The main forms of graphene are small-layer graphene and large-area thin films (as shown in Figure 4), and the preparation methods vary depending on the form. The properties of graphene depend on its preparation method and microstructure, and the main synthesis methods currently include mechanical exfoliation, solvent liquid-phase exfoliation, external growth, oxidation-reduction, and intercalation exfoliation, etc^[39-40].

Among these graphene preparation methods, obtaining graphene with a single atomic layer thickness is the focus of the preparation process. Exfoliation from bulk graphite is the main method for mass-producing small pieces of graphene. Mechanical exfoliation involves directly exfoliating in liquid or functionalizing the edges of graphite under solid-state conditions before exfoliation. The solvent-phase exfoliation method is an effective way to prepare non-redox graphene, which disperses graphite in specific solvents or surfactants and uses energy input methods such as ultrasonication to peel single-layer or multi-layer graphene from the graphite surface, resulting in graphene dispersion. However, the graphene dispersion obtained by this method has poor stability and requires subsequent purification and separation, potentially increasing the complexity and cost of the preparation process^［42-43］. It is also possible to exfoliate by intercalating chemicals between graphene layers^［44］, but the exfoliation operation may lead to poor structural integrity and uncontrollable size of the prepared graphene. In addition, the stripping solvent is an important factor affecting the efficiency of liquid-phase exfoliation of graphene. Commonly used solvents include water-based solvents^［45-47］, organic solvents^［48-49］, and mixed solvents^［50-51］, among others. When selecting a solvent, it should be considered whether it can prevent graphene from re-aggregating, environmental protection, price, and whether the prepared graphene product is controllable.

The main steps of the redox method are to first oxidize flake graphite to obtain graphene oxide containing a large number of oxygen-containing functional groups and then reduce it chemically or at high temperatures to obtain graphene. However, this preparation method still cannot effectively solve the problems of incomplete structure, multiple layers, and poor uniformity in graphene^［52］. Therefore, how to effectively obtain graphene with single-atom layer thickness and low defects is the key direction for optimizing the current graphene preparation process.

The size of graphene is positively correlated with its phonon wavelength, and the larger the size, the more conducive it is to heat transfer^［53］. The main method for preparing large-area graphene films is chemical vapor deposition (Chemical Vapor Deposition, abbreviated as CVD). Graphene prepared by the CVD method has fewer defects and controllable layers, and can produce larger-sized graphene films, but there are also issues such as difficulties in transferring deposited graphene^［54-55］.

Researchers at home and abroad have mainly studied large flake graphene films prepared at different temperatures. Sui et al^［56］ used the CVD method to prepare graphene films on Cu and thermally treated the graphene films at different temperatures (800 ℃, 900 ℃, and 1000 ℃). At a voltage of 60 V, the steady-state temperatures of the films treated at 800 ℃, 900 ℃, and 1000 ℃ reached 42 ℃, 150 ℃, and 206 ℃, respectively, indicating that the thermal conductivity of the graphene film was enhanced after heat treatment. Cui et al^［57］ also successfully prepared graphene films on glass using the CVD method. The prepared films exhibited excellent electrothermal performance (maximum heating rate of 2.6 ℃/s, maximum temperature up to 160 ℃) and their thickness could be controlled by adjusting the growth time, temperature, and gas flow rate, thereby changing their resistance. The prepared graphene films had the advantages of uniform heating, rapid temperature rise, and high heating efficiency, showing good application prospects in the field of manufacturing automobile defrosting and defogging devices. Traditional CVD methods for growing graphene require high temperatures to initiate a series of chemical reactions, which are costly, and the preparation size of graphene is limited by equipment. Kulczyk-Malecka et al^［58］ successfully prepared vertically aligned graphene at 300 ℃ using plasma-enhanced chemical vapor deposition. Wang et al^［59］ discussed the preparation of graphene films by different CVD methods under low-temperature conditions (<600 ℃) from the aspects of precursor types (gas, liquid, and solid) and substrate types. The main factors for synthesis are the precursors (solid, liquid, and gas) and substrate types (Cu, Ni, Co, and Fe, etc.). Liquid aromatic hydrocarbons and solid hydrocarbons are the main carbon sources for preparing graphene, and gaseous precursors have high reaction temperatures; Cu substrates have high catalytic activity and low carbon solubility, which can better control the number of graphene layers and obtain high-quality monolayer graphene.

The strong interlayer interaction between graphene sheets makes them prone to agglomeration and stacking, weakening the interfacial bonding with the polymer matrix, which limits the application of graphene^［60-61］. Therefore, grafting long-chain polymers onto graphene for chemical modification allows the graphene surface to contain highly reactive functional groups (such as hydroxyl, carboxyl, and epoxy groups). Subsequently, by means of polymerization, the corresponding groups at the ends of the polymer chains are polymerized with the activated functional groups, adjusting the interfacial bonding of graphene in different media. This confers significant importance to the functionalization of graphene and the preparation of composite materials^［62-64］. The main methods for modifying the surface of graphene are covalent modification and non-covalent modification.

Non-covalent modification mainly utilizes physical connection forms such as electrostatic interaction and π-π between the modifier and graphene. In the aspect of non-covalent modification Teng et al^[65] functionalized pyrene molecules with functional segment polymer chains which uniformly dispersed GNSs in the polymer matrix improved interfacial interaction and enhanced the thermal conductivity coefficient of GNS/epoxy composites. Wan et al^[66] selected polyethylene glycol octylphenyl ether (POPE) surfactant to treat graphene and successfully prepared highly dispersed graphene-filled epoxy resin composites. Transmission electron microscopy showed that there was a single-layer structure at the edges of the treated graphene with good dispersibility. Li et al^[67] used polystyrene sulfonate (PSS) to treat graphene where PSS formed non-covalent bonds with graphene preventing the agglomeration of graphene. Non-covalent modification mainly uses high-quality graphene as a conductive filler to improve the interfacial bonding between graphene and the resin matrix. However due to the discontinuous thermal conduction pathways of graphene sheets phonons cannot vibrate and propagate along the continuous graphene network structure within the polymer system resulting in still unsatisfactory thermal conductivity of the composite^[68]. The poor stability of non-covalently modified graphene also limits its application.

Covalent modification involves grafting some chemical functional groups onto the graphene surface through chemical reactions in a covalent manner, and reacting the modified polymer with the carbon-carbon double bonds in graphene in the form of covalent bonds to enhance interfacial bonding and participate in curing reactions^［69］. Due to the chemical inertness of graphene, its filling amount in polymers is not large. Usually, the oxygen-containing functional groups on the surface of graphene oxide are covalently modified to prepare graphene with better dispersibility^［70］. Zhu et al.^［71］ utilized the hydrogen bond interaction between polyvinyl alcohol (PVA) and graphene oxide (GO) to obtain well-dispersed GO and prepared PVA/GO nanocomposite films with good mechanical and thermal stability. Loh et al.^［72］ summarized the chemical routes for the functional modification of graphene and graphene oxide from the perspectives of diazo reaction, esterification reaction, acylation reaction, nucleophilic ring-opening reaction, etc. (as shown in Fig. 5). By considering the interactions of modification types, reaction types, and modification molecules, the surface functional modification of graphene can be improved. Covalent modification can enhance the dispersibility and interfacial bonding of graphene and reduce phonon scattering^［12］. Modifying graphene through chemical bonding connects the modifying groups grafted onto the graphene surface, improving the dispersibility and reactivity of graphene. However, the presence of grafted functional groups disrupts the original hexagonal ring structure of graphene, indirectly affecting its inherent properties.

In addition to the above two methods, inserting a rigid filler between graphene layers also serves as a way to enhance the thermal conductivity of graphene. The bridging effect of the filler connects adjacent graphene sheets, which not only prevents the stacking of graphene layers but also improves the thermal conductivity of graphene. It is essential to ensure the connection stability between the layers and the filler.

The electrothermal effect of composites prepared with graphene as filler is related to the amount of graphene added^［73］. Currently, the common method of blending modification is often used to add highly thermally conductive fillers into polymer materials (mainly resin-based materials), giving the composites characteristics such as high thermal conductivity, low expansion coefficient, and lightweight. The preparation of graphene-added electrothermal composites mainly involves in-situ polymerization, melt blending, and solution blending methods.

In the study of in-situ polymerization, Polschikov et al^［74］ prepared polypropylene/graphene nanoplatelets (GNP) composites through in-situ polymerization and tested their properties. The results showed that graphene nanoplatelets could significantly increase the crystallization temperature, thermal stability, and elastic modulus of the composites. In addition, ultrasonic treatment could improve the dispersion effect of GNP particles in the composites, enhancing the comprehensive performance of the composites. Kazerouni et al^［75］ used graphene oxide (GO) as raw material and utilized in-situ polymerization to prepare PSP/GO nanocomposites by the polycondensation reaction of dichloroethane and sodium tetrasulfide; adding only 0.5 wt% of GO increased the melting point of the composites by more than 16 ℃. The in-situ polymerization method has high compatibility requirements for the two raw materials, introducing graphene into the resin matrix will also increase the viscosity of the polymerization products, making the in-situ polymerization process complicated, and it will affect other properties of the composites.

In the study of melt blending, Zhou Xing et al^［76］ used thermoplastic polyurethane (TPU) as the matrix and polyvinyl alcohol (PVA) functionalized modified graphene oxide (GO) as the filler to prepare GO-g-PVA/TPU composites by melt blending method, and tested the properties of the composites. The results showed that when the content of GO-g-PVA was 4 wt%, the crystallization peak temperature of the composite was increased by 28.8 ℃ compared with pure TPU, and the addition of GO-g-PVA improved the tensile properties of the composite. Tewatia et al^［77］ prepared graphene/polyetheretherketone (PEEK) composites by high shear melt blending method and characterized the thermal properties of the composites. The results showed that there was a good interaction between graphene and PEEK, which increased the crystallinity of PEEK, improved the thermal stability of the composites before 550 ℃, and enhanced the thermal properties of the composites. In addition, Zhang et al^［78］ prepared polymer/graphene oxide nanocomposites by a two-step method of in-situ polymerization and melt blending. They first used silica as a stabilizer to prepare polystyrene (PS)/graphene oxide (GO) composite microsphere preforms, and then melt blended these preform microspheres into the polymer matrix to obtain nanocomposites and tested their properties. GO can be uniformly dispersed in the PS matrix, the coefficient of thermal expansion of the composite decreased, the tensile strength and elongation at break of the composite significantly improved, and the impact resistance increased by 64%.

In the study of solution mixing methods, Wu et al.^[79] used a one-step method to prepare polystyrene (PS)/graphene nanocomposites. They performed non-covalent functionalization of graphene by in-situ exfoliation of graphene nanoplatelets directly in PS solution, utilizing π-π interactions between the benzene rings on PS and the basal planes of graphene. The grafting rate was as high as 85%, and the prepared graphene composites exhibited good electrical properties. Ding et al.^[80] prepared styrene/graphene composites (PG) using solution mixing and hot pressing processes. Graphene oxide (GO) was reduced to graphene nanoplatelets using p-phenylenediamine, and the properties of the resulting composites were analyzed. The results showed that when the graphene content was 10%, the thermal conductivity of the composite increased by 66%, reaching 0.244 W·m^-1·K^-1. Moreover, the thermal conductivity performance of the hot-pressed PG composite was anisotropic, due to the ordered arrangement of graphene and styrene chains after the hot pressing process. Ren et al.^[81] used a combination of solution and melt blending methods to prepare graphene/PA6 composites. When the graphene content was 20 wt%, the thermal conductivity of the composite reached 3.55 W·m^-1·K^-1, which is an increase of 1167% compared to pure PA6. Wang et al.^[82] improved the dispersion effect of graphene nanoplatelets (GNP) with cellulose nanofibers (CNF), and used ice templating-induced orientation method to prepare GNP aerogels. The aerogels were encapsulated with paraffin wax (PW) to make composites, and the electrothermal properties of the composites were tested. The results showed that when the GNP content was 4.1 wt%, the thermal conductivity of the composite could reach 1.42 W·m^-1·K^-1. Furthermore, under conditions of 5 V voltage and 1.18 A output current, the short current change response time indicated that the prepared material had very high electrothermal conversion efficiency (as shown in Figure 6).

The main research results mentioned above are based on graphene as a conductively filling material, which is compounded with the matrix to modify and enhance the thermal conductivity of the matrix, significantly improving the thermal performance of the composites. The main heating mechanism is as follows: after being electrified, the electrothermal film generates a voltage difference, which in turn produces an electromagnetic field. Carbon atoms undergo Brownian motion in a strong alternating electromagnetic field, and the mutual friction and collision between atoms generate thermal energy. As the heating temperature continues to increase, the lattice vibration amplitude of the conductive filler becomes larger, contributing more strongly to the heat generation. However, structural issues such as agglomeration, wrinkles, and defects in the graphene filler itself prevent the composite electrothermal material from truly leveraging the high thermal conductivity characteristics of graphene, posing a challenge to the preparation methods for low-defect graphite. In addition, the use of various high thermal conductivity materials in combination to prepare composites with even better thermal and mechanical properties is also a major direction for future development.

Electric heating film utilizes resistance heating method to achieve the purpose of temperature control and heat preservation. The resistance can make nonlinear changes with temperature, automatically adjusting the thermal output power. According to the materials used in electrothermal film, it can be divided into metal and non-metal categories. Metal materials mainly adopt printed circuits or insert metal material resistance wires or resistance wires made of metal oxides between insulating films. The heating metal materials of each piece of electrothermal film are connected in series, and the electrothermal films are connected in parallel circuits. Non-metal electrothermal films mainly use carbon-based materials as heating materials, adding film-forming agents, flame retardants, and other additives into carbon-based conductive materials, coating them onto the surface of insulating materials, and then obtaining the electrothermal film after high-temperature treatment. Although metal electrothermal films and non-metal electrothermal films have their own processing technology characteristics, both have some limitations. The processing cost of metal electrothermal films is high, and the heating is uneven; inorganic electrothermal films have high brittleness and poor flexibility, only suitable for rigid films. In addition, the random entanglement of polymer molecular chains and molecular chain vibration cause phonon scattering that prevents crystallization, reducing the thermal conductivity of the polymer^［83］. Therefore, by using substances with better thermal conductivity to fill polymer materials, highly thermally conductive polymer electric heating films can be prepared.

The free electrons in graphene heating films can move directionally under the action of an external electric field, converting electrical energy into thermal energy. Meanwhile, the high thermal conductivity of graphene also allows heat to be transferred rapidly, making the film heat evenly and possess good flexibility^［84］. Thermally conductive films can generally be divided into pure graphene films, graphene hybrid films, and graphene polymer composite films. The preparation methods of graphene films should vary according to the different preparation methods of graphene materials.

Graphene as a conductive filler to prepare new composite electrothermal films that balance flexibility and uniform heating characteristics^[85]. By utilizing the performance features of graphene, composite films with good thermal conductivity can be prepared. An et al.^[86] used solution casting and thermal curing methods to prepare graphene/epoxy resin composite films. The resistance of the resulting composite film decreases as the amount of graphene increases. At 30 V voltage, the maximum temperature of the composite film can reach 126°C, and the prepared composite film has the characteristics of rapid temperature response, high electric power, and stability. Li et al.^[87] prepared graphene nanosheets by electrochemically exfoliating graphite in sodium tungstate aqueous solution, and formed an electrothermal film on a polyethylene terephthalate (PET) substrate through filtration, transfer, and heat treatment. The electrothermal response of the electrothermal film is 20 seconds. As the voltage increases from 5 V to 30 V, the steady-state temperature of the film increases from 32°C to 149°C. After 100 bending tests, there was no significant change in the electrothermal performance. Zhou et al.^[88] successfully prepared highly stretchable flexible electrothermal films using reduced graphene oxide (rGO), poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS), and waterborne polyurethane (WPU). A PEDOT:PSS solution was mixed with a 20% WPU solution by weight, then 5% rGO by weight was added to the mixed solution. After drying and curing, the composite electrothermal film was obtained. The electrical conductivity of the composite film is 18.2 S·cm^-1, the resistivity is approximately 0.055 Ω·cm, and it exhibits high electrothermal performance. Yang et al.^[89] blended GO with carbon nanofibers (CNF) and reduced them using hydroiodic acid to prepare ultra-flexible rGO/CNF composite films with a thickness of 23 μm, providing the composite films with good thermal conductivity. Song Yuqing^[90] studied the heating performance of novel graphene-based electrothermal films prepared by ion-induced gelation processes by applying voltage and heating rates. Results showed that under a 15 V DC voltage, the high steady-state temperature of the graphene-based film can reach 136°C. Under an 8 V electrothermal voltage, it can continuously heat for 1.5 hours with excellent heat retention. Wang et al.^[91] utilized thermoplastic polyurethane (TPU) as the matrix and nano-crystalline cellulose (NCC)-modified reduced graphene oxide (rGO) as the conductive filler to prepare flexible conductive films by blending methods. When the rGO/NCC addition ratio is 5 wt% and the initial concentration of TPU is 30 wt%, the thermal conductivity of the composite film can reach 0.3464 W·m^-1·K^-1, increasing the thermal conductivity of pure TPU by 56% and the maximum tensile strength by 1.5 times compared to pure TPU material, and the electrothermal response of the composite film is rapid. Tang et al.^[92] utilized polyimide (PI) as the film-forming material and graphene (GE) as the conductive filler to prepare GE/PI flexible electrothermal films. This composite film rapidly heats to 390°C at 24 V when the GE content is 8 wt%, and possesses comprehensive mechanical properties. Nie et al.^[93] prepared an embedded graphene grid flexible electrothermal film by combining laser induction and transfer printing techniques. For a flexible electrothermal film with a line width of 1575 μm, it can reach a high temperature of 165°C and maintain good stability under an external voltage of 15 V.

The graphene films after compounding still have structural defects, and the low carrier mobility and high resistivity of the films are current problems. The main reason is the lattice defects of the prepared graphene. Some studies have shown that after high-temperature repair, graphene can transform agglomerated disordered graphene sheets into continuous graphene layers, providing an effective path for phonon transmission and improving thermal conductivity^［94］. Graphene presents different performance characteristics when compounded with different polymers. The current main research directions are the properties of products of graphene and carbon chain polymers^［95］, heterochain polymers^［96］, and element - organic polymers^［97］, and the tensile properties, thermoelectric properties, and thermal stability of the composite films have been improved to varying degrees^［98］. At present, most of the prepared graphene composite films cannot precisely control the internal structure of the graphene composite films to make them change directionally to achieve the required performance, and the preparation process is random. Customizing graphene composite films with different functions is the main research direction in the future. In order to realize the customization of graphene composite films with specific functions, researchers are exploring more precise control methods in the hope of being able to prepare graphene composite films with specific functions in the future.

In general, the graphene film itself is regarded as a fixed resistor^［99］. The electrothermal film is thin, lightweight, and exhibits good electrothermal performance after high-temperature annealing, which is widely used in defrosting and anti-fogging fields^［100］. Scholars from both domestic and international backgrounds have conducted relevant research. For instance, Wang et al.^［101］ utilized the Meyer rod coating process to apply graphene oxide (GO) onto a PET substrate, preparing flexible rGO films after annealing. These flexible films were placed in a refrigerator for 2 hours to simulate car window frosting, and under an applied voltage of 10 V, they could remove frost within 80 seconds. Currently, resistance wire heating films made of nichrome alloys are mainly used for de-icing and heating purposes, but their flexibility and durability are not high. In contrast, graphene-based flexible electrothermal heating films provide surface heating, exhibiting not only strong flexibility but also enabling uniform heating, making them more effective in electric heating de-icing systems. Shao et al.^［102］ prepared nanofibrillated cellulose (NFC)/graphene composite electrothermal films using ultrasonic dispersion and vacuum filtration methods. This composite film reached a saturation temperature above 80°C at a power density of 2000 W/m ², showing good temperature uniformity. Additionally, NFC can improve the flexibility and mechanical properties of graphene composites, making it suitable as a de-icing and defrosting material. Muhsan et al.^［103］ used a vacuum bagging process to prepare graphene films and glass fiber epoxy resin matrix composites, conducting de-icing performance verification. The results showed that under the same applied voltage conditions, the composite material corresponding to the graphene film had a higher temperature. Zhu Hongwei's team at Tsinghua University^［104］ synthesized an electrothermal defogger on a copper-zinc alloy substrate using graphene films with defects and uniformly distributed wrinkles prepared by chemical vapor deposition. At a safe voltage of 28 V, the defogger could completely eliminate fog within 5 seconds. Qian Mengshuang^［105］ combined numerical simulation and experimental verification to study the de-icing effect of graphene electrothermal films on aircraft. Using a graphene electrothermal system for aircraft de-icing resulted in more effective heat transfer, faster ice melting speed, and more uniform temperature distribution after heating. Vertuccio et al.^［106］ placed flexible graphene films between carbon fiber laminates to create an aircraft surface de-icing system. This de-icing system melted a 1 mm thick layer of ice in less than 7 minutes, demonstrating the rapid de-icing effect of flexible graphene films.

Graphene electrothermal film features low voltage, high thermal conductivity, and is safe, lightweight, and thin, making it applicable in the smart wearable field. Ge et al^［107］ mainly introduced the application scope of graphene and its derivative materials in smart textile thermal management (personal thermal management and flame retardancy), covering mechanics, material development, fabric design, and vehicle body applications, and believed that various 2D materials (boron nitride, MXene, and TMD, etc.) can be combined in the future to precisely adjust the performance of next-generation smart textiles. Zhang et al^［108］ introduced graphene oxide (GO) and polyethylene glycol (PEG) into the carbon nanotube (CNT) network to form composite phase change fibers with a multi-level skeleton; the presence of GO increased interface contact and space, and the CNT/GO/PEG phase change fiber possessed excellent thermal conductivity and thermal stability. Wang et al^［109］ prepared rGO/PET fabric through filtration adsorption and reduction methods, and the prepared fabric exhibited unchanged resistance after 100 bending tests, and could reach a stable temperature of 67 ℃ under an external voltage of 10 V. Utilizing the characteristics of high electrical conductivity, high thermal conductivity, and good mechanical properties of graphene, heaters that conform to human joints can be prepared, and the heating area is evenly distributed. Kwon et al^［110］ compounded low-resistance silver nanowires (AgNWs) with graphene materials, and improved the contact between AgNWs and graphene by introducing polymers containing disulfide groups. The graphene/AgNWs composite film obtained by vacuum filtration has excellent flexibility and can adapt to surfaces of various shapes, showing good adhesion on surfaces such as glass and plastic materials, and is expected to be applied in wearable heating devices, with the application range shown in Figure 7.

The graphene electric heating element has the characteristics of uniform temperature distribution, fast heating speed, and strong flexibility, which can enhance the protective effect on the human body in cold environments and meet the needs of human thermal comfort, showing a good application prospect. Moreover, the large surface energy and chemical inertness of graphene make it necessary to modify graphene or use special coating methods when preparing films, which inevitably increases the complexity of the processing technology for preparing electrically heated graphene films. Due to space limitations, readers interested in the latest research on temperature regulation of graphene in wearable fabrics can refer to existing papers and reviews^{［111-115］}.

Graphene heating film materials also have relevant research in the medical field, such as Wang et al.^[116]To prevent complications of peritoneal adhesion after abdominal surgery, a flexible graphene composite film capable of generating far-infrared radiation was studied and its anti-adhesion effect was evaluated. The results showed that after infrared irradiation treatment, the incidence area of postoperative peritoneal adhesion decreased by 67%, with no skin damage observed. This research achievement can be applied to wearable devices for the prevention of postoperative complications in clinical medicine. Cao et al.^[130]By simulating the self-regulation of human skin, an interactive temperature regulation electronic system was designed using thermoresponsive composites and laser-induced graphene arrays. The graphene array can reach 40°C at a power of 0.15W, and the bionic skin can self-regulate within the range of 35~42°C. In comparative wound healing speed tests, the healing speed of this system is 10% faster than the control group, and inflammation is also somewhat reduced. Graphene can utilize its electrothermal performance advantages during the postoperative recovery process of surgical operations, thereby achieving the dual role of preventing postoperative complications and reducing postoperative recovery time.

The battery thermal management system is crucial for alleviating thermal runaway of batteries. By controlling the heating and cooling of individual battery cells, it reduces the temperature differences among cells, thereby decreasing performance variations among them. Currently, lithium-ion batteries are primarily used in electric vehicle power systems. However, when the temperature is below -10 ℃, the power attenuation of lithium-ion batteries is severe, and the discharge capacity significantly decreases; at temperatures below -40 ℃, the battery capacity is only one-third of its nominal value, limiting the use of batteries in cold climates such as high altitudes and latitudes^{［117-118］}. At present, auxiliary external means (thermal resistance heaters or heat pumps) are mainly used to optimize the low-temperature characteristics of batteries by preheating with heating films laid at the bottom or side walls of the battery pack. The arrangement method of the electric heating film also affects its heating efficiency. Graphene electric heating films used for preheating electric vehicle batteries in low temperatures have the characteristics of high thermal conversion efficiency and fast heating speed, and can be arranged within the confined space inside the battery pack. Liu Yangkun^［119］ fabricated a composite heating film from graphene nanoplatelets (GNP)/polyurethane (PU) hybrid paste, which was applied to preheat electric vehicle batteries, with a power area density of 2000 W/m², heating a single cell to 20 ℃ in 6.6 minutes. The 5000 W/m² heating film arranged on the side of the battery can heat from -10 ℃ to 0 ℃ in 14.4 minutes, with the maximum temperature difference of various parts of the battery not exceeding 10 ℃. When preheating the battery, the arrangement method, power area density, and battery adaptability of the electric heating film also need to be considered. Currently, the heat sources of the studied battery heating systems mainly come from the battery itself, indirectly increasing the battery's power consumption, affecting the cruising range of electric vehicles. Electric heating films are one of the most direct and efficient heating methods under current low-temperature conditions. Additionally, the arrangement of electric heating films on the battery surface can also affect the heat dissipation performance of the battery module under high-temperature conditions. Mao et al.^［120］ utilized the high thermal conductivity of expanded graphite/phase change materials and the radiative cooling performance of graphene coatings to prepare expanded graphite (EG)/phase change material (PCM)/graphene (GE) composite phase change materials. Compared with traditional battery thermal management materials, the temperature was reduced by 26% (as shown in Figure 8), and this material is expected to be applied in projects with strict thermal management requirements. Moreover, how to arrange the installation position of the heating film and balance the low-temperature heating and high-temperature heat dissipation functions of the battery is an urgent problem to be solved for heating the battery pack based on the thermoelectric effect method.

According to the different ways of driving stimulus response, actuators can be divided into various types such as electrothermal actuation, photothermal actuation, and electrochemical actuation. Electrothermal actuation can achieve a series of reversible movements or deformations by connecting to a power supply and changing the applied voltage or current magnitude. Due to differences in material composition, the Joule heat generated by the materials after electrification varies, resulting in a certain regular deformation^{［121-122］}. Electrothermal actuation must possess characteristics such as being lightweight, highly flexible, having large deformation and multiple degrees of freedom^［123］. Carbon-based composite materials prepared into electrothermal actuators do not contain electrolytes and can achieve high-performance actuation under low-voltage stimulation^［124］. Graphene has excellent electrothermal conversion performance and mechanical strength, making it an ideal material for smart actuators^{［125-126］}. Chang et al.^［127］ fabricated a bimorph actuator using graphene and polypropylene, which can be driven by both light and electricity, with low driving voltage (≤7 V), rapid response (8 s), and large-angle reversible deformation (change angle >100°). Chen et al.^［128］ developed a multi-responsive actuator based on graphene oxide/biaxially oriented polypropylene; the actuator produced a bending curvature of 3.1 cm^-1 under humidity drive, while under near-infrared light drive, it generated a bending curvature of 2.8 cm^-1. The biomimetic robotic arm made from this actuator can grasp objects 2.9 times its own weight. Yang et al.^［129］ fabricated a bilayer actuator composed of porous graphene film and polyimide film, which can produce a bending curvature of 0.55 cm^-1 within 5 seconds under low voltage (16 V) conditions and provide an output force 20 times greater than its own weight. This actuator can be applied in soft robotics, artificial muscles, and wearable electronic devices. Due to the inherent properties of the materials themselves, the development of smart actuators still lacks good flexibility, durability, and adaptability to shape changes. Future research needs to overcome challenges like achieving large deformation and high driving force in self-actuated performance.