Thermal runaway of lithium-ion batteries is a key bottleneck restricting their application in new energy vehicles, large-scale energy storage and other fields — once triggered, it may cause catastrophic consequences such as fire and explosion. The occurrence of thermal runaway is directly related to the performance of the four core materials: separator, electrolyte, cathode, and anode.
How to build a safety line of defense from the source of material design? This article will systematically disassemble the safety optimization technologies of the four core materials, detail various protection strategies from surface modification to intelligent response, and provide comprehensive reference for the research, development and production of high-safety lithium batteries.
1. Separator Protection: The “First Physical Barrier” of Battery Safety
The core mission of the separator is to isolate the positive and negative electrodes and prevent short circuits. Its thermal stability and mechanical strength directly determine the safety bottom line of the battery. Currently, mainstream protection technologies can be divided into four directions:
1. Surface Modification: Low-Cost and Efficient Upgrade
By coating inorganic particles or organic polymers on the surface of polyolefin separators, heat resistance and electrochemical performance are quickly improved:
Inorganic Particle Coating: Al₂O₃, SiO₂, TiO₂, ZrO₂ and other materials are commonly used, while boehmite (AlOOH) has greater application potential due to its higher heat resistance temperature, lower density and lower internal resistance. Experiments show that based on a 9μm PP separator, the B1 separator coated with boehmite has a shrinkage rate of less than 3% at 140℃ (unmodified PP separator exceeds 57%), remains intact at 180℃, and its tensile strength is increased by 18.8%; the puncture strength of B2 separator is increased by 54.4%, and it can be completely wetted by electrolyte within 30 seconds;
Organic Polymer Coating: To solve the problem that inorganic coatings are prone to clogging pores, polymers such as PVDF, PVDC, ANF, and PAN have become preferred choices. Among them, PVDF and its copolymer coating technology are mature, which can improve separator stability without affecting ion transport.
2. New Separator System: Breakthrough from Material Essence
Develop separator materials with inherent high heat resistance to replace traditional polyolefins:
Polyimide (PI)-Based Separators: With a melting point as high as 500℃, they have excellent chemical stability and mechanical strength. The PI aerogel (PIA) separator prepared by sol-gel method has a porosity of 78.35% and an electrolyte absorption rate of 321.66%. The assembled LiFePO₄-Li half-cell can cycle stably more than 1000 times at 120℃ with a capacity retention rate of over 80%; compared with the traditional Celgard 2400 separator, the thermal runaway temperature is increased from 131℃ to 170℃, and the safety margin is greatly expanded;
Other New Separators: Separators such as polyethylene terephthalate (PET), bacterial cellulose, and fluoropolymers are superior to traditional PP/PE separators in terms of thermal stability, liquid absorption rate, and ionic conductivity, providing more options for safety design.
3. Thermal Shutdown Separators: Intelligently Cut Off Ion Channels
Modify with temperature-responsive materials to make the separator actively “close pores” at high temperatures:
PP separators are coated with ethylene-vinyl acetate copolymer microspheres (particle size about 1μm, thermal response temperature 90℃). In short-circuit tests, the maximum surface temperature of the modified battery is only 57.2℃, much lower than 131.2℃ of the traditional PP separator battery. The principle is that the microspheres melt and collapse at high temperatures, forming a dense insulating layer, disconnecting the lithium ion transport channel, and preventing the intensification of thermal runaway.
4. Heat-Absorbing Separators: In-Situ Absorb Internal Heat
Integrate phase change materials (PCM) into the separator to achieve active temperature regulation:
When the battery is abused, the PCM in the phase change temperature-regulating separator melts when heated and absorbs a lot of latent heat, quickly reducing the internal temperature. The 63mAh lithium iron phosphate-graphite battery equipped with this separator can return to room temperature within 35 seconds after the needle puncture test, providing inherent overheating protection for high-energy-density batteries.
2. Electrolyte Optimization: Essential Upgrade from “Flammable” to “Flame-Retardant”
Electrolyte is the main “fuel” for thermal runaway. Its safety improvement focuses on developing flame-retardant/non-flammable systems while ensuring electrochemical performance:
1. Ionic Liquid Electrolytes: A Completely Non-Flammable Choice
Ionic liquids are composed of cations and anions, with a melting point below 100℃, and have the advantages of good thermal stability, low volatility, and non-flammability. However, they have problems such as high viscosity, poor wettability, and poor compatibility with graphite anodes. It is necessary to optimize by adding film-forming additives, mixing carbonate solvents, etc., to balance safety and rate performance.
2. Fluorinated Solvent Electrolytes: Compatibility with High Safety and High Voltage
Fluorinated solvents have high flash point, flame retardancy and good electrode wettability due to their strong electronegativity and low polarizability:
Perfluoroalkyl ethers have significant flame retardant effects, but poor miscibility and lithium salt solubility, and are mostly used as additives; some fluorinated ethers can be used as co-solvents, and when the volume fraction exceeds 70%, the electrolyte is completely non-flammable;
Commercial fluorinated solvents such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2), when used in combination with other fluorinated carbonates, can develop non-flammable electrolytes resistant to high voltage, suitable for high-performance batteries such as high-nickel ternary, lithium cobalt phosphate and lithium metal batteries.
3. Organophosphate Solvents: Low-Cost Flame-Retardant Solutions
Organophosphates have the characteristics of high boiling point, low viscosity, and easy synthesis. Similar in structure to carbonates, they are potential flame-retardant solvents. However, it is necessary to solve the compatibility problem with graphite anodes — currently, they are mostly used as flame-retardant additives (flame-retardant effect is significant when the concentration is above 20%), and molecular design is needed in the future to break through application bottlenecks.
4. Phosphazene Flame Retardants: Effective with Small Addition
Phosphazene compounds (including linear and cyclic structures), as composite flame-retardant additives, can make the electrolyte non-flammable with a mass fraction of 5%-15%, and have good compatibility with electrode materials, with little impact on battery electrochemical performance, so they are widely used in high-voltage batteries.
3. Cathode Protection: Inhibiting Surface Phase Transition and Side Reactions
Cathode materials are prone to decompose and release reactive oxygen at high temperatures, triggering violent reactions. The core of protection is to isolate their direct contact with the electrolyte:
Surface Coating Technology: Building a Stable Protective Layer
Coat the cathode surface with materials with high chemical inertness, such as phosphates (AlPO₄, Mn₃(PO₄)₂), fluorides, solid oxides, etc.:
Using a “coating + perfusion” strategy, NCM811 cathode is coated with cobalt boride (CoB) metallic glass. After 500 cycles at 1C, the capacity retention rate is increased from 79.2% to 95.0%, effectively inhibiting microstructural degradation and interface side reactions;
NCM622 cathode coated with Mn₃(PO₄)₂ nanocrystals can reduce the contact between electrolyte and active surface, reduce the intensity of exothermic side reactions, and improve thermal stability.
4. Anode Modification: Stabilizing SEI Film and Inhibiting Lithium Plating
The safety hazards of the anode mainly come from the decomposition of the SEI film at high temperatures, the reaction between lithiated graphite and electrolyte, and the growth of lithium dendrites:
1. Constructing a High Thermal Stability SEI Film
Add film-forming additives such as ammonium perfluorooctanoate (APC), vinylene carbonate (VC), and vinyl ethylene carbonate (VEC) to the electrolyte to form a uniform and dense polymer film on the surface of the graphite anode, improving the thermal stability of the SEI film;
Construct an artificial SEI film through metal/metal oxide deposition, polymer or carbon coating, etc., to actively isolate the direct reaction between the anode and the electrolyte.
2. Optimizing Lithium Ion Diffusion Efficiency
Shortening the diffusion path of Li⁺ between graphite layers and increasing the interlayer spacing can accelerate the solid-phase diffusion rate, avoid lithium plating reactions caused by excessive Li⁺ flux during high-current charging, and reduce the risk of lithium dendrites piercing the separator.
5. Future Development Direction: Multi-Dimensional Synergistic Protection
Lithium battery safety design has evolved from single material improvement to “multi-component synergy and full-life cycle protection”. The key future directions include:
Develop separators with both high temperature resistance and reactive oxygen barrier functions to block the crosstalk reaction between reactive oxygen released by the cathode, electrolyte and anode;
Develop electrolyte systems that can real-time in-situ repair the SEI film to inhibit heat accumulation in the early stage of thermal runaway;
Design reactive oxygen capture coatings to quench reactive oxygen released by cathode thermal decomposition, cutting off the thermal runaway chain reaction from the source;
Promote the application of single-crystal ternary cathode materials to reduce structural degradation caused by lattice gaps and improve thermal stability.
For more in-depth research on lithium battery safety material design and thermal runaway prevention, you can refer to the research published by the Journal of Power Sources. Our previous articles onhigh-temperature lithium battery thermal stability and LATP coating for solid-state battery interface further elaborate on battery material performance and protection technologies. For detailed industry standards and high-safety battery testing specifications, refer to the report released by the Institute of Electrical and Electronics Engineers (IEEE).