I. Applications of diaphragms
It isolates the positive and negative electrodes and prevents electrons from passing freely; it allows ions in the electrolyte to pass freely between the positive and negative electrodes. Its performance determines the battery's interface structure, internal resistance, etc., and directly affects the battery's capacity, cycle life, and safety performance.
II. Diaphragm Characteristics
The separator material must possess good insulation properties, good affinity for the electrolyte, good temperature resistance and wettability, and good electrolyte retention. The separator prevents short circuits caused by contact between the positive and negative electrodes or by punctures from burrs, particles, lithium dendrites, etc. The separator must have high tensile and puncture strength, be resistant to tearing, and be stable against thermal shrinkage at high temperatures, preventing short circuits and thermal runaway caused by thermal shrinkage.
In cases of overcharging or elevated temperature, the separator limits the increase in current, preventing short circuits and potential explosions. By sealing the pores, it blocks current conduction within the battery, providing micropore self-sealing protection and safeguarding users and equipment. The separator must have high porosity and uniform micropore distribution. The inherent properties of the material and the porosity characteristics after film formation constrain lithium-ion migration within the battery, i.e., high ionic conductivity.
III. Diaphragm Classification
Based on the structural characteristics and production technology of lithium battery separators, they can be classified into microporous polyolefin membranes, modified polyolefin membranes, non-woven membranes, coated composite membranes, nanofiber membranes, and solid electrolyte membranes.
1. Microporous polyolefin membrane
Through continuous technological updates and practical applications, polyolefin microporous membranes have become the best-performing and most industrially available lithium-ion battery separators. Depending on the production process, they can be divided into single-layer and multi-layer membranes, namely polypropylene (PP) single-layer membranes, polyethylene (PE) single-layer membranes, and PP/PE/PP three-layer composite membranes. Polyolefin microporous membranes, represented by polypropylene (PP) and polyethylene (PE), possess superior performance, good chemical stability, and low cost, and therefore dominate the lithium-ion battery separator market.
2. Modified polyolefin membrane
PE and PP membranes exhibit poor affinity, temperature resistance, and wettability for electrolytes. The current trend in high-performance membrane fabrication involves adding or compounding materials with hydrophilic and high-temperature resistance properties to single-layer polyolefin membranes, grafting hydrophilic monomers onto the surface of PE or PP microporous membranes, or altering the organic solvent in the electrolyte. Processes include coating, dip coating, spraying, and lamination to obtain high-performance composite membranes.
SONG et al. coated a PE separator with a polyarylate material exhibiting good heat resistance, forming a composite separator with a porous polymer melting temperature as high as 180℃. Cheng Hu et al. coated a Celgard 2400 monolayer PP membrane with polyoxyethylene doped with nano-SiO2, improving the wettability of the separator and significantly enhancing its cycle performance. RYOU et al. coated a PE separator with dopamine using a dip-coating method, obtaining a modified separator with higher electrolyte adsorption performance, effectively improving the high-rate cycle performance of the separator. KIM et al. modified a polyolefin separator with a mixture of PVDF/SiO2, obtaining a composite separator possessing the electrolyte-loving properties of PVDF and the high-temperature resistance of SiO2, achieving a charge-discharge efficiency of 94% at a 2C discharge rate. FANG et al. modified a PP membrane using a polyethylene glycol-grafted polydopamine coating, resulting in increased liquid absorption and reduced interfacial resistance.
3. Non-woven fabric diaphragm
Compared to polyolefin membranes, nonwoven membranes offer superior thermal dimensional stability, safety, wettability, and porosity. Nonwoven materials are typically prepared by orienting or randomly arranging specialized fibers into a mesh structure, which is then reinforced through mechanical, thermal bonding, or chemical cross-linking methods.
Fibers include natural and synthetic fiber materials, such as natural cellulose and its derivatives, synthetic polyolefin fibers, polyamide (PA) fibers, and polyethylene terephthalate (PET) fibers; nonwoven membranes possess good mechanical properties and high melting temperatures, and maintain dimensional stability well during use. Zhang Song et al. prepared a BC/TiO2 composite membrane with polarity, porosity, and good thermal stability by combining bacterial cellulose nanofibers and nano-TiO2 particles, which improved ionic conductivity and battery cycle performance.
4 nanofiber membrane
MIAO et al. prepared nanofiber membranes with extremely high thermal stability using polyimide as a raw material. These membranes exhibited no thermal shrinkage at 250°C, and their 10C discharge capacity was 60% of that at 0.2C, significantly higher than the discharge capacity of polyolefin membranes. JUNG et al. fabricated a battery with an electrochemical stability window of 4.7V using a PMMA/PVC composite fiber membrane. In a lithium-ion half-cell system, the capacity showed almost no decay after 100 cycles at 0.5C.
5-coating composite film
Nonwoven membranes are relatively thick, with large and uneven pore sizes, resulting in poor tensile mechanical strength. Coated composite membranes are typically fabricated using transfer coating or impregnation methods to improve their overall performance. Composite membranes are multilayer membranes that use dry-process, wet-process, or nonwoven fabrics as the substrate, coating it with layers of inorganic ceramic particles or composite polymers.
Based on the different components of the coating, it can be divided into four types: organic coating composite film, inorganic coating composite film, organic/inorganic hybrid coating composite film, and in-situ composite film.
Inorganic coated inorganic composite membranes, also known as ceramic membranes, are porous membranes composed of a small amount of binder and inorganic particles. Inorganic composite membranes possess good flexibility, high mechanical strength, high thermal stability, excellent high-temperature resistance, and excellent electrolyte wetting and adsorption properties. Some membrane companies have already industrialized these membranes. Ceramic materials have high thermal resistance, which can prevent the expansion of thermal runaway at high temperatures and improve the thermal stability of the battery.
Al2O3 Surface Coating Series: Yang Baoquan used polyethylene (PE) wet-process membrane as the substrate and uniformly coated both sides with Al2O3 particles to obtain a composite-coated PE lithium battery separator, which significantly improved the thermal safety performance, ionic conductivity, and cycle performance of lithium batteries. JEONG et al. used atomic layer deposition technology to deposit an Al2O3 ceramic layer with a thickness of approximately 6 nm on the surface of a PP microporous membrane, effectively improving the heat resistance and hydrophilicity of the PP base membrane. X. Huang prepared a composite separator by mixing fibers and Al2O3, and then coated it with a PVDF membrane using a dip-coating method. The resulting composite separator exhibited stable cycle performance and almost no shrinkage at 250℃. J. Lee et al. studied the surface coating of polyimide membranes with Al2O3/PVDF-HFP, which improved the wettability of the separator and delayed the increase in battery impedance.
Surface-coated SiO2 series: YOO et al. used a coating process to coat nano-SiO2 onto PE membranes, obtaining ceramicized PE membranes with a SiO2 layer, increasing the heat resistance temperature to 170℃ (PE 135℃). HSJeong et al. studied the effect of different SiO2 particle sizes on the performance of composite membranes, finding that the composite membrane prepared with 40nm SiO2 had the highest porosity, and the SiO2 did not dissolve after 200 cycles. Yang Yunxia's team at East China University of Science and Technology coated PE membranes with boehmite monohydrate; the treated membrane showed almost no thermal shrinkage at 140℃, and a thermal shrinkage of <3% after 0.5h treatment at 180℃, significantly improving the thermal stability of the membrane. A TiO2/BaTiO3 composite membrane was obtained by coating a uniformly mixed slurry onto the surface of a base membrane using specific machines or equipment.
Organic coatings, like inorganic coatings, suffer from drawbacks such as severe pore blockage and high ion transfer resistance, affecting the membrane's wettability with the electrolyte and the battery's cycle performance. To address these issues, researchers have explored using polymer nanoparticles, polymer fibers, PVDF, PAN, PMMA, and PEO as coating materials to replace traditional dense coatings. The resulting high-porosity nanoporous structure aims to improve the membrane's wettability with the electrolyte and enhance the battery's ionic conductivity.
The team led by Hu Jiwen at the Chinese Academy of Sciences used a multiple impregnation method to coat aramid fibers (ANF) onto the surface of PP membranes. The coated membranes exhibited good dimensional stability and significantly improved rate and cycle performance.
Organic/inorganic composite coatings for separators involve mixing inorganic nanoparticles and organic polymers, and then coating the resulting homogeneous slurry onto a separator substrate. The research group of Li Weishan at South China Normal University prepared composite separators by coating the surface of PE separators with a quaternary polymer P(MMA-BA-AN-St) incorporating CeO2 ceramic particles. The effects of different ceramic contents (0%, 10%, 50%, 100%, 150%, and 200%) on electrolyte retention and ionic conductivity were compared, with approximately 50% ceramic content showing the optimal effect.
In-situ composite membranes are produced by pre-dispersing ceramic particles or polymer fibers into a film-forming slurry, followed by wet biaxial stretching or electrospinning to form a membrane. Compared to organic or inorganic coatings, in-situ composite membranes solve the problem of coating peeling off the surface, forming a uniform open-pore structure.
Donghua University of Technology proposed using a vacuum filtration method to add ceramic nanoparticles to electrospun PVDF/PAN membranes. The resulting composite membrane had a ceramic loading of 67.5%, uniform ceramic particle distribution, and excellent comprehensive performance.
6 Solid electrolyte membrane
Traditional lithium batteries use volatile organic electrolytes, which pose safety hazards. All-solid-state lithium batteries use solid electrolytes (mainly inorganic electrolytes and polymer electrolytes), which are safer.
Inorganic solid electrolytes include crystalline and amorphous forms. Currently, LiPON electrolytes and sulfide electrolytes show promising prospects for practical applications. These electrolyte materials are generally prepared using sputtering or powder sintering processes. LI et al. prepared a thin-film battery with a structure of Pt/LiCoO2/LiPON/SnxNy/Pt and a thickness of only 7.6 μm using sputtering. The battery exhibited high capacity retention at ≤150℃ and good high-temperature performance, with a discharge capacity at 150℃ being 87% of that at 20℃.
Polymer electrolytes are ion-conducting composite systems composed of polymers and lithium salts. In recent years, they have been mainly classified into four categories: all-solid-state polymer electrolytes, gel electrolytes, microporous gel polymer electrolytes, and composite polymer electrolytes. All-solid-state polymer electrolytes (SPEs) are composed of a polymer that enables lithium salt dissolution and ion migration, combined with lithium salts.
IV. Diaphragm Preparation
The main lithium battery separator manufacturing processes on the market are divided into two categories: dry process and wet process. The dry process (melt-draw process) and the wet process (thermal phase separation process) have different pore formation mechanisms.
1. Dry process
The dry process involves melting, extruding, and blowing polyolefin resin into a crystalline polymer film. After crystallization and annealing, a highly oriented multilayer structure is obtained. Further stretching at high temperature peels off the crystalline interface, forming a porous structure, which can increase the pore size of the film.
Dry stretching can be divided into dry uniaxial stretching and biaxial stretching based on the stretching direction. The core patents for uniaxial stretching are primarily held by companies in the United States and Japan; the Institute of Chemistry, Chinese Academy of Sciences, holds domestic patents for biaxially stretched PP. The dry uniaxial stretching process prepares films by first stretching at low temperatures to create defects such as silver streaks, and then stretching these defects at high temperatures to form micropores. Currently, Celgard Corporation in the United States and Ube Industries in Japan both use this process to produce single-layer PE, PP, and three-layer PP/PE/PP composite films.
The separator produced by this process has a flat, elongated microporous structure. Due to uniaxial stretching, the lateral strength of the separator is relatively poor, but there is almost no lateral thermal shrinkage. The product is thicker than the separator produced by the wet process and is prone to longitudinal tearing. Compared with uniaxial stretching, the dry biaxial stretching process improves the lateral strength. Moreover, the lateral and longitudinal stretching ratios can be appropriately adjusted to obtain the desired performance according to the strength requirements of the separator. Furthermore, the micropore size of biaxially stretched separators is more uniform, resulting in better air permeability. The dry stretching process is relatively simple and pollution-free, making it a commonly used method for preparing lithium battery separators. However, this process has drawbacks, including difficulty in controlling pore size and porosity, a smaller stretching ratio, and a tendency for separator perforation during low-temperature stretching, resulting in a thicker product.
2 Wet process
The wet process, also known as the phase separation method or thermally induced phase separation method, involves mixing liquid hydrocarbons or some small molecules with polyolefin resin, heating and melting them to form a homogeneous mixture, then cooling to separate the phases and pressing the mixture to obtain a membrane. The membrane is then heated to near its melting point and subjected to biaxial stretching to orient the molecular chains. After holding at this temperature, solvent extraction is used to form micropores, thus preparing a microporous membrane material. Companies such as Asahi Kasei, Tonen, and Nitto in Japan, and Entek in the United States use this method to produce single-layer PE battery separators.
Separators produced using the wet biaxial stretching method exhibit uniform pore dispersion, good wettability to electrolytes, are isotropic, have high transverse tensile strength and high puncture strength, and are not prone to perforation or tearing under normal process conditions. This allows for thinner products and higher battery energy density. In China, PP separators are primarily used in power and energy storage batteries, while PE separators are mainly used in 3C batteries. Considering both cost and technology, the dry process will dominate the domestic power separator market in the short term, but in the long run, the wet process is the mainstream technological trend.
3. Electrospinning process
Electrospinning can produce uniform fibers and fibrous felt-like materials with small pore sizes, high specific surface areas, and high porosity. Fiber diameters range from tens to thousands of nanometers, and the fiber diameter affects the membrane pore size. Electrospinning technology involves uniformly mixing polymers and ceramic materials to form a slurry, which is then used to prepare ceramic membranes using electrospinning equipment. Ceramic particles are embedded in the fibers, significantly improving the membrane's thermal stability and electrolyte wettability. Zhang Zihao reviews the techniques for preparing single polymer membranes, modified multi-polymer membranes, and organic/inorganic composite membranes using electrospinning nanofiber membrane substrates. Important materials include PVDF, PA, PET, polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and polyethersulfone ketone (PPESK).
4. Wet papermaking process
Wet papermaking is a common method for manufacturing diaphragm materials. Short, fine fibers and a binder are mixed and dispersed in a slurry, which is then transferred onto a carrier using transfer coating. Finally, the diaphragm is obtained through dehydration/solventizing, drying, and winding. Zhang et al. used a wet papermaking process to prepare flame-retardant cellulose composite diaphragms with excellent wettability and liquid absorption, significantly reducing the cost of diaphragm preparation. Cui Guanglei et al. invented a papermaking process for preparing nonwoven diaphragms, which is simple, low-cost, and suitable for large-scale production.
5. Meltblown spinning process
The meltblown process is a method of directly spinning resin into a web to produce microfiber nonwoven fabrics, which have excellent impermeability and filtration performance. Deng Rongjian introduced that the meltblown spinning process has advantages such as mature technology, good safety, and low cost. Polyester or polyamide nonwoven membranes prepared by the meltblown method have excellent stability.
6-phase conversion method
Phase inversion method utilizes the mass transfer between solvent and non-solvent in the casting solution to induce a phase transition in the original steady-state solution, ultimately solidifying the phase-separated structure into a film. Sangweina prepared a series of cross-linked composite gel polymer electrolyte films of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), and polystyrene (PS) using the phase inversion method and in-situ cross-linking of polystyrene. The results showed that when the PS content reached 25% of the total PEO/PS content, the film exhibited higher porosity, liquid absorption rate, and conductivity.
Looking ahead, with the expansion of production capacity, lithium battery separators with low cost, high wettability, good thermal stability, high safety, and excellent cycle life will dominate the industry. In recent years, domestic separator companies have made rapid technological progress, and domestically produced separators have gradually replaced imported ones in the low-to-mid-range lithium battery market. With the maturation of various separator manufacturing technologies, the future separator market will see a flourishing of diverse products and technologies, especially solid-state electrolytes, which will become an important development direction. Solid-state batteries are expected to be industrialized within the next 10 years.
