Application of Flame-Retardant Separators And Flame-Retardant Electrolytes in Lithium-Ion Batteries

Application of Flame-Retardant Separators and Flame-Retardant Electrolytes in Lithium-Ion Batteries

With the rapid development of new fields such as new energy vehicles and high-end electronic products, lithium-ion batteries have been widely adopted due to their excellent overall performance. However, traditional lithium-ion batteries suffer from poor thermal stability. During charging and discharging, thermal runaway can occur, leading to combustion and even explosion. This paper first introduces several different types of flame-retardant separators for lithium batteries, which demonstrate significantly improved flame-retardant performance compared to conventional separators. It then discusses the performance advantages of flame-retardant electrolytes from the perspectives of flame-retardant additives and structural modifications. Therefore, the research and application of flame-retardant lithium battery separators and electrolytes to improve battery safety has become a current research hotspot.

I. Research Background on Flame-Retardant Separators for Lithium Batteries

In recent decades, lithium-ion batteries (LIBs) have developed rapidly due to their advantages of high specific energy density, high cycling efficiency, no memory effect, and environmental friendliness. They are widely used in many aspects of life, particularly as energy storage units for mobile devices like smartphones and laptops. They not only provide power sources for new energy electric vehicles but also show great potential in storing wind, solar, and tidal energy.

However, lithium batteries also have inherent limitations. For instance, overheating, overcharging, or mechanical damage can trigger thermal runaway. If uncontrolled, this can lead to the combustion of battery materials, resulting in serious accidents like explosions.

For lithium batteries, the separator, as a crucial component, is a polymer film situated between the cathode and anode. It facilitates rapid transport of ionic charges and prevents direct contact between the electrodes to avoid short circuits. The quality of the separator directly impacts battery performance and manufacturing costs. Its properties determine the battery's interface structure, internal resistance, etc., directly affecting capacity, cycle life, and safety. The electrolyte, serving as the medium for ion transport, conducts ions between the cathode and anode. It is typically composed of high-purity organic solvents, lithium salt electrolytes, and necessary additives, prepared under specific conditions and proportions. The performance of the electrolyte also directly impacts battery safety. Consequently, the development of separators and electrolytes with excellent flame-retardant properties has become one of the key research focuses.

This paper introduces several different flame-retardant separators for lithium batteries, comparing their performance advantages over traditional separators. It then discusses the performance and advantages of flame-retardant electrolytes in terms of additives and structural improvements, and provides an outlook on the future development direction of lithium batteries.

II. Flame-Retardant Separators

Generally, the positive and negative electrode materials of lithium batteries are relatively stable at 150–200℃, which imposes higher requirements on the flame-retardant properties of separator materials. Currently, traditional separators are mainly made from materials like polyethylene (PE) and polypropylene (PP). Although they possess good electrochemical stability, mechanical strength, and thermal shutdown properties, under high charge/discharge currents or elevated operating temperatures, these traditional separators exhibit poor electrolyte wettability, leading to high ion transfer resistance, which reduces battery rate capability. Additionally, dimensional instability of the separator can cause internal short circuits, potentially leading to thermal runaway. In contrast, flame-retardant separators offer excellent electrolyte wettability and heat resistance, effectively preventing such accidents, thus garnering significant attention.

1. Modified Polyolefin Separators

To overcome the inherent drawbacks of polyolefin materials, researchers have modified traditional polyolefin-based separators. This aims to reduce their melt fluidity at certain temperatures, maintaining good overall dimensional stability, thermal stability, and low thermal shrinkage, thereby effectively isolating the battery electrodes.

The Daeyong Yeon group in Korea used two typical metal hydroxides, aluminum hydroxide Al(OH)₃ and magnesium hydroxide Mg(OH)₂, as functional materials to prepare composite lithium battery separators. A slurry containing the metal hydroxides and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was coated onto a polyethylene (PE) separator. This coating endowed the composite separator with good flame retardancy, significantly shortening the self-extinguishing time (SET), which helps enhance the dimensional stability of the PE separator at high temperatures.

Jongchan Song et al. from the Korea Institute of Science and Technology applied an aromatic polyimide (PI) coating onto a polyethylene (PE) separator via a simple dip-coating method. This coating method preserves the porous structure of the separator and the battery's original excellent performance while significantly improving the membrane's thermal stability, avoiding thermal shrinkage at high temperatures. The melting point of a pure PE separator is only 135℃. As shown in Figure 1, it exhibits significant thermal shrinkage at 140℃. After coating with polyimide at different concentrations, under the same temperature conditions, the thermal shrinkage rate decreases with increasing PI amount, stabilizing at 3 wt.% concentration where the thermal shrinkage rate is optimal. Further increasing the PI amount causes the thermal shrinkage rate to increase again. This simple and effective method addresses the thermal stability issue of polyolefin separators without sacrificing their inherent excellent battery performance.

2. Organic/Inorganic Composite Separators

To improve the thermal stability of polypropylene membranes, simple polymer surface modification can effectively enhance the wettability of polyolefin separators but does not significantly improve their thermal stability. Simple inorganic nanoparticle surface modification can improve both wettability and thermal stability but still presents some issues.

The team of Jinbao Zhao at Xiamen University coated SiO₂ particles on both sides of a polyethylene separator, then introduced polydopamine (PDA) via dip-coating. This process encapsulates the ceramic particles and PE with PDA, forming a self-supporting film on the surface. This organic-inorganic composite coating layer enhances the separator's thermal and mechanical stability. The self-supporting membrane backbone resists thermal shrinkage, particularly showing no shrinkage even at 230℃. Additionally, the PDA-treated separator exhibits better electrolyte wettability, improving cycling and rate performance.

Weidong He et al. from the University of Electronic Science and Technology of China modified the separator preparation process, using both coating and electrophoresis methods to fabricate high-performance lithium-ion battery separators. They synthesized garnet-type LLZTO (Li₆.₇₅La₃Zr₁.₇₅Ta_0.25O₁₂) nanoparticles via solid-state reaction, then incorporated them into PVDF-HFP at different mass fractions. Five types of LLZTO/PVDF-HFP composite separators with varying ratios were prepared by the coating method. This three-layer composite separator structure (PVDF-HFP/LLZTO/PVDF-HFP) exhibits excellent flame retardancy and thermal stability. The outer PVDF-HFP layers improve the compatibility between the separator and the battery electrodes, reducing interfacial resistance at the cathode and anode. The internal LLZTO layer enhances thermal stability, allowing the composite separator to maintain its geometric integrity even at 300°C. Compared to pure PVDF-HFP membrane, this three-layer structure retains its original shape and size under the same conditions with no thermal shrinkage, confirming that LLZTO imparts heat resistance and flame retardancy to the composite separator. Therefore, this structured separator can effectively reduce the safety risk of short circuits during high-temperature operation of lithium batteries.

3. Polyimide Separators

Aromatic polyimide (PI), due to its low dielectric constant, excellent chemical stability, good thermal stability, and mechanical properties, is an ideal separator material. Consequently, electrospun PI nanofiber membranes with different molecular structures, controllable fiber diameters, and membrane thicknesses have been extensively studied.

Miao et al. from Fudan University prepared polyimide nanofiber membranes via electrospinning, which exhibited good thermal stability and ion transport capability. Chuan Shi et al. systematically studied PI nanofiber separators and PI composite nanofiber separators, including PI/SiO₂ and PI/Al₂O₃ composite nanofiber separators. These can provide high thermal stability and ideal electrolyte absorption and retention. However, electrospun nanofiber membranes often have excessively large pore sizes, which can lead to lithium dendrite growth causing battery self-discharge and internal short circuits. High porosity also results in poor mechanical strength, limiting large-scale application. While compounding with inorganic nanoparticles can effectively improve heat resistance and adjust pore size, long-term immersion in electrolyte may cause nanoparticle detachment from the polymer matrix, leading to battery degradation.

Lingyi Kong et al. from South China University of Technology prepared fluorinated polyimide (FPI) nanofiber membranes with excellent performance using electrospinning and thermal crosslinking methods. Compared to traditional PE membranes, FPI nanofiber membranes exhibit superior heat resistance and flame retardancy. PE membranes ignite quickly upon exposure to flame and shrink immediately, whereas FPI nanofiber membranes neither ignite nor shrink. Compared to the original FPI nanofiber membrane, the thermally crosslinked FPI nanofiber membrane has higher mechanical strength, smaller average pore size, narrower pore size distribution, enhanced ability to block lithium dendrite growth and penetration, comprehensively improving the charge/discharge performance and safety of lithium batteries.

4. Non-Woven Fabric Separators

Alternatively, using high-temperature-resistant polymer-based non-woven fabrics to construct separators can also maintain good dimensional stability and enable high-power lithium batteries. Some reported non-woven fabrics are based on polymer materials like polyethylene terephthalate (PET), polyimide (PI), and cellulose. However, due to their relatively insufficient flame retardancy, there is a need to develop new non-woven separators based on other high-performance engineering plastics to further enhance lithium battery safety.

The team led by Hua Wang from the Sichuan Textile Science Research Institute pioneered the preparation of a novel non-woven fabric using polyphenylene sulfide (PPS) as the base material, providing a foundation for developing new high-efficiency and safe separators. By coating the PPS non-woven fabric with a slurry mixture of PVDF-HFP and SiO₂ nanoparticles, they successfully obtained a PPS non-woven based composite separator (PPSC). Compared to commercial polyolefin separators, PPSC has higher porosity and better electrolyte wettability. This composite separator also exhibits good dimensional stability and flame-retardant performance even after heat treatment at 250℃. Combustion tests on three types of separators show: conventional PP/PE/PP separator immediately shrinks significantly upon contact with flame and completely burns within a very short time. PPS separator shows slight shrinkage upon contact with flame and has self-extinguishing properties. PPSC separator is difficult to ignite, shows slight shrinkage, and exhibits self-extinguishing characteristics. The excellent performance of the PPS non-woven based composite separator provides a good material foundation for preparing high-power lithium-ion batteries.

5. Flame-Retardant Separators Prepared via Wet Papermaking Process

Currently, lithium battery separators are typically produced using dry and wet processes. The performance of dry-process separators is relatively low, insufficient to meet the demands of high-power lithium batteries. Therefore, using wet papermaking technology to prepare safety-enhanced separators has become mainstream.

The team led by Guanglei Cui at the Chinese Academy of Sciences successfully prepared aramid lithium battery separators with good electrolyte wettability, high ionic conductivity, and excellent heat resistance and flame retardancy through a simple papermaking process, improving the safety of lithium-ion batteries. The aramid separator endows lithium cobalt oxide (LiCoO₂)/graphite batteries with excellent cycling performance and better interfacial compatibility. Comparing aramid separators with traditional PP separators, the PP separator shows an endothermic peak at 165℃, whereas the aramid separator shows no significant endothermic peak below 300℃, indicating better thermal stability. Furthermore, in lithium iron phosphate batteries, this aramid separator also exhibits stable charge/discharge performance. These excellent battery performance characteristics, heat resistance, and the simple papermaking process endow aramid separators with considerable application potential. Given aramid's excellent mechanical properties, flame retardancy, heat resistance, and electrical insulation, aramid separators are expected to provide excellent safety performance for power and energy storage batteries.

In summary, these flame-retardant separators are compared in terms of their thickness, porosity, Gurley value, thermal shrinkage rate, and combustibility, as shown in Table 1.

III. Flame-Retardant Electrolytes

Commonly used organic solvents for lithium-ion battery electrolytes are alkyl carbonate compounds, such as diethyl carbonate (DEC), ethylene carbonate (EC), etc. These are highly flammable compounds. If batteries are used improperly (overcharging, short circuit, etc.), it can easily cause battery overheating, leading to electrolyte combustion, fires, or even explosions.

Therefore, given that the main battery materials are unlikely to be replaced in the short term, improving the flame retardancy of electrolytes is an effective way to enhance lithium-ion battery safety. Adding flame retardants to the electrolyte can effectively improve the flame retardancy of organic electrolytes and reduce the battery's self-heating rate and heat release. This method almost does not increase battery cost, does not alter the production process, and has minimal impact on battery electrochemical performance, thereby improving battery safety.

Over the past decades, researchers have primarily focused on several major aspects to address lithium-ion battery safety: adding flame-retardant additives, structural modifications, and developing new electrolyte materials.

1. Flame-Retardant Additives

Flame-retardant additives for electrolytes are mainly categorized into organic phosphorus-based, organic halogen-based, and composite electrolyte systems. Among them, phosphorus-based organic flame-retardant additives are the most researched and widely used. These mainly include phenyl and alkyl phosphate esters (such as triphenyl phosphate and tributyl phosphate) and cyclic phosphazene compounds.

Xu et al. conducted in-depth research, finding that adding organic phosphorus-based additives to electrolytes increases viscosity and reduces conductivity. Using sodium methoxide (NaOCH₃) as a raw material, they reacted it with ethylene oxide to synthesize the flame retardant hexamethylphosphonitrilate (HMPN) as an additive, which improves the thermal stability of the electrolyte without compromising its inherent performance, making HMPN an ideal flame-retardant additive.

Yao et al. from the Department of Materials Science and Engineering, University of Science and Technology of China, employed 4-isopropylphenyl diphenyl phosphate (IPPP) as a flame-retardant additive in LiPF₆ electrolyte. Through combustion tests and microcalorimetry, they found that adding IPPP reduces the flammability of the electrolyte and delays the onset temperature of the main exothermic reaction.

Daiying Zhou et al. from South China Normal University added dimethyl methylphosphonate (DMMP) as a flame retardant to LiPF₆ electrolyte. The addition of DMMP reduces the electrolyte's flammability and self-heating rate, alleviating the balance between electrolyte flammability, thermal stability, and battery performance.

Xiang Hongfei et al. from the University of Science and Technology of China also used DMMP as a flame retardant to modify the 1M LiPF₆/EC+DEC system, studying the flammability, electrochemical stability, and cycling performance of electrolytes containing DMMP. Based on measurements of self-extinguishing time and limiting oxygen index, they similarly found that adding DMMP significantly inhibits electrolyte flammability. Flammability tests on electrolytes with different DMMP contents showed that with 5% DMMP, the self-extinguishing time was shortened by 70 s/g, with 10% DMMP, the self-extinguishing time was 0 s/g, meaning the electrolyte could not be ignited, resulting in a highly flame-retardant electrolyte. Simultaneously, the addition of DMMP had little impact on the battery's electrochemical performance.

2. Strategies for Structural Improvement

Microcapsule structures are particle-like structures where the capsule wall is made of natural or synthetic polymer materials and inorganic materials. The core encapsulates solid or liquid flame retardants. When a lithium battery experiences thermal runaway, the rising temperature causes the capsule wall to rupture, releasing the encapsulated flame retardant, thereby ensuring lithium-ion battery safety. Baginska M et al. in the USA successfully encapsulated the flame retardant tris(2-chloroethyl) phosphate (TCP) within core-shell poly(urea-formaldehyde) microcapsules using in-situ polymerization.

There are also some novel design schemes currently. Yi Cui et al. developed a novel intelligent electrospun separator with thermally triggered flame-retardant properties for lithium-ion batteries. The electrospun fibers consist of a triphenyl phosphate (TPP) core and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) shell. When the lithium-ion battery operates normally, TPP does not affect the cycling performance. However, when the battery overheats, the copolymer shell melts and ruptures, releasing the TPP flame retardant, which captures free radicals, thereby preventing thermal runaway combustion and explosion in lithium-ion batteries. Encapsulating TPP within a protective polymer shell prevents the direct dissolution of the flame retardant into the electrolyte, avoiding negative impacts on battery performance. The thermal melting of the polymer shell releases the flame retardant, effectively suppressing the combustion of highly flammable electrolytes under conditions of lithium-ion battery thermal runaway. It is anticipated that such intelligent separators will find effective applications in the future.

IV. Conclusion

China's lithium battery industry is in a stage of rapid development, and the application of lithium batteries is becoming increasingly widespread. However, in practical applications, incidents of battery thermal runaway and explosions have sparked significant debate about battery safety. Researching and preparing new composite separators with good thermal stability and new, efficient electrolyte additives will be the focus of future industry development.

In recent years, domestic separator production has made significant progress in capacity building, cost reduction, and technological improvement. However, most high-end separators are still imported, and there are relatively few domestic manufacturers with comparable performance. Much of the advanced research remains at the university laboratory and research institution stage. Therefore, we still need to continue tackling scientific challenges, gradually transitioning experimental achievements into industrialization. In the future, under the collaborative research efforts of government, academia, and industry, the safety performance of lithium-ion batteries will be more effectively guaranteed. This will also play a promoting role in China's economic development, and China in the new era is poised to lead global economic growth.