Recent Progress of High Safety Separator for Lithium-Ion Battery

In general, separator failure can be caused by three factors: high temperatures, accidental interference or damage from extrusion and collision, and puncture by metal dendrites[56]. To overcome those situation mentioned above, the ideal high-safety separator must have the following features: (1) Unique characteristics to avoid metal dendrites formation and growth. (2) Remarkable thermostability at higher temperatures. (3) Robust mechanical characteristics match the cell-assembly process. (4) Excellent chemical and electrochemical stability. (5) The structure and other characteristics of the separator should be adequate to provide outstanding electrochemical performance. From the perspective of high-safety separators, the research on thermal safety separators (heat-resistant and flame-retardant separators), thermal-shutdown separators, anti-dendrite separators, high mechanical strength separators, and good compatibility separators have entered the field of view of scientific researchers. To improve the thermal safety of separator, there are two different strategies: using high thermal stability materials and increasing the separator combustion energy barrier. The thermal safety separator focuses on having a higher melting point and lower thermal shrinkage than the commercial LIB polyolefin separator. Thermal-shutdown separators would prevent the thermal runaway by shutting down the battery's electrochemical reaction. Commonly, thermal-shutdown separators consist of at least two kinds of materials: one with an appropriate melting point to close the separator pore after heating abnormally and others with a high melting point to prevent anode/cathode from short circuit. The appropriate melting point should be lower than the battery thermal-runaway temperature. To increase separator combustion energy barrier, the proposed strategy involves incorporating the flame retardant such as triphenyl phosphate (TPP), polyphenylene sulfide (PPS), PBI, or inorganic non-flammable ceramic additives inside the protective polymer shell of microfibers, avoiding the flame retardant from directly contacting with the electrolyte, which would not deteriorate the electrochemical performance of LIBs. To inhibit the growth of metal dendrites, starting with the separator, there are two efficient strategies: one is the separator components chemically react with metal dendrites; and another is that a separator with high modulus and unique pore structure can weaken local current density to obtain uniform deposition. The high mechanical strength of the separator can prevent structure breakage from mechanical abuse. The separators mechanical strength can be deeply affected by the thickness, porosity, pore size, and preparation method. Good chemical compatibility of the separator can ensure its inertness, maintain its structural integrity, and store enough electrolytes to prevent electrolyte leakage. Here, from the perspective of material preparation, the recent progress of high-safety separator is reviewed. 5.1. Inorganic Material Modification As one of the most widely used separators in rechargeable LIBs, the relatively low-cost polyethylene separators show excellent mechanical properties and good chemical/electrochemical stability. The excellent mechanical properties of the inorganic material-modified separator provide sufficient strength to resist both external damage during the assembly process and internal damage caused by lithium dendrite punctures during use. The excellent thermal stability of inorganic material-modified separators ensures low thermal shrinkage, preventing short-circuit contact between the positive and negative electrodes during heating. The inorganic material modified separator with automatic shutdown function can prevent further release of energy, and high film breaking temperature can prevent the separator fracture at high temperature resulting in short circuit contact between positive and negative electrodes, and good flame retardancy helps nip thermal runaway in the bud to improve the battery safety. However, the non-polar surface and low melting point of the PE separator are the significant invisible threat to the battery safety. The hydrophobic surface and poor surface energy of commercial PE separators result in their low liquid electrolyte wettability [57]. From its glass transition (T_g = −68 °C) and melting point (T_m = ~135 °C), the low thermal stability of PE separators at elevated temperatures makes it undergo sizeable thermal shrinkage, which may lead to a battery internal short circuit and finally cause thermal runaway [58]. Modifying separators with inorganic materials to enhance safety is one of the most actively researched methods and has been commercially applied [59]. Various strategies are used in inorganic material modification, including surface coating, surface grafting, fillers, self-assembly, atomic layer deposition, and chemical vapor deposition. Various selected representative inorganic ceramic particles were used for the modification, including silicon dioxide (SiO₂), aluminium oxide (Al₂O₃), aluminium oxide hydroxide known as boehmite (AlOOH), titanium dioxide (TiO₂), cerium oxide (CeO₂), magnesium dioxide (MgO), nickel oxide (NiO), and zirconium dioxide (ZrO₂). Inorganic ceramic particles can significantly improve the thermal, mechanical, and electrochemical properties of modified separators through various strategies [60,61]. The hydrophilic nature of inorganic ceramic nanoparticles improves the separator’s affinity for the electrolyte, resulting in enhanced electrolyte uptake and ionic conductivity. In addition, the high melting temperature of inorganic ceramic nanoparticles and the adhesive nature of the binder can produce a coating layer that is thermally stable and offers superior heat resistance for the separator [59]. 5.1.1. Surface Coating Inorganic ceramic nanoparticles can be coated onto the surface of PE separators with binders to fabricate composite membranes named inorganic particle-coated composite separators. In general, the directly coated inorganic ceramic nanoparticles construct an organic-inorganic network structure layer on the surface of the PE substrate. Different coating methods include solution casting with a blade, deep coating via deposition, and sol-gel coating with composite gel polymers to introduce multifunctional and thermostable inorganic-organic particles. The inorganic particle-coated composite separators consist of a polymer substrate and ceramic layer, which combine the inorganic powder heat resistance and the polymer substrate characteristics. The coated ceramic layer helps protect the battery from high temperatures and maintains the stability of its structure. At the same time, the organic-inorganic network structure layer can enhance the separator electrolyte uptake to improve ionic conductivity. In addition, the inorganic ceramic particles need to be mixed with binders (such as PVDF) and additives before coating [62,63], and the coating percentage should be adjusted according to application scenarios [37]. The general process to develop the inorganic particle-coated composite separators is as follows: Firstly, disperse inorganic ceramic nanoparticles into a specific organic solvent, then add the specific polymer binder and additives to make a homogeneous solution by stirring, finally use different coating methods to build a coated separator using the resultant composite solution. In the end, dry it under a vacuum. Furthermore, inorganic aerogel can be used as another coating material because of its rich pores, high thermal insulation, and electrolyte affinity [37]. The inorganic particle-coated composite separators have the following characteristics: (1) The excellent mechanical strength and chemical stability from polyolefin separators; (2) The improved heat-resistant and thermal stability due to the coated ceramic layer; (3) The reasonably controlled cost for large-scale application due to the simple preparation process and cheap commercial materials [56]; and (4) The coated ceramic layer can enhance the wettability and electrolyte uptake of separator which may affect the Li ions diffusion [64]. Furthermore, many other factors affect the characteristics of inorganic particle-coated composite separators according to the various coating categories. There are different coating preparation methods to enhance the performance of inorganic particle-coated composite separators. Among these coating methods, the deep-coating process via deposition has been used to create a uniform coating layer with varying thicknesses. By combining a polyolefin matrix with a ceramic layer, most inorganic particle-coated composite separators exhibit excellent thermal stability, with the ceramic coating acting as a robust supporting layer. The most adaptable and wieldy employed ceramic particles are SiO₂ [65,66], Al₂O₃ [67,68], ZrO₂ [63], TiO₂ [69]. There are some other used ceramic particles with promising functionality and excellent comprehensive properties, including AlOOH, MgO, CeO₂, NiO, magnesium hydroxide Mg(OH)₂, calcium phosphate (Ca₃(PO₄)₂), and borate-based particles. It has been proved that a hybrid inorganic ceramic coating layer is an effective way (Figure 6A) [70,71]. However, specific difficulties that exist in inorganic particle-coated composite separators: (1) The inevitable thickness increase and porosity decrease of the separator after coating leads to higher battery internal resistance and decreased energy and power density. (2) The weak adhesion strength between the coated ceramic layer and the polymer matrix. (3) The readily chip off of the coated ceramic layer during charge-discharge cycles due to different expansion coefficients. Designing and preparing unique inorganic particle-coated composite separators to solve the above three problems is still a major challenge that requires unique strategies and multifunctional and thermally stable materials. 5.1.2. Surface Grafting Inorganic ceramic particles can also be grafted onto the surface of polymer separators using irradiation treatments to create composite membranes, known as inorganic nanoparticle-grafted composite separators. Inorganic or/and organic materials grafting on the surface of commercial PE-based separators to construct composite separators is another typical strategy to overcome the drawbacks mentioned before [56,78,79]. As the non-polar surface of polymer separators, the polymer separator surface must be activated by irradiation before grafting. Irradiation treatments are advanced and valuable in modifying the structure and performance of PE separators [72,74,80,81,82,83]. The irradiation treatments include plasma, γ-ray, electron beam, ultraviolet, and so on. Although the widely studied inorganic particle-coated composite separators have quite enough mechanical strength and thermal stability to ensure safety and prevent battery internal short circuits, the increased thickness and blocked porous structure would reduce the energy and power density of the battery. In addition, the uneven distribution and weak binding of the coated ceramic particles result in particle detachment during cell construction and operation [81]. Accordingly, without causing significant impairment of the polymer matrix, it is a potential modification method to prepare thermostable separators via surface grafting under appropriate low irradiation [81,84]. The free radicals generated by high-energy irradiation would trigger the polymer crosslinking reaction, which improves the separator's physiochemical properties [85]. As a primary superiority, the irradiated composite separators show stronger chemical bonding without any significant change in thickness [72,80,81]. The general process to develop the inorganic nanoparticle-grafted composite separators is as follows: Firstly, active the polymer separator surface by irradiation treatments, then graft the specific additives on the activated surface to make a middle layer in a homogeneous solution, finally graft the uniformly dispersed ceramic particles on the treated separator surface in the solution under stirring. In the end, dry it under a vacuum. The SiO₂-grafted PE separators show a much stronger adhesive strength (>2.5 N/cm, Figure 6B) [72]. Both ceramic particles’ physical coating and chemical grafting on PE separators generally could improve mechanical strength, thermal stability, and wettability, thereby enhancing electrochemical performance [80]. Although the surface grafting method effectively avoids some shortcomings of the surface coating, preparation conditions are relatively harsh, and it needs to be further improved in large-scale commercial preparation. 5.1.3. Blending as Filler Besides coating and grafting, inorganic ceramic particles can be mixed directly as a filler into the polymer matrix to fabricate composite membranes named inorganic particle-filled composite separators. Adding ceramic micro/nano-fillers in polymer separators like PE or PP to improve their mechanical strength, wettability, thermal stability, and ionic conductivity is another main strategy of inorganic material modification. Directly incorporating inorganic particles into the polymer matrix enhances the separator’s physical and chemical stability. At the same time, the improved electrolyte affinity of the separator would reduce the battery’s internal resistance and increase the ionic conductivity. In addition, the polymer matrix crystallinity decreases, and Lewis acid-base type interactions between the added inorganic nanoparticles and electrolyte solution result in the better overall performance of the inorganic particle-filled composite separators compared with the inorganic particle-coated composite separators. As an effective and straightforward method, blending inorganic nanoparticles directly into the PE matrix as a filler is used to produce composite separators. From the introduction of inorganic fillers to considering the trade-off between the basic parameters used to develop the ideal separator for high-performance lithium-ion batteries, the role of appropriately assigning certain characteristics is conducive to achieving the best balance between various functions [86]. The reduced crystallinity after adding inorganic nanofillers into PE matrixes can promote rapid migration of lithium ions [87]. Regardless of separator characteristics and types, based on their key parameters in batteries, the purpose of developing inorganic nanoparticle-filled composite separators can be classified into five categories: (i) enhancing Li ions diffusion, (ii) improving mechanical performance, (iii) increasing thermal stability, (iv) decreasing interfacial resistance, and (v) suppressing Li dendrite formation [86]. Mixing inorganic nanoparticles as a filler into the PE matrix is expected to improve inorganic material modification and overcome the limitations of inorganic nanoparticle-coated composite separators. In addition, it can combine the PE separator preparation and the inorganic ceramic NPs modification in one step, reducing the cost and time of preparation and post-treatment, and the method is an industrially scalable and universal preparation method [73]. Without additional post-modifications, Li and co-workers successfully manufactured a series of inorganic particle-filled composite separators through the scalable biaxial stretching technique using ultra-high molecular weight polyethylene (UHMWPE) and SiO₂ nanocomposite, named UHMWPE/SiO₂ nanocomposite separators (Figure 6(C1)) [73]. Among them, the UHMWPE/SiO₂ nanocomposite separator consisting of 80 wt% UHMWPE and 20 wt% SiO₂ has excellent thermal stability of MD (1.7%) and TD (1%) at 120 °C. In addition, without the pores blocking, the filled SiO₂ NPs improved the permeability, porosity, electrolyte uptake, and ionic conductivity, which lend to outstanding electrochemical performance. In another study [74], Li and co-workers developed another advanced nanocomposite separator using UHMWPE and SiO₂ NPs by combining an industrially scalable biaxial stretching process and an E.B cross-linking post-modification procedure (Figure 6(C2)), which shows significant commercialization possibility. On the premise of not sacrificing the separator’s microporous structure and increasing its thickness, combined with other methods to further improve the separator performance, it is an efficient modification way. Despite the excellent performance of inorganic particle-filled composite separators, the local non-uniformity caused by the polymerization of ceramic nanoparticles should not be ignored to improve its dispersion. 5.1.4. Self-Assembly In addition to the three conventional modification methods mentioned above, the continuous development of material modification techniques has led to the use of many new methods for preparing high-safety separators. Among these, layer-by-layer (LbL) self-assembly is considered the simplest, most versatile, and most promising method for preparing multilayer materials on polymer substrate surfaces, as it can combine multiple functional structures for specific applications [88]. Compared with other technologies, the unique advantage of LbL is that it can adjust the composition, surface properties, and coating thickness of multilayer structures at the molecular level, with adjustable structure and properties [89,90]. This modification method improves the wettability and electrochemical performance of the PE separator without affecting the micropore structure [91]. Given the inherent advantages of the LbL method, many researchers have used LbL self-assembly to prepare organic-inorganic hybrid multilayer-modified PE separators. For instance, Yuan and co-workers developed a unique polyhedral oligomeric silsesquioxane (POSS)/ZrO₂ multilayer on PE separators surface by LbL self-assembly (Figure 6D) [75]. The POSS/ZrO₂ multilayer on PE separators improves ionic conductivity, Li+ transference number, and Li+/electrolyte interfacial stability.These favorable properties lead to excellent electrochemical performance and high safety. Xu et al. also used self-assembly to build ultrathin poly(acrylic acid) (PAA)/ZrO₂ multilayer on the surface of PE separators for LIBs [88]. Zhu and co-authors used the oppositely charged polyethyleneimine (PEI) and inorganic SiO₂ NPs to assemble an ultrathin layer on the surface of the PE separator by LbL for LIBs [92]. Most modified PE separators by LbL self-assembly show an improved hydrophilic nature, greatly increased electrolyte uptake, and excellent electrochemical stability [93]. 5.1.5. Atomic Layer Deposition Atomic Layer Deposition (ALD) is a film preparation process using vapor deposition, which has the characteristics of self-limiting and saturated surface reaction and is an ideal surface chemical modification and energy-related material preparation technology, which is widely used in the manufacture of supercapacitors, solar cells, fuel cells and rechargeable batteries [94,95]. ALD is essentially a thin-film coating deposition technique that typically requires alternating feeding of two precursors into a vacuum deposition chamber. The vacuum chamber holds the substrate that needs to be modified and is preheated to accelerate the chemical process of film deposition [96]. In general, the ALD process to modify the separator is as follows: sequentially expose the gas-phase precursors to the substrate (polymer separator), trigger the self-limited and saturated surface reactions, resulting in a modified separator after a certain time. Several reviews have reported the unique growth mechanism of ALD using dual compounds, involving a two-cycle, self-terminating half-reaction via ligand exchange[96,97,98]. Figure 6E shows the general mode of the ALD growth mechanism, which assumes that water serves as an oxygen precursor for metal oxide (MO) and that ML₂ (M = metal and L = ligand) serves as a metal precursor [99]. This reaction mechanism provides several advantages for inorganic films deposited with ALD, including precise angstrom thickness control, excellent uniformity and consistency, adjustable composition, and relatively low deposition temperature [95,100]. These ALD thin films can be used as surface coatings for cathodes and anodes, on the one hand, to solve structural and interface problems and on the other hand, to prepare modified separators with a uniform surface distribution of nano-fillers. In addition, the nucleation sites on the substrate surface can be controlled by pre-set functional groups, allowing for more precise deposition of nanoparticles via ALD. This is because functional groups at nucleation sites, such as hydroxyl groups, defect sites, and heteroatom doping sites, are prerequisites for ALD surface reactions to occur [94,95]. ALD has been investigated for a few decades and made remarkable progress. Through material nanostructuring, crystallization optimization, and angstrom-scale size control, ALD can significantly enhance battery performance and facilitate the discovery of new materials. For example, using titanium isopropoxide and water as precursors, Chao et al. developed the TiO₂ layer uniformly coated PE separator by a Roll-to-roll ALD process for LIBs (Figure 6E) [76]. Due to the self-limiting nature of the reaction, the thickness of the TiO₂ layer increases with the number of ALD cycles. Linear speed plays a key role in engineering and designing the TiO2 gravimetric loading. The uniform deposited TiO₂ ceramic nanolayers significantly improve thermal and dimensional stability, resulting in high safety during high-temperature operation. In addition, Park’s group proposed a hybrid organic-inorganic coating strategy using ALD, which enhances the thermal and dimensional stability of PE battery separators without increasing their thickness [101]. By first subjecting PE separators to a plasma treatment, the ALD approach uniformly coated a few-nanometer-thick inorganic Al₂O₃ NPs layer on the PE separator and followed by PDA coated layer through the dip-coating process. 5.1.6. Chemical Vapor Deposition Chemical vapor deposition (CVD) is a powerful surface modification technique, including plasma polymerization, molecular layer deposition, oxidative chemical vapor deposition (oCVD), and initial chemical vapor deposition (iCVD). The chemical vapor deposition technique allows the formation of conformal polymer coatings at the micro and/or nanoscale, which cannot be achieved by solution treatment due to dehumidification, liquid thinning, and surface tension effects [102]. The iCVD technique uses vaporized monomers and initiators for radical polymerization in the vapor phase. The general polymerization mechanism of the iCVD process is as follows: vaporized monomers and initiator molecules are introduced and mixed; free radicals are generated through initiator decomposition, which then transfer to the surface of the substrate along with the reactive monomers. These radicals polymerize, forming a solid thin film on the cooled substrate[103]. The vaporized initiator molecules thermally disintegrated and produced radicals after touching a hot filament, then the radicals activated the vaporized monomer to start the chain reaction, and resulted in the polymer film deposition on the chamber substrate kept at a mild temperature (15~40 °C). This is a one-step, solvent-free process in which the deposition of polymer films can maintain uniform morphology on multiple complex surfaces of the substrate [104]. The reaction consists of three steps: initiation, propagation, and termination, the number of free radicals in these three stages first rises, then maintains, and finally declines [105]. Typically, chain reactions are terminated by radical-radical recombination or by transfer of reactive radicals [106]. CVD polymerization directly polymerizes gas-phase monomers into large molecular chains bound to the substrate surface, making it the preferred method for manufacturing specific polymers and substrates. Due to the low surface temperature of the substrate, polymers can form modified layers directly on fragile things such as thin paper and porous polymer membranes through CVD. Yoo et al. [77] applied the iCVD technique for modifying PE separators to increase the separator’s thermal stability and other performance. The highly cross-linked polyhexavinyldisiloxane (pHVDS) coated on the PE separator surface through iCVD process with enhanced thermal stability at 140 °C and basically unchanged microporosity and thickness (Figure 6F). The strength of the binding force between the deposition layer and the substrate is the direction that needs to be improved. 5.2. Organic Material Modification Aimed at the high safety separator, to improve the thermal stability, mechanical properties, and electrochemical performance, the composite separators have been done by using inorganic or/and organic material for modification through coating/grafting, blending, polymer functionalization or other methods [107,108]. These methods all have their advantages and disadvantages. Separators can be divided into single layer, multi-layers or composite separators depending on the processing technique and material components [109,110,111]. As commercial battery separators consist of polyolefins with inferior thermal stability and low electrolyte wettability [57,112], and some organic material (PDA, PAN, PI, PVDF-HFP, etc.) with higher melting points and better mechanical properties can be used to enhance thermal stability, to prepare high safety separator through inorganic material modification is considered another most actively researched procedures and has the commercial application prospect. 5.2.1. Surface Coating/Grafting Besides inorganic material modification, another typical tactic is constructing composite separators by coating/grafting organic materials on the commercial PE-based separators’ surface [56]. Organic compounds (PDA, PI, PVDF-HFP, etc.) canbe synergistically coated on the PP and PE surface to improve the polyolefin-based separators thermal stability. The general process to develop the organic material-modified separators through surface coating/grafting is similar to the inorganic material. During grafting, the polymer separator’s electric properties and structure would change or destroy after high-energy radiation because of the thermal properties. To solve this problem, some chemical groups can be grafted by ionizing radiation, ultraviolet radiation, and chemical initiator onto the polymer surface to enhance the electrolyte wettability and electrode compatibility [79]. With the consideration of the flammability of polyolefin-based materials, there is still a risk of separator burning. Therefore, using other flameretardant materials to achieve the flame-retardant property of the separator is necessary. Li and co-workers fabricated a sandwich-like composite separators by coating a porous polybenzimidazole (PBI) layer on both sides of a PE separator through the phase inversion process (Figure 7A) [113]. The modified composite separator (the PBI/PE/PBI referred to as “PBIE”) shows greatly improved thermal stability with no thermal shrinkage up to 200 °C and obtained non-inflammability from the PBI intrinsic flame retardancy, and the shutdown function at 140 °C from melting the PE. The modified composite separator has high safety and outstanding electrochemical performance simultaneously. Another study describes the preparation process of an HDPE wax@AO-coated composite separator with shutdown performance. [114]. Figure 7B shows that free radicals generated under high-energy irradiation initiate the polymer chain crosslinking reaction, significantly improving the separator's mechanical strength [38,115]. Besides, several studies have reported the fabrication of high-performance MOFs-based composite separators for batteries with different functionalities [31,32]. 5.2.2. Blending Blending is another efficient way to prepare organic material modification composite separators. Blending refers to the preparation technology in which one polymer with good properties is used as the substrate to physically mix with another polymer or multiple polymers with complementary properties to improve the comprehensive properties of composite materials. The complementary properties and interactions between different polymers provide great possibilities for improving the overall performance of the modified separator. Blending is greatly beneficial since it combines the advantages of each component to improve the overall performance. However, as each component usually shows a huge difference in physicochemical properties, it is vital to select a suitable solvent for all components and determine the weight ratio of each component for preparing high-safety separators of Li-ion batteries. Blending polymers is an easy and cost-effective method to improve separator performance, depending on the materials and processing conditions. It can be divided into two categories: blending stretching and blending electrospinning. Blending PP/PE [116], PVDF-HFP/PE [117], PVDF-TrFP/PE (poly(vinylidene fluoride hexafluoropropylene) [118,119,120,121], HDPE/MC (polyethylene/methylcellulose) [122] and electrospun PVDF/PAN [119] separators show the enhanced thermal stability and good electrochemical performance. Many studies have focused on balancing the separator’s microporous structure and its stability[52,123,124,125,126,127,128,129]. Pore volume affects the crystallinity of the PVDF separator [130], the ordered porous structure improves the charge/discharge capacity [131] and hinders dendrite growth to enhance battery safety [132,133]. For the blending stretching, the stretched polymer chains widely accepted thermal shrinkage [134]. Li and co-authors developed a suitable microporous structure of ultra-high molecular weight polyethylene/poly(4-methyl-1-pentene) (UHMWPE/PMP) blending separator via sequential biaxial stretching. The UHMWPE/PMP separator shows a lower thermal shrinkage of 0.7% in a transverse direction and 1.6% in the machine direction at 120 °C (Figure 7C) [135]. 5.3. New Separator Material Innovative ideas for overcoming the above challenges of commercial separators include modified separators and engineered designing of new materials. Some new organic materials are used as raw materials to process into nonwoven separators by electrospinning technology or other technology, such as PI, PVDF-HFP, PAN, PMIA, PVP, PTFE, PEI, PBI and PEEK, etc. Nonwoven separators typically exhibit a porous structure, good electrolyte affinity, and high thermal stability, compared to polyolefin separators. More importantly, some heat-resistant or flameretardant polymers, such as PVDF, PVDF-HFP, PI, PTFE, PBI, PEEK, and other inorganic materials, can make the separator exhibit excellent heat-resistant and flame-retardant properties. Among many organic polymers, with melting points of about 165 °C and good electrochemical spinning ability, PVDF is a popular candidate for nonwoven separators [56,136]. Besides PVDF, Li et al. prepared a series of fluorinated PEEK polymers and first fabricated them into nonwoven separators with excellent anti-shrinkage performance and negligible thermal shrinkage at 150 °C (Figure 8A) [137]. For excellent electrochemical performance in high-temperature environments, Polyimide (PI) materials are chosen as a new raw material with superior heat resistance to prepare high-safety separators. Using the electrospinning and thermal imidization processes, Park and co-workers developed a novel PI nonwoven separator with excellent thermal stability (Figure 8B)[138]. Yao and co-workers reported a polytetrafluoroethylene (PTFE) nanofiber separator prepared using the electrospinning process[139]. Polybenzimidazole (PBI) polymer has excellent electrolyte wettability and outstanding thermal dimensional stability, thus showing great potential as a separators for high-performance and high-safety batteries. Li et al. successfully prepared a novel and highly symmetrical spongy porous PBI separator to ensure battery safety with ultra-high thermal stability using the water-vapor induced phase separation (Figure 8C) [140]. Liu et al. investigated the electrospinning OPBI separators on battery safety and the inhibitory effect of fiber OPBI separators on the growth of lithium dendrites [141]. Tuo and co-workers reported an aramid nanofiber membrane fabricated by vacuum-assisted filtration of the nanofiber dispersion as a separator. [142]. Liu et al. constructed PEI porous membrane separator with a high curvature, three-dimensional heat-resistance skeleton. The well-adjusted three-dimensional multi-porous skeleton greatly improves the ion transport capacity and provides good short-circuit protection [143]. Besides synthetic polymers, natural polymer materials have also been used to prepare separators [144,145,146]. As the most abundant natural polymer on Earth, cellulose has the advantages of being cheap, environmentally friendly, renewable, and easy to obtain. The cellulose structure contains abundant hydroxyl functional groups, which can be chemically modified. At the same time, its higher porosity can improve the electrolyte uptake of the separator, which is the most potential substitute for polyolefin separator [147]. Compared with ordinary cellulose, nanocellulose has higher crystallinity and mechanical strength, thereby preventing battery short circuit problems caused by lithium dendrites. In addition, combined with inorganic material modification, the thermal properties and flame retardant properties of nonwoven composites will be further improved, such as SiO₂@(PI/SiO₂), PVDF-CA/Al(OH)₃, ZrO₂ fiber/PVDF-HFP. Recently, a novel SiC fiber-supported alumina separator (A@S ceramic separator) was reported by solution casting [148]. Yao and co-workers designed a kind of heatproof–fireproof bifunctional separator by coating ammonium polyphosphate (APP) particles on a ceramic-coated separator modified with phenol-formaldehyde resin (CCS@PFR). The PPA layer not only isolates the combustible material from the highly reactive oxygen species released by the cathode but also converts the violent combustion reaction into a mild, gradual exothermic reaction by carbonizing the combustible material in the battery [149]. Furthermore, the ceramic membrane is another choice for the high safety separator. Liu et al. reported a nonwoven ZrO₂ ceramic membrane with a robust nanofiber microstructure via polymeric electrospinning followed by a high-temperature organic burn-off. The ZrO₂ separator can withstand higher current densities and have longer cycling lives than the state-of-the-art separators, and it is regarded as a promising alternative separator for high-power batteries [150]. For the safety of high energy/power density LIBs during their widespread practical applications, it is extremely important to systematically study high-safety separators, which remains under-researched. The heat-resistant/flame-retardant properties, mechanical strength, and puncture resistance to prevent metal dendrites are the most important research topics for high-safety separators. These properties can be achieved through inorganic/organic material modifications or by using new materials. In addition, with the rapid development of power vehicles and energy storage power stations, high-specific energy batteries available for use require higher battery capacity, lighter weight, and smaller component volume. Therefore, the demand and application prospects of ultra-thin and ultra-light separators are broad. However, the ultra-thin separators have poor mechanical strength and puncture strength to prevent metal dendrite, which would cause safety problems when used at low temperatures and high discharge state. The more serious safety issues of high specific energy batteries put forward higher requirements for high safety separators, which are summarized as follows: (i) Thermal properties: excellent thermal stability to prevent thermal shrinkage and special flame retardant from preventing thermal runaway; (ii) Mechanical properties: high tensile strength and strong puncture strength to prevent separator damage leading to the battery short circuit; (iii) Structure: optimized separator porosity, pore structure, and composition are designed and implemented to induce uniform metal deposition and further avoid the excessive growth of metal dendrites; (iv) Compatiblity: outstanding adaptability and excellent stability of separators to other battery components are required to reduce the battery internal resistance and prevent from generating excessive heat at high current density. Meeting the separator performance requirements for both high energy density and high safety in batteries remains a significant challenge [56]. Based on extensive research of high safety separators, the current main challenge is performance balance, which means it the difficult to possess all the desired performance parameters simultaneously. And The performance of a high safety separator is closely related to its preparation method. Each separator preparation method has its own advantages and disadvantages, and different materials are selective in the preparation methods that can be used. As the transport path for lithium ions, the micropores of the separator have a significant impact on battery performance. It is possible to prepare a high-safety separator with excellent performance only by selecting suitable materials and optimizing the preparation process and process parameters through suitable preparation methods and modification methods (Table 2). 5.4. Electrolyte Membrane Separators Electrolyte membrane separators, including ceramic-based separators or solid-state electrolytes (SSEs), are the future of separators that can address the root cause of battery thermal runaway. These new solid-state electrolytes, with enhanced mechanical resistance, are sandwiched between both electrodes. The main strategy for developing solid-state electrolytes is to create new electrolyte systems that are safer, lighter, and non-flammable, replacing traditional liquid electrolytes and effectively improving the battery's resistance to dendrite formation. All-solid-state batteries (SSBs) are expected to have higher storage capacities and enhanced safety by using solid-state electrolytes as the solid ion conductor [157]. SSBs are a safer battery product than traditional batteries but costly and difficult to custom design [158], and there is still some distance from commercialization. The use of solid-state electrolytes instead of traditional liquid electrolytes opens up new possibilities for the development of flexible LIBs. All-solid-state batteries (SSBs) can withstand larger mechanical deformations without electrolyte leakage or short circuits. Based on the material system, solid-state electrolytes (SSEs) can be classified into five categories: solid polymer electrolytes (SPEs), gel polymer electrolytes (GPEs), solid ceramic electrolytes (SCEs), composite polymer electrolytes (CPEs), and hybrid solid-state electrolytes (HSSEs). Solid polymer electrolytes (SPEs) are ion-conducting polymer materials with both electrolytes and separator properties [159]. SPEs comprise organic polymer matrices and dissolved Li⁺/Na⁺ salts, ionic liquids, or other inorganic materials. The optional organic polymer matrix materials include cellulose derivatives, PEO, PMMA, PVDF, PVDF-HFP, and PAN. Common organic solvents are ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) [160,161]. The SPEs combine the advantages of polymer’s flexibility, good contact with the electrode, and high ionic conductivity of the inorganic solid electrolyte, greatly improving its comprehensive performance. The uniform dispersion of inorganic ceramic particles in SPEs helps to prevent the ordered aggregation of polymer chain segments, reduce the crystallization degree of polymer, increase the amorphous region, and benefit Li⁺ migration, thereby improving the overall ionic conductivity. Some inorganic fillers in the SPEs can also improve the stability of the interface. Gel polymer electrolytes (GPEs) are a gel system composed of a polymer matrix with electron-donating groups, lithium salt (LiPF₆), and low molecular weight organic solvent (plasticizer) [161]. The optional organic polymer matrix materials include PVDF, PEO, PAN, and PMMA. Common organic solvents are the same as SPEs. GPEs combine the advantages of liquid and solid polymeric electrolytes. The polymer forms a complex with metal cations, and its complexation degree, existence form, and state have a crucial influence on the performance of GPEs. According to the effective medium theory, the main factors affecting the electrical conductivity and mechanical properties of GPEs are temperature and composition. As the working temperature increases, the crystalline phase in GPEs decreases, the continuity of the amorphous phase increases, and the flexibility of polymer molecules increases, thus making the conduction of lithium ions easier and the ionic conductivity increases. However, due to the uncontrollable operating temperature of LIBs, the current research generally regulates the performance of GPEs by changing their composition. According to the preparation process and the bonding state between the polymer and the solvent, GPEs can be divided into two categories [162]: (1) physical GPEs and (2) chemical GPEs. Mechanical strength and conductivity are a pair of contradictions in GPEs. By increasing the polymer/solvent ratio, the mechanical strength can be improved, though the conductivity is reduced. At the same time, although adding a large amount of liquid plasticizers improves the ionic conductivity of GPEs, it sacrifices the chemical stability of the polymer matrix and reduces the safety and service life of the GPEs. This contradiction makes it difficult for the currently studied GPEs to match high-performance LIBs, and the research on GPEs is still in the laboratory stage. In addition, due to the existence of liquid electrolytes, the conduction mechanism of GPEs cannot be explained by the mechanism of explaining the conduction of SPEs. Solid ceramic electrolytes (SCEs), which are inorganic solid electrolytes (typically oxide or sulfide solid electrolytes), are usually single Li+ ion conductors. Oxide solid electrolyte fillers mainly include NASICON, perovskite, and garnet structures. Among them, NASICON structure electrolytes and garnet-type electrolytes are the most widely used with stable structure, excellent performance, high conductivity at room temperature (~10⁻³ S/cm), and stability for lithium metal. Sulfide solid electrolyte has good mechanical ductility, excellent interface contact performance, and higher ionic conductivity at room temperature (~10⁻² S/cm), but it has poor stability and low safety against lithium metal. Despite the advantages of SCEs, their practical application is still limited by their mechanical rigidity and fragility, which result in poor interface contact and complex and difficult material processing [163]. In contrast, polymer-based SSEs generally have excellent processability and flexibility. Therefore, it is easy to make thin structures and flexible films using polymer-based SSEs with better interfacial contact between the soft polymer and the electrode, resulting in lower interface resistance [164]. Composite polymer electrolytes (CPEs)/hybrid solid-state electrolytes (HSSEs) are generally obtained by the combination of high-surface-area inorganic filler and SPEs or GPEs CPEs combine the advantages of SPEs or GPEs and inorganic solid electrolyte to achieve high lithium-ion conductivity, thermal stability, mechanical strength, and electrochemical stability, and have become one of the current research hotspots [165,166]. The solid electrolyte obtained by adding inorganic filler to the polymer solid electrolyte has excellent comprehensive properties. These inorganic filler particles are ceramic metal oxide with the size of micrometers or nanometers [118]. The inorganic filler can play three roles: (1) Reduce the crystallinity and increase the amorphous phase region, which is conducive to Li⁺ migration; (2) A fast Li⁺ channel can be formed near the packing particles; (3) Increase the mechanical properties of the polymer matrix, making it easy to form films. According to the ability of inorganic fillers to conduct ions, inorganic fillers can be divided into inert and active fillers. Inert fillers do not transport lithium ions, mainly silica, alumina, and zirconia. Active materials can participate in the ion conduction process, mainly oxide solid electrolyte filler and sulfide solid electrolyte filler. The polymer matrix can significantly improve flexibility, reduce the resistance at the electrode-electrolyte interface, and lower the large-scale manufacturing costs of the solid composite electrolyte. It provides an important idea for designing solid-state batteries that meet the requirements of high energy density and high safety. HSSEs show great potential further to combine the properties of inorganic and polymer electrolytes while overcoming the disadvantages of each component when used alone [165]. However, there is still a series of technical problems that need to be solved. The microscopic mechanism of organic/inorganic composite electrolytes, inorganic fillers, and polymer materials is still unclear, and the challenge of evenly dispersing inorganic particles in the polymer matrix remains unsolved. Obtaining CPEs/HSSEs with good overall performance is an important prerequisite for Realizing the application of solid-state batteries requires the design of CPEs/HSSEs with high ionic conductivity, a wide electrochemical window, high mechanical strength, and consideration of interface contact and compatibility. This is the focus of current research. Currently, solid-state batteries are rapidly advancing, with solid-state electrolytes offering enhanced safety and higher energy density. They may also serve as a substitute for separators in the future. But, electrolyte membrane separators for solid-state batteries are not the main content of this review. Readers are encouraged to refer to the recent review works [86,162,163,164,167,168,169] for more detailed information.