disadvantages, including high subcooling, susceptibility to phase separation, and low thermal conductivity. In contrast, this issue can be effectively solved by metals and their alloys, which possess thermal conductivities hundreds of times greater than those of salt hydrates. This enables faster heat conduction and higher rates of energy storage and release. Additionally, compared to salt hydrates, metals and alloys can serve as PCM at high temperatures, thereby extending the range of temperatures and expanding potential application scenarios [15].

Owing to their exceptional properties, metals and alloys have emerged as promising materials for high-temperature applications. However, in addition to their application potential, the high cost and toxicity of metals and alloys are critical considerations. Furthermore, metallic materials impose specific environmental requirements for their use. For thermal energy storage, the durability of metallic materials and the corrosion resistance of the operating environment must be considered, posing significant challenges for future research. Ryan et al. [93] explored a metal–organic compound as an innovative solid-liquid PCM for thermal energy storage. They systematically examined the impact of ligand bonding, hydrogen bonding networks, organic ligands, and outer sphere anions on the thermodynamic properties of the metal–organic PCM (MOPCM). Their findings highlighted the critical role of high-density ligand bonding and hydrogen bonding in achieving high energy density in PCMs. Due to space constraints, the properties and synthetic modifications of metals and alloys are not discussed in detail here.

3.3. The Composite PCM

Unlike traditional organic and inorganic PCMs, composite PCMs are synthesized by integrating original materials with other substances, thereby yielding materials with superior performance to meet the demands of production and application. Composite materials not only outperform the original materials in terms of performance but also address the need for an appropriate phase change temperature. Given that the melting point of synthetic composites can be adjusted over a wide range (depending on the composition ratio) and that composites produced through eutectic reactions generally exhibit lower melting points than their individual components, composites become a reliable choice when a single PCM cannot achieve the desired phase change temperature [94]. However, in some cases, composites may have melting points that lie between those of their constituent materials, which is also a factor to consider. In any case, controlling the phase change temperature is easier with composites than with a single PCM [95]. As previously stated, a crucial property of PCM is the phase change temperature. The development of composites offers researchers the opportunity to select new materials for practical applications.

Currently, research on composite PCM primarily focuses on enhancing their overall thermodynamic properties and energy storage capacity. In Figure 4d, Dai et al. [96] introduced an innovative composite PCM designed for efficient battery thermal management. They achieved this by encapsulating SiO₂ with Na₂HPO₄ and incorporating carbon nanotubes to augment overall heat transfer capabilities. A subsequent characterization revealed a decrease in the battery’s peak temperature from 76 °C to 61.2 °C at a discharge rate of 2 °C. This reduction not only mitigated the temperature escalation within the module but also effectively suppressed the initiation and progression of TR. Sun et al. [97] proposed a composite material consisting of sodium thiosulfate pentahydrate, sodium acetate trihydrate, and deionized water. It was integrated with a power battery, resulting in a 6.8% increase in the battery’s discharge capacity at −20 °C. In Figure 4e, Huang et al. [98] synthesized a core–shell structured microcapsule through in-situ polymerization by incorporating alumina nanoparticles, zinc oxide nanoparticles, and carbon nanotubes, aiming to enhance the thermal conductivity of the material. The study revealed that the composite containing carbon nanotubes delivered superior thermal conductivity and achieved a maximum latent heat effect of 139.64 J/g, surpassing the other two fillers. The study indicated that carbon nanotube composites hold significant promise for applications in battery thermal management. Wen et al. [99] prepared and measured a ternary composite of lauric, palmitic, and stearic acids for thermal stability. The phase transition temperature remained stable after 100 thermal cycles. Zhang et al. [100] utilized lauric, myristic, palmitic, and stearic acids to prepare a binary eutectic mixture by reacting with 1-hexadecanol, and subsequently added titanium dioxide to form the composite. The phase change temperature, latent heat, thermal stability, and thermal conductivity were significantly enhanced.

Figure 4. (a) Synthesis routes for aqueous salt composite-based PCM, (I) the mixture was stirred for 15 min under a water bath at 60 °C, (II) SiO₂ and TiO₂ nanoparticles were combined to create a layered structure, (III) EG was synthesized with the binary SiO₂@TiO₂, (IV) the Na₂HPO₄·12H₂O/Na₂SiO₃·9H₂O/MEG-based form-stable composite PCMs were synthesized using the melt blending method; reproduced with permissions from ref. [90], copyright 2024, Elsevier. (b) Expanded Vermiculite (EVM) and Na₂HPO₄·12H₂O encapsulated on alumina surface principle; reproduced with permissions from ref. [91], copyright 2018, ACS. (c) MXene and MXene/MPCM preparation process; reproduced with permissions from ref. [92], copyright 2024, Elsevier. (d) Schematic diagram of the synthesis of composite PCM for thermal management of LIBs and with enhanced heat transfer; reproduced with permissions from ref. [96], copyright 2024, Elsevier. (e) Diagram of the synthesis mechanism of CPCM and leakage of different components of CPCM with paraffin wax at different temperatures [98].

Figure 4. (a) Synthesis routes for aqueous salt composite-based PCM, (I) the mixture was stirred for 15 min under a water bath at 60 °C, (II) SiO₂ and TiO₂ nanoparticles were combined to create a layered structure, (III) EG was synthesized with the binary SiO₂@TiO₂, (IV) the Na₂HPO₄·12H₂O/Na₂SiO₃·9H₂O/MEG-based form-stable composite PCMs were synthesized using the melt blending method; reproduced with permissions from ref. [90], copyright 2024, Elsevier. (b) Expanded Vermiculite (EVM) and Na₂HPO₄·12H₂O encapsulated on alumina surface principle; reproduced with permissions from ref. [91], copyright 2018, ACS. (c) MXene and MXene/MPCM preparation process; reproduced with permissions from ref. [92], copyright 2024, Elsevier. (d) Schematic diagram of the synthesis of composite PCM for thermal management of LIBs and with enhanced heat transfer; reproduced with permissions from ref. [96], copyright 2024, Elsevier. (e) Diagram of the synthesis mechanism of CPCM and leakage of different components of CPCM with paraffin wax at different temperatures [98].

4. Barrier-Type Insulating Materials

Barrier-type insulation materials are a class of materials that achieve thermal insulation by effectively blocking or reducing heat transfer (including conduction, convection, and radiation). They usually have low thermal conductivity, high reflectivity, or special structural design and are able to provide thermal insulation, fire protection, and thermal insulation in both high and low-temperature environments. Generally, these materials form a barrier to prevent the transfer of heat from one area to another by physical or chemical means, thus maintaining the temperature difference between the two areas. In addition, the design of the material can be multifunctional in order to meet the state of thermal insulation performance; fire, moisture, corrosion resistance, and other functions can also be added to the design. An essential metric for assessing thermal insulation efficacy lies in thermal conductivity, with the typical approach or objective revolving around attaining minimal thermal conductivity, thereby endowing the material with heightened thermal resistance.

Nevertheless, relying solely on thermal conductivity for material selection fails to offer a comprehensive solution to all challenges. Hence, the context of the application must be factored into when choosing insulating materials. For batteries, the choice of insulating materials frequently involves considering the material’s pliability, requisite mechanical robustness, resistance to fire, and voltage tolerance. Currently, barrier-type insulating materials LIBs are broadly categorized into organic thermal insulating materials, inorganic thermal insulating materials, and composite materials, each of which will be examined in the ensuing discourse.

4.1. Organic Thermal Insulating Materials

Organic thermal insulating materials, which prioritize carbon as a critical evaluation criterion, are predominantly manufactured through the processing of organic polymer materials. Typically derived from biomass, wood, cotton, straw, and other natural sources, these materials exhibit low density, scalability, and lightweight characteristics [101,102,103]. Additionally, organic insulating materials are considered renewable resources and environmentally friendly due to their natural material origins. When reaching the end of their lifecycle, these materials can be decomposed or repurposed, depending on the available energy and ecological resources. Given the aforementioned characteristics of organic thermal insulating materials, the utilization of organic thermal insulating materials to prevent the TR behavior of batteries represents a significant research topic in the field of battery technology. Organic insulating materials may be categorized into polymer insulating film, foam materials, fiber materials and polymer composites, which will be described in the following sections.

4.1.1. Polymer Thermal Insulating Film

Polymer films, comprised of organic, non-metallic compounds derived from polymers, are fabricated. Various preparation techniques enable the fabrication of these materials with diverse structures. Typically, their thickness ranges from 0.1 to 100 μm. They have outstanding mechanical strength, chemical stability, and effective thermal insulation [104,105,106]. Predominantly, commonly used polymer films include polyimide, polyamide, polyethylene, and polypropylene.

• Properties

The thickness of the polymer separator profoundly influences both the safety and electrochemical performance of the battery. Thicker separators are typically correlated with increased thermal resistance, effectively mitigating heat transfer within the battery, thereby crucially averting overheating and enhancing battery safety. Simultaneously, the thickness of the separator influences its overall mechanical strength to some extent. Thicker separators exhibit higher impact and compression resistance compared to thinner counterparts, thereby reducing the risk of the battery being susceptible to external forces. Nevertheless, the thickness of the separator must fall within a specific range. Excessive thickness can impede the internal energy release efficiency and energy density of the battery [107]. Therefore, to ensure the safety of batteries, a judicious reduction in separator thickness to enhance the energy density is imperative. Currently, polymer separators in batteries typically range from approximately 20 to 50 μm in thickness.

Considering that batteries are susceptible to various external forces during transportation, installation, and usage, ensuring the sufficient mechanical properties of separators is crucial to mitigate potential hazards. These properties encompass tensile and compressive strength, impact resistance, pinch resistance, and dimensional stability. Tensile strength pertains to the separator’s capacity to endure external tension or pressure without undergoing deformation. It serves as a fundamental indicator of the separator’s strength, typically ranging between 50 and 100 MPa. Under standard conditions, a value ranging from 5 to 10 MPa is deemed typical. Besides impact resistance, needle puncture resistance, dimensional stability, and other factors are also taken into account when assessing the performance separators. Consequently, when selecting a suitable separator, it is necessary to integrate various mechanical properties [108].

Local overheating often arises during the charging or discharging of batteries, resulting in separator shrinkage, deformation, or even thermal decomposition. The extent of shrinkage can be computed using the following formula [109].

$$Thermal shrinkage={\displaystyle \frac{S_0-S}{S_0}}\times 100\%$$

(2)

wherein S and S₀ denote the sample’s surface area at room temperature and the elevated temperature, respectively. Likewise, the extent of sample thermal decomposition can be ascertained using a thermogravimetric analyzer (TGA) and a differential scanning calorimeter (DSC). Most polymer separators exhibit significant changes between 200 and 500 °C. These separators exhibit reduced effectiveness at elevated temperatures, making the investigation of high-temperature-resistant separators a primary research focus for contemporary scholars.

• Synthesis

The synthesis method for polymer membranes usually impacts both the internal resistance and energy density of the battery. The predominant synthesis methods include the stretching method, the phase separation method and electrostatic spinning [110,111,112,113]. In the stretching method, the polymer material is initially dissolved in a suitable solvent to create a polymer solution, which is then uniformly coated onto the mold’s surface to produce a thin film via spinning or impregnation. Subsequently, the coated film undergoes stretching and heating to attain a specific tensile strength, followed by heat treatment or annealing to achieve the thin film with designed properties. Stretching represents a prevalent approach in the production of synthetic commercial LIB separators, allowing precise control over separator thickness to meet the requirements of high safety and excellent electrochemical performance.

The phase separation method commences with dissolving the polymer in a diluent at the polymer’s melting point to create a homogeneous solution. This solution is then cooled to generate a two-phase structure, with the polymer serving as the continuous phase and the solvent as the dispersed phase. Subsequently, solvent extraction using an appropriate extractant enhances the structural stability of separators, followed by necessary subsequent processing steps to obtain the final product. In comparison to the stretching method, this method requires higher precision in process control accuracy, and the particular solvents and extractants necessary for the phase separation method moderately escalate the overall preparation cost.

Utilizing an electric field of high voltage, the electrostatic spinning method elongates a polymer solution or melts it into exceedingly delicate fibers. Subsequently, these fibers are deposited onto a receiving device to prepare a separator. Despite its capacity to produce a substantial quantity of separators swiftly, the method unavoidably escalates the intricacy and challenge of the entire process owing to factors such as voltage, solution concentration, spinning speed, and other parameters.

In summary, the aforementioned methods possess unique merits, exhibiting diverse levels of thickness, mechanical robustness, and thermal resilience. Within the realm of practical production endeavors, it is imperative to contemplate the ramifications of alterations in production conditions when selecting an optimal production pathway.

• Modification of polymer thermal insulating film

The majority of polymer separators commercially accessible for LIBs comprise microporous polyolefin membranes derived from polyethylene and polypropylene. These materials retain a substantial share of the separator market owing to their commendable chemical stability, mechanical properties, and electrochemical stability. Nevertheless, the low melting point and softening temperature inherent to these polyolefin membranes render them vulnerable to separator shrinkage under conditions of overcharge, over-discharge, or even high-power charge and discharge, potentially culminating in severe TR. Furthermore, the presence of polyethylene, polypropylene, and other non-polar macromolecules serves to elevate the internal resistance of the battery, thereby impeding the attainment of requisite performance standards for high-power charging and discharging. Henceforth, researchers persist in their quest for separator materials endowed with superior thermal stability in order to align with the performance specifications mandated for batteries.

Mu et al. [114] employed electrostatic spinning technology to fabricate a polyimide hybrid membrane enriched with polar functional groups, boasting ultra-high porosity (75%), exceptional electrolyte absorption (775%), robust mechanical properties (15.7 MPa), and commendable thermal stability. Furthermore, Cu/NMC cells employing this hybrid membrane exhibit enhanced robustness compared to primary cells, retaining satisfactory discharge performance even after one hour of exposure to a temperature of 140 °C. In Figure 5a, Li et al. [115] synthesized high-safety EVA/PEEK/EVA composite membranes via dip coating and thermally induced phase separation, wherein the EVA layer is transformed into a barrier film, effectively obstructing ion transport to avert subsequent battery reactions upon surpassing an internal temperature of 80 °C. The PEEK substrate within the material retains its dimensional stability even at temperatures reaching 240 °C, with the ionic conductivity and discharge capacity exhibiting enhancements of 42% and 77.1%, respectively, surpassing those of the polyolefin membrane, the prevailing choice on the market. Yu et al. [116] designed and prepared a new type of polyimide (PI) microsphere coating material, which was plated on Celgrad polypropylene (PP) separators. Through a simple leaf coating process, a PP@PI microsphere composite separator with excellent thermal stability, flame retardancy, high liquid retention capacity, and high ionic conductivity was prepared. Subsequently, the synthesized composite membrane was integrated into a battery for electrochemical characterization, with the battery assembled using the PP@PI microsphere composite membrane showcasing notably superior discharge capacity and capacity retention rate compared to its pure PP membrane counterpart. Hu et al. [117] pioneered the development of a flame-retardant and heat-resistant porous composite membrane comprising poly (ether imide) (PEI) and alumina nanowires (NMs), culminating in the creation of a porous composite membrane characterized by commendable ductility, flame retardancy, and thermal stability. Particularly, its superior shrinkage and tensile properties at temperatures could be up to 200 °C, surpassing those of conventional polyolefin separators, alongside substantial enhancements in ionic conductivity and electrolyte uptake.

In addition, polymerization modification, chemical modification, and physical modification methods are also noteworthy in the realm of polymer membrane modification. By introducing novel side chains or branching into the polymer chain, the characteristics of the polymer can be effectively controlled. Alternatively, incorporating inorganic and organic fillers enhances the polymer’s mechanical strength, thermal stability, and barrier performance. Ultimately, material modification must be contextualized within specific application scenarios, necessitating the judicious selection of appropriate modification techniques tailored to the particularities of each application.

4.1.2. Fiber and Foam Materials

Foam insulating materials, comprising polyester, polyether, and polyurethane as raw materials, have been synthesized with the addition of foaming agents and co-solvents, followed by high-speed stirring to generate tiny bubbles and rapid curing. The presence of a large number of closed pore structures filled with air within its interior effectively retards heat conduction, thereby imparting a favorable thermal insulation effect. Furthermore, the density and thickness of the foam material constitute important factors influencing the thermal insulation effect. In contrast to the aforementioned foam material, fiber material primarily consists of natural or synthetic polymer-based fibrous materials, delivering a unique fibrous network structure that effectively impedes heat conduction and radiation, thereby achieving desirable thermal insulation effects. Polystyrene foam and polyurethane foam are two typical kinds of foam insulating materials, while polyimide foam is commonly employed for battery insulation.

• Properties

Density constitutes a pivotal metric for both foam and fiber materials, profoundly influencing cellular insulation and its corresponding strength index. For foams, decreased density engenders a material characterized by augmented pore volume and air strata, mitigating heat conduction and convection, culminating in heightened insulation capabilities. Nevertheless, a diminishing foam density corresponds to amplified compromises in strength and durability. Under external stressors on the battery, the holistic thermal insulation efficacy is compromised, which heightens the susceptibility to TR and affects the energy density and electrochemical safety of the battery. The density of fiber materials exhibits an approximately inverse relationship with thermal insulation, while strength and durability equally warrant consideration. Elevated density implies a tighter interconnection among fibers, thereby enhancing tensile and compressive attributes. Nonetheless, this denser configuration entails a diminished proportion of pore structure, compromising the efficacy of thermal insulation [118,119,120,121,122]. Consequently, during material selection, meticulous consideration should be accorded to the interplay between battery electrochemical performance and safety, alongside density and strength, to ascertain the optimal insulation material. Typically, the optimal density for foam and fiber insulating materials used in battery insulation ranges between 0.03 and 0.1 g/cm³. While higher density may enhance insulation performance to some extent, the associated increase in cost and weight hampers its application in batteries.

Mechanical strength serves as a fundamental metric for gauging a material’s capacity to withstand external forces and prevent deformation and damage. In the context of battery-insulating materials, a deficiency in mechanical strength renders the material susceptible to deformation or damage upon encountering external impacts or extrusion, consequently compromising its thermal insulation efficiency and longevity. Concurrently, heat generation is inherent during battery operation. Inadequate mechanical strength predisposes the material to melting and softening at elevated temperatures, compromising the battery’s overall insulation efficacy and safety performance. Elevated mechanical strength facilitates the material in retaining a certain level of structural stability at high temperatures, thereby enhancing the overall safety performance of the battery.

Nevertheless, the current challenge lies in the low mechanical strength of pure foam materials, posing a formidable obstacle to overcome. Scholars have discovered that a composite of foam and fiber materials exhibits enhanced mechanical strength due to the inclusion of fiber as a filler material. This underscores the rationale behind examining the synergy between these two materials in addressing inherent performance deficiencies through complementary filler reinforcement [123,124,125]. However, the hydrophilic nature of fibers poses challenges in achieving proper adhesion to the polymer surface. Various surface modification techniques have been employed to enhance filler-substrate adhesion, including alkali treatment, silane treatment, acetylation, peroxide treatment, and permanganate treatment, among others. However, achieving a general mechanical strength within the range of 0.1–1.0 MPa may be excessive. Increasing mechanical strength enhances safety, but it may compromise thermal insulation effectiveness.

Good insulation performance is a crucial factor in inhibiting heat transfer from the battery to its surroundings, ensuring that the battery’s operating temperature remains within a specified range, which significantly affects both the longevity and performance of batteries. At the same time, excellent insulating materials mitigate energy losses, redirecting the battery’s energy outward rather than dissipating it in the form of heat to the surroundings. Current research primarily concentrates on optimizing the structures of materials and employing heat reflection technologies for the aforementioned materials. For instance, strategies such as incorporating foaming agents to induce porosity and reduce thermal conductivity or employing multi-layer composite structures with varied thermal conductivities can contribute to the enhanced overall thermal insulation and heat dissipation capabilities of the materials.

Simultaneously, the minimization of heat transfer by reflecting thermal energy away from the material can be realized by coating the material surface. Moreover, beyond enhancing thermal insulation, flame retardancy represents an equally crucial property for these materials. Flame-retardant materials exhibit resistance to ignition when exposed to fire sources and, if ignited, have the ability to self-extinguish rapidly, significantly mitigating the risk of fire propagation. This property is vital in preventing major fire incidents triggered by battery TR. However, achieving both flame retardancy and thermal insulation simultaneously is challenging. Researchers have explored the incorporation of flame-retardant additives into insulating materials to enhance their flame-retardant properties without compromising thermal insulation. Halogenated compounds, phosphorus-based compounds, boron flame retardants, and other researched products have demonstrated promising outcomes [126,127,128].

• Synthesis

Various methods have been employed for the synthesis of foam and fiber-insulating materials, which will be discussed separately below for clarity and differentiation. Currently, the predominant technological pathways for foam materials primarily encompass two main categories, including physical foaming methods and chemical foaming methods [129,130,131]. The fundamental concept behind physical foaming methods involves the formation and stabilization of gas bubbles within solid or liquid substances to produce foam materials with a multi-cavity structure. Under high pressures, gases are dissolved into solid or liquid substances. Upon pressure reduction, these dissolved gases are liberated from the substance, forming bubbles. Additionally, certain substances produce volatile components during heating, which generate bubbles within the substance. Variations in heating speed and temperature result in changes to bubble size and distribution, thus influencing the performance of the foam material. In contrast, the chemical foaming method generates gas through controlled chemical reactions within rubber, resin, etc., to yield foam materials characterized by the porous structure. A typical example is the reaction between isocyanate and polyol, wherein the isocyanate group (-NCO) reacts with the hydroxyl group (-OH) in the polyol. The isocyanate group (-NCO) within the isocyanate reacts with the hydroxyl group (-OH) in the polyol, resulting in the formation of a urethane prepolymer.

Subsequently, a blowing agent is introduced. Upon decomposition during the reaction, the blowing agent releases gases, which diffuse through the urethane prepolymer, facilitating the expansion of the overall structure to establish the foam structure. Cellulose-based insulation synthesis entails a sol–gel process followed by solvent removal through drying. Traditional evaporation drying often results in adhesion due to the solvent surface tension, consequently leading to internal network structure collapse. Therefore, freeze-drying and supercritical drying methods are employed in production. Freeze drying, a prevalent technique for fabricating cellulose-based insulating materials, involves cooling the suspension to its freezing point, freezing the solvent within the pores, and subsequently reducing the pressure below the sublimation pressure under vacuum conditions. Vacuum sublimation thereafter diminishes capillary forces and significantly safeguards the internal porous network. Supercritical drying operates on the principle that in a supercritical state, which contains no gas–liquid interface, but rather a homogeneous fluid phase between gas and liquid. Substituting a supercritical fluid for the original solvent, typically water, can prevent the formation of a liquid–vapor interface, thereby obviating meniscus formation and capillary pressure arising from solvent removal, effectively averting the collapse of the three-dimensional porous structure and the formation of low-density porous materials. Nevertheless, both methods mentioned above suffer from high costs, posing an insurmountable challenge. Addressing this challenge may necessitate researchers and scientists to explore more economical and efficient drying methodologies.

• Modification of foam and fiber materials

As previously discussed, achieving optimal density, mechanical strength, and thermal properties concurrently remains a challenge. Material selection and parameterization must align with specific application requirements. Researchers continually strive to explore diverse high-performance materials, resulting in ongoing modifications to foam and fiber-insulating materials. In Figure 5b, Wang et al. [132] successfully synthesized cross-linked polyimide (PI) fiber (c-PI) heat-resistant separators featuring a narrow pore size of 0.78 µm via electrospinning of poly(amido acid) mixed with polystyrene, followed by thermos-amination. These separators exhibited significantly higher porosity and ionic conductivity compared to conventional PI separators. Furthermore, the incorporation of the c-PI separator notably enhanced the electrochemical performance of the battery, offering valuable theoretical insights for the design and development of advanced separators in future battery technologies. In Figure 5c, Cha et al. [133] utilized the surfaces of porous zirconia (YSZ) microspheres to fabricate stable and porous YSZ ceramic foam, successfully generating internal pores within individual hollow spheres derived from YSZ.

Furthermore, perforations were introduced into the shells to enhance the thermal insulation effect by mitigating continuous heat transfer pathways. Concurrently, the incorporation of apertures onto sphere surfaces yielded porous foam utilizing hollow spheres, delivering a maximum porosity of 80.69%. This substantially enhanced its thermal insulation efficacy, boasting low thermal conductivity (0.10 W/m·K) alongside ample compressive strength (5.7 MPa) for insulation integrity. In Figure 5d, Lee et al. [134] synthesized a fiber-based composite insulating material by incorporating pyrolyzed cellulose nanofibers (p-CNF) and carbon fibers (CF). The p-CNF/CF composite leveraged its ultra-low thermal conductivity and high degradation temperatures to significantly enhance thermal insulation performance. Beata et al. [135] introduced another innovative polyurethane material by incorporating fly ash microspheres. They investigated the impact of microsphere incorporation ranging from 5% to 20%, revealing a notable enhancement in the mechanical properties of polyurethane composites. This improvement stemmed from the predominant presence of silica and chrysotile oxides within the microspheres. With the addition content of 15%, microsphere inclusion led to delayed sample ignition, necessitating the increased oxygen for material combustion, which further contributed to the flame-retardant capabilities of polyurethane foam composites to a certain extent. Bruno et al. [136] employed an environmentally conscious thermal barrier comprising a natural polymer reinforced with flame-retardant fiber material and a hydrogel with high water content. Positioned amidst 50 Ah LIB cells, the barrier with a thickness of 2 mm effectively halted heat propagation within the 2-cell module. Upon reaching approximately 100 °C, the barrier exhibited substantial energy absorption capabilities attributed to water evaporation. The remaining low thermal conductivity fiber material then functioned as the thermal insulation between the cells. Furthermore, the ease of manufacture and biodegradability of the barrier contributed to enhancing the safety of LIB packs while simultaneously minimizing the cost and environmental effects. Given that the performance of individual foam and fiber insulating materials significantly lags behind that of multi-material composites, they are often utilized as additives or cooperated with other materials to form new composites.

Figure 5. (a) EVA/PEEK/EVA separator construction and thermal barrier mechanism; reproduced with permissions from ref. [115], copyright 2023, Elsevier. (b) Synthesis of cross-linked PI and PI heat-resistant membranes; reproduced with permissions from ref. [132], copyright 2022, Elsevier. (c) Schematic representation of different morphologies of spray-dried particles under different conditions and the corresponding SEM images, (i) low solid content and high drying rate, (ii) low solid content and low drying rate, (iii) high solid content and high drying rate, and (iv) high solid content and low drying rate; reproduced with permissions from ref. [133], copyright 2023, Elsevier. (d) Preparation of cellulose-based thermal insulation composites, SEM images of (i) I-CNF/CF and (ii) p-CNF-CF); reproduced with permissions from ref. [134], copyright 2023, Elsevier.

Figure 5. (a) EVA/PEEK/EVA separator construction and thermal barrier mechanism; reproduced with permissions from ref. [115], copyright 2023, Elsevier. (b) Synthesis of cross-linked PI and PI heat-resistant membranes; reproduced with permissions from ref. [132], copyright 2022, Elsevier. (c) Schematic representation of different morphologies of spray-dried particles under different conditions and the corresponding SEM images, (i) low solid content and high drying rate, (ii) low solid content and low drying rate, (iii) high solid content and high drying rate, and (iv) high solid content and low drying rate; reproduced with permissions from ref. [133], copyright 2023, Elsevier. (d) Preparation of cellulose-based thermal insulation composites, SEM images of (i) I-CNF/CF and (ii) p-CNF-CF); reproduced with permissions from ref. [134], copyright 2023, Elsevier.

Organic insulating materials exhibit both advantages and disadvantages in comparison to their inorganic counterparts. The advantages involve the lower density and enhanced flexibility, enabling superior accommodation of irregularly shaped cells within the material, thus facilitating the improved thermal insulation effect. However, they typically have reduced stability at elevated temperatures and are susceptible to chemical corrosion. Compared to them, inorganic insulating materials generally demonstrate superior resistance to high temperatures and chemical stability. Thus, the selection between organic and inorganic materials considers various factors to mitigate energy resource consumption and enhance energy efficiency.

4.2. Inorganic Thermal Insulating Materials

Inorganic thermal insulating materials constitute a category of materials composed of inorganic substances or mixtures employed to insulate between distinct electrochemical systems or dissimilar materials to impede the thermal transfer of energy. Specifically encompassing materials such as alumina aerogel, silica aerogel, and carbon aerogel, in comparison with the aforementioned organic materials, first and foremost, they typically exhibit superior high-temperature resistance, effectively safeguarding battery components from the deleterious effects of elevated temperatures [137,138,139]. Besides, inorganic materials generally boast enhanced chemical stability, capable of withstanding potential corrosive substances in the battery environment. Moreover, certain inorganic thermal insulating materials also demonstrate elevated mechanical strength and durability, thus preserving their functionality over extended periods of utilization. Based on this, this section will focus on inorganic thermal insulating materials utilized for LIBs, with particular emphasis on aerogel, ceramics, and other materials.

4.2.1. Aerogels

Owing to their extraordinary characteristics, silica aerogels, alumina aerogels, and carbon aerogels stand as prominent choices in contemporary markets. Furthermore, owing to their exemplary porosity and assorted attributes, aerogels have enhanced thermodynamic, optical, and physical traits. Nevertheless, these attributes contribute to their lightweight nature. However, their limited mechanical robustness renders them susceptible to external forces. Its high porosity renders aerogel the lightest solid material, boasting a skeletal density of approximately 2200 kg/m³. However, another consequence of this high porosity is a notably low bulk density, sometimes as minimal as 3 kg/m³ [140]. For the sake of simplicity, the evaluation of aerogel performance can be concentrated on their thermal efficiency and cost-effectiveness.

• Properties

Owing to its highly porous structure characterized by nanoscale pore dimensions, energy transfer of aerogels is constrained, leading to a reduction in thermal conductivity. The volume of the solid component of aerogels constitutes merely one percent, or even one thousandth, of the volume occupied by air in most cases. The thermal conductivity of the material is limited due to the relatively low thermal conductivity of the air component compared to other solid constituents. For instance, silicon dioxide aerogel, a type of silicate material, exhibits low thermal conductivity in comparison to metals and other materials, owing to its abundant internal gas-solid interfaces, thereby substantially attenuating thermal radiation conduction. The light within the porous aerogel materials undergoes multiple reflections or refractions, leading to the absorption of infrared energy, which impedes transmission to the other side, consequently diminishing thermal radiation conductivity. Presently, its thermal conductivity stands as low as 0.01 W/m·K, contributing to the highly effective thermal insulation effect compared to the majority of commercial battery-insulating materials. This garners considerable attention from researchers. For instance, Yu et al. [141] devised and fabricated a novel aerogel-based thermal insulating material to mitigate the propagation of TR hazards in LIBs. This material entailed a mechanically reinforced architecture comprising hollow glass microspheres (HGMs) and acrylic emulsions augmented with aerogel particles (APs), with an inquiry into the impact of HGMs and APs supplementation on the collective material attributes. A positive correlation between hollow glass microspheres (HGMs) and mechanical properties, as well as between aerogel particles (APs) and thermal insulation properties, was intensively discerned. Additionally, the influence of AP size on material characteristics was scrutinized. Diminishing the particle size from 5 μm to 50 μm resulted in a decrease in the overall mechanical robustness of the material while the enhanced thermal insulation capabilities, thereby offering a fresh material alternative for bolstering the safety of LIBs. Currently, research endeavors solely dedicated to enhancing the thermal insulation performance of aerogels appear boundless. Scholars have made significant strides in reducing thermal conductivity to attain elevated levels of thermal insulation. However, substantial progress towards mass production remains imperative.

Low-cost products, alongside traditional insulating materials, continue to dominate the insulation market, leveraging their inherent attributes. Due to the aspects such as the raw materials, synthesis process and market dynamics, the current price of aerogel lacks a definitive competitive edge when compared to traditional materials, resulting in a significant price disparity. However, it is indicated that the unit cost of aerogel will decrease to below USD 500 in the foreseeable future. Furthermore, for materials such as aerogels, despite the lack of a cost advantage compared to traditional materials with similar insulation capabilities, enhancing its lifecycle to minimize the cost per unit time represents a potent competitive strategy.

• Synthesis

The preparation of aerogels encompasses three primary stages, including gel formulation, gel maturation, and gel desiccation [142,143,144,145]. For instance, when producing silica aerogel, an appropriate silica precursor such as sodium silicate, silica gel, or TEOS should be appropriately chosen. Under catalyst mediation, the silica precursor undergoes hydrolysis and condensation in either water or an organic solvent, yielding a sol wherein silicate or siloxane moieties progressively assemble into a three-dimensional matrix in the form of gel via condensation reactions. Subsequently, unreacted monomers within the gel’s network structure persist in further reacting, thereby engendering a denser network. Common procedures for aging employ ethanol–siloxane mixtures. It is noteworthy that due to the presence of numerous organic solvents or water within the gel matrix, it becomes imperative to substitute the solvent via displacement reactions, particularly for highly volatile solvents like ethanol or acetone, to avert the formation of turbid and excessively dense aerogels. Afterward, the resultant gel must undergo a final step of desiccation. This constitutes the most pivotal stage in aerogel preparation, with prevalent methodologies encompassing atmospheric pressure drying (AFD) and supercritical drying (SCD). The evaporation of the dehydrating agent induces surface tension, causing certain structural units to contract and deform, thereby impacting the aesthetics and functionality of the aerogel. Furthermore, in the case of AFD, excessively rapid drying rates may engender a potential pseudo-drying phenomenon.

• Modification of Aerogel

Despite possessing excellent thermal insulation properties, aerogel materials pose an increased risk of TR when integrated into batteries due to the constraints inherent to the application scenario and the susceptibility of the battery to external forces, such as punctures, extrusion, and other external conditions encountered during the application process. This issue has garnered significant attention from researchers, thus making the enhancement of mechanical strength in aerogel materials a prominent area of research focus. Owing to the inherently brittle nature of pure aerogel, its application in battery insulation proves challenging due to its susceptibility to rupture. Hence, attention has shifted towards composite materials that hybridize aerogel with other materials. Typically, a composite comprising silica and aerogel, characterized by a molecular-level entanglement with chitosan in a random “cluster-cluster” aggregated silica structure, has yielded a three-dimensional semi-interpenetrating network. This structure significantly has enhanced the mechanical properties of silica aerogel, contributing to a high degree of mechanical flexibility (fracture elongation > 80%) and increased yield strength (>7 MPa), prompting researchers to pursue further modifications for enhancing the mechanical properties of aerogels.

Wu et al. [137] devised and fabricated a silica aerogel composite comprising oriented and layered silica fibers (SFs), SiC nanowires (SiCNWs), and silica aerogel. Figure 6c illustrates the synthesis pathway and its microscopic morphology in detail. The oriented and layered SF structure enabled static air to serve as a barrier, impeding solid-phase heat transfer. Concurrently, SiCNWs uniformly grew in situ on the surface of SFs, forming a compact network structure. This structure mitigated thermal conductivity by sealing cleavage cracks in SiCNWs/SFs/SA, while SiCNWs attenuated high-temperature radiative heat transfer, endowing the material with superior thermal insulation properties at elevated temperatures. The composite’s varied network structure enhanced the overall mechanical strength of the material without dimensional alteration, achieving a tensile strength of 0.75 MPa after 1800 s of heat treatment at 1000 °C, thereby positioning the synthesized material as a promising thermal insulator. Li et al. [146] amalgamated 9,10-Dihydro-9-oxa-10-phosphophenanthrene-10-oxide-3-aminopropyltriethoxysilane (DOPO-KH550)-modified hollow glass microspheres (HGM) doped with polyvinyl alcohol (PVA) to fabricate a novel composite aerogel, denoted as PVA-DKHGM, as a flame-retardant material. The detailed synthesis procedure is shown in Figure 6c.

The hollow structure of the HGM and the high porosity of the composite material delivered a thermal conductivity lower than 0.0187 W/m·K to the synthesized PVA-DKHGM composite, surpassing that of most market-available composites. Additionally, the composite exhibited traits such as light weightiness, high mechanical strength, good thermal stability, ease of preparation and porosity, rendering it highly promising for practical applications. Yang et al. [147] accomplished the synthesis of a novel C/SiOC composite aerogel (CSA) via the straightforward copolymerization of bis-sol, followed by the atmospheric pressure drying and heat treatment, with carbon aerogel (CA) serving as the precursor material. Figure 6a illustrates the synthesis pathway and microscopic morphology of the material. By modulating the relative content of CSA in SiOC, notable effects on both the morphology and properties of CSA were observed. This manipulation not only enhanced the micro-absorption characteristics of the composite but also significantly bolstered its mechanical properties compared to CA of similar density, nearly doubling the maximum compressive strength. Furthermore, the results demonstrated that CSA exhibited superior antioxidant properties and remarkable flame retardancy compared to CA, suggesting its potential for specific application contexts.

4.2.2. Ceramics

In contrast to aerogels, ceramics represent a more conventional thermal insulation material attributed to their exceptional heat resistance, corrosion resistance, thermal conductivity, and other properties. These advantages contribute significantly to the role of ceramic materials in battery thermal management by mitigating temperature elevation and heat dissipation within the battery system, thereby enhancing the battery’s cycle life and safety. Concerning battery thermal management, the utilization of ceramic materials encompasses advancements in thermal barrier coatings and fillers. For instance, certain battery configurations have incorporated nanoceramic materials at the electrode interface to serve as a super-insulating layer, capable of substantially enhancing the robustness of the electrode interface and promptly mitigating the temperature escalation in the event of cell runaway. Furthermore, ceramic materials can be employed in the fabrication of heat sinks, heat pipes, and other heat dissipation components to enhance the thermal management capabilities of the battery system through design optimization [101,148,149].

• Properties

Ceramics exhibit exceptional heat resistance, characterized by a high melting point, enabling prolonged stability even under elevated temperatures. For instance, alumina ceramics demonstrate a thermal conductivity ranging from 30 to 35 W/m·K, delivering a marginal decline with increasing temperature. Furthermore, their chemical stability is noteworthy, which can be highlighted by their outstanding resistance to oxidation and commendable resilience against acids, alkalis, salts, and other corrosive agents. Consequently, ceramic materials employed in batteries ensure prolonged operational stability, remaining resilient against chemical corrosion. Besides, ceramic materials exhibit a high Young’s modulus alongside low tensile strength but demonstrate remarkable flexural and compressive strength. Despite their limited plasticity at room temperature, the toughness of ceramic materials can be enhanced through meticulous design and precision machining. Within batteries, ceramic materials find utility as supportive structures or encapsulating materials to provide essential mechanical reinforcement.

• Synthesis

There are several methods to synthesize ceramics, primarily relying on thermal sintering, the sol–gel method, and freeze-drying. Thermal sintering methods commence by molding powder particles into a desired shape. This process may entail pre-forming or directly loading the powder into the mold to ensure precise shaping and density. Subsequently, the molded powder particles are subjected to heating in a furnace, accompanied by the application of pressure to enhance the particle cohesion, thereby minimizing pores and defects and enhancing overall material density and strength. Thermal sintering can produce ceramic products with ultra-high purity. However, it demands stricter production control, resulting in lower synthesis efficiency. Ceramics production via the sol–gel process mirrors the steps of aerogel production aforementioned, with the distinction lying in the removal of solvents and volatile components post-gel formation through processes like drying and sintering, resulting in the formation of dense ceramic products. In contrast to thermal sintering, this method can be conducted at lower temperatures, boasting a high purity level, thereby facilitating precise control over the microscopic morphology and properties of the ceramic material. Unlike them, freeze-drying also referred to as the ice template method, is applied to synthesize ceramics with varying pore sizes and architectures. The process primarily encompasses freezing and solidifying ceramic slurry at low temperatures. Subsequently, during the freezing of water, rearrangement of powder particles occurs within the slurry, followed by the sublimation of ice under vacuum, resulting in the formation of a porous structure characterized by directional alignment. Due to the rearrangement of ceramic particles to construct an ordered pore structure during freezing, the resulting material exhibits heightened mechanical strength. In practical production, it is imperative to comprehensively assess material application areas, production costs, and other factors to determine the most economical and energy-efficient synthesis route.

5. Application of Thermal Insulating Materials in Batteries

In this chapter, we will discuss the application of the above-mentioned thermal insulating materials in LIBs and their performance in suppressing battery TR.

5.1. Performance of PCM

The application areas of PCM in LIBs can be summarized as follows: battery cell thermal management, battery module integration and electric vehicle applications. Battery cell thermal management, in general terms, utilizes the ability of PCM to absorb and release heat to effectively control the temperature of LIBs. Battery module integration is embedded in the battery monomer between the PCM so that the heat is evenly distributed to avoid local overheating. Besides, PCM can also be used as a structural support material to a certain extent, which can effectively reduce the influence caused by external shock. When the LIBs are rapidly charged or discharged, PCM can effectively control the temperature of the battery to the appropriate temperature range, which can reduce thermal stress and material aging, thereby extending battery life.

For LIBs applied in different scenarios, PCM tends to regulate the temperature of LIBs by controlling heat absorption and heat release. Not only that, the high heat capacity and latent heat of PCM themselves can delay the occurrence of TR and extend the reaction time, providing the battery system with ample time to take countermeasures and reduce losses. On the other hand, the PCM embedded between the battery single cells acts as a thermal isolation in the battery module, preventing the TR from spreading from one single cell to another single cells. Locally controlling the TR protects the whole battery module. Rajan et al. [150] investigated the application of 1-tetradecanol as the PCM in lithium iron phosphate batteries, which made the temperature reduction of the 5 mm PCM because of the low thermal conductivity of the PCM. Additionally, with the introduction of copper foam, the latent heat was absorbed more uniformly, and the increase in thermal conductivity led to a 3 °C reduction in battery temperature.

A new encapsulated inorganic PCM (EIPCM) was synthesized and characterized by Ping et al. [151]. The EIPCM was tested for its melting temperature of 51 °C and latent heat of 111.69 kJ/kg, and the effect of EIPCM on battery thermal management was further investigated at different discharge rates. The results showed that at 3 C high discharge rate, the peak battery temperature was reduced from 86.6 °C to 66.1 °C, and the temperature difference was also kept within ±3 °C. In addition, it was found that EIPCM could effectively inhibit TR propagation and delay the occurrence of TR by 495 s, which had a better prospect in practical battery applications and energy storage. Zou et al. [152] prepared a composite PCM (CPCM) with high thermal conductivity using graphene, carbon tubes, EG, and paraffin wax as raw materials and investigated the locally enhanced heat transfer of the battery module. It was concluded that CPCM had a similar local enhancement of heat transfer with composite PCM with similar thermal conductivity and that the temperature distribution of the battery module became more uniform when the heat transfer enhancement region was narrower. Compared with the 16-cell CPCM, the 4-cell CPCM-enhanced battery module improved the uniformity and had certain advantages in cost as well as quality.

Organic PCMs’ high latent heat of phase transition and good stability allow them some applications in small- and medium-power battery systems. Inorganic PCMs have more potential than organic PCMs in some scenarios that require high thermal management of batteries due to their high thermal conductivity and adjustable phase transition temperature. Of course, composite PCM synthesized by combining the advantages of both organic and inorganic materials are currently the focus of research and battery applications. They can be optimally formulated to achieve a combination of high thermal conductivity and high latent heat to satisfy a wide range of applications.

5.2. Performance of Barrier-Type Insulation

Similar to PCM, the application of barrier-type insulation materials in LIBs can be summarized in the following areas: insulation between cells, module and pack insulation, and the important component of battery thermal management systems. These applications are precisely the key means to inhibit TR.

For the battery cell insulation area, the porous nature of the barrier-type insulation material is used to control heat conduction, convection and radiation to reduce the transfer of heat between battery cells. When TR occurs in one cell, the insulation can significantly reduce the impact on other neighboring cells and prevent the chain reaction caused by TR. For the module and battery pack insulation, the barrier-type insulation material acts as a barrier between the battery module and the battery pack to prolong the path of heat transfer, thus preventing TR in one cell from being transmitted to the entire battery pack. For battery thermal management system applications, wrapping the battery in the entire battery module system or adding a certain thickness of insulation layer on the outside of the battery based on the above two types of applications in order to achieve the temperature control of the entire battery system, improve the efficiency of the thermal management system and extend the life of the battery. The battery thermal management system can quickly cut off the power supply and start the cooling system when it detects signs of TR.

MP et al. [153] synthesized polyimide-Al₂O₃ membranes using electrospinning technology. The synthesized membranes began to decompose at 525 °C. Compared with the complete decomposition of polyolefin membranes at 475 °C, the polyimide-Al₂O₃ membranes had better thermal stability. Yan et al. [154] prepared ceramic fiber sponge aerogels with high thermomechanical properties by electrospinning technology. The subsequent test found that without any protective battery pack, when the TR occurred in a single unit, the peak temperature was 867 °C, and the peak temperature of adjacent batteries reached 460 °C, triggering the TR of adjacent batteries. The peak temperature of the battery pack protected by the thermal insulation material was only 652 °C when the unit was thermally out of control. The adjacent batteries reached the peak temperature after 1000 s, and the maximum temperature was only 180 °C, effectively protecting the whole battery pack. Yu et al. [155] designed a composite heat insulation board with a sandwich structure (copper/silica aerogel/copper). Low surface radiation emissivity and good thermal convection inhibition effect make the thermal conductivity as low as 0.031 W/m·K. Results showed that the time of TR of neighboring cells increased from 1384 s to more than 6 h, which greatly inhibited the propagation of TR.

The barrier-type thermal insulation materials mentioned in Section 4 have been widely used in restraining the TR of lithium-ion batteries. Organic barrier-type materials often have disadvantages such as flammability, easy aging and easy water absorption. Inorganic barrier-type materials usually have high density, which may have a great impact on the quality of the whole battery pack. Although the composite barrier-type material combines the advantages of the two, the increasing manufacturing cost also affects its use on specific occasions. Therefore, it is still necessary to select appropriate materials in combination with specific use scenarios, which is a problem that researchers and manufacturers should pay attention to.

6. Conclusions and Prospect

In this review, we describe the thermal insulating materials for LIBs and provide a detailed account of their properties, synthesis, and modification. Analysis of different contributions to the source of heat leading to TR in batteries shows the necessity of thermal management for LIBs. In order to study the properties and synthesis of thermal insulating materials, we have discussed two typical materials, including PCMs and barrier-type insulation materials. For the preparation and modification of PCMs, leakage, phase transition temperature and latent heat are primary characterizations that determine whether PCM can be applied to specific working scenarios. In addition, heat storage capacity is an important parameter used to measure the performance of PCM. For the preparation and improvement of barrier-type insulation materials, thermal conductivity, mechanical strength, and flame-retardant properties are essential criteria for judging the thermal insulation performance of the material.

Comparing these two types of materials, the following differences have been summarized. Firstly, the two types of materials have different working mechanisms. PCMs store or release a large amount of heat through their own physical state changes, while barrier-type materials achieve the blocking of heat through the reduction of the three ways of heat transfer. Secondly, the application of PCMs needs to consider a variety of factors such as latent heat, phase change temperature and leakage, resulting in a narrower range of materials to choose from. Barrier-type materials have a wider range of options than the former, which makes them well-suited to battery module scenarios. Both types have important applications in the suppression of TR in LIBs. However, it can be concluded that there is no single type of insulation material that can satisfy all the crucial requirements for the suppression of TR in LIBs.

Although the above-mentioned two types of materials have made significant contributions to the security of LIBs, some challenges still exist. Firstly, the optimal operating temperature range for batteries is usually very narrow (e.g., 20–40 °C for LIBs), so most thermal insulating materials are difficult to satisfy in terms of the whole temperature range. Secondly, thermal insulating materials usually undergo volume changes during practical applications, which can affect the structural stability of the battery when used in thermal management systems. In addition, low thermal conductivity has been a challenge for some PCMs, which can lead to unbalanced heat distribution in the battery, resulting in localized overheating. Barrier-type insulation materials are usually characterized by large thicknesses and high mass, which will take up too much space and increase the weight of the battery pack design, thus affecting the overall performance of the battery pack. In addition, the balance between thermal insulation and heat dissipation is a major difficulty in battery thermal management.

Based on these challenges, thermal insulation materials should be optimized properly. For example, suitable modification of materials at the micro-scale through nanotechnology, doping of appropriate metal ions, and combining PCMs with barrier-type materials to improve the thermal performance of thermal insulating materials are accessible methods.