1. Overview of electrolyte additives
Electrolyte additives are functional components that are small in content (usually <5%) but crucial in lithium-ion batteries. They can form a stable interface film (SEI/CEI) on the electrode surface, significantly improving the battery’s cycle life, rate performance and safety. According to functional differences, additives can be divided into film-forming additives, flame retardant additives, overcharge protection additives, wetting agents, acid neutralizers and other types. With the continuous improvement of the energy density of power batteries, the electrolyte additive system has become one of the key factors to break through the bottleneck of battery performance.
2. Mainstream additive types and mechanisms of action
1. Film-forming additives
Vinylene carbonate (VC): The most commonly used positive electrode film-forming agent, which preferentially reduces to form a dense SEI film at around 1.8V (vs. Li+/Li), effectively inhibiting the continuous decomposition of the electrolyte. The recommended addition amount is 1-2%, and excessive addition will lead to increased interfacial impedance.
Fluoroethylene carbonate (FEC): Suitable for silicon-based negative electrodes (addition amount 3-5%), and the LiF-rich interface layer formed can buffer the volume expansion of silicon materials. However, it should be noted that decomposition may occur at high temperatures (>60°C).
DTD: a dual-functional additive (0.5-1%) that forms films on both the positive and negative electrodes, especially suitable for high-nickel ternary systems.
2. Flame retardant additives
Trimethyl phosphate (TMP): flame retardant through gas-phase free radical capture mechanism, but it will reduce the conductivity of the electrolyte (the amount added must be controlled within 10%).
Fluorophosphates (such as TFEP): have both flame retardant and film-forming functions, and when used in conjunction with FEC, the flash point of the electrolyte can be increased by more than 50°C.
3. Other functional additives
Lithium difluorophosphate (LiPO₂F₂): a positive electrode CEI film modifier (0.5%) that can inhibit the dissolution of transition metals and increase the cycle life of NCM811 batteries by 30%.
Lithium nitrate (LiNO₃): a special additive for lithium-sulfur batteries (1%), which inhibits the polysulfide shuttle effect by forming a Li₃N protective layer.
III. Additive selection and compatibility principles
1. System adaptability principle
High nickel ternary system: VC+LiPO₂F₂+DTD combination is recommended (total addition amount 3-4%), and stable positive electrode CEI film is constructed first
Silicon-carbon negative electrode system: FEC+sulfur-containing additives (such as PS) composite solution (5-7%) is required to alleviate the volume effect of silicon materials
High voltage system (>4.5V): Electropolymerization additives such as biphenyl (BP) (0.5%) must be added to prevent electrolyte oxidation
2. Compatibility taboo warning
When VC and LiNO₃ coexist, redox reaction will occur, causing both to fail at the same time
Mixing P-containing additives with B-containing additives (such as LiDFOB) may produce precipitates
HF generated by FEC decomposition under high pressure will corrode the positive electrode and needs to be used with HF scavenger
IV. Key points of engineering application
1. Adding process control
Must be added in the final stage of electrolyte preparation (temperature <30°C)
Adopt step-by-step addition method: first add film-forming agent, then add functional improver
More than 48 hours of aging treatment is required to fully dissolve the additives
2. Performance verification method
Electrochemical window test: verify the effect of additives on electrolyte stability
HPLC monitoring: ensure that additives are effectively consumed during the formation process
TOF-SIMS analysis: characterize the distribution of chemical components of the interface film
3. Failure case analysis
Case 1: A manufacturer added excessive VC (3%) to the LFP battery, resulting in a 50% decrease in low-temperature performance
Case 2: NCM811 battery did not add LiPO₂F₂, and the capacity retention rate after 200 cycles was only 82%
Solution: Establish an additive-performance response database and implement DoE (experimental design) optimization
V. Development trends of new additives
Polyfluorophenyl ether (FPE): 4.8V ultra-high voltage stability additive (industrialization in 2023)
Ionic liquid additives: such as PYR₁₄TFSI (3% addition can increase the starting temperature of thermal runaway by 40°C)
Smart response additives: temperature-sensitive polymer additives that automatically form a protective layer when overheated
VI. Usage recommendation list
Basic formula reference:
General power battery: 2%VC + 1%DTD + 0.5%LiPO₂F₂
Long cycle energy storage: 1%VC + 0.3% vinyl sulfate
High safety type: 5% FEC + 3% TFEP
Supplier selection criteria:
Purity requirement ≥99.9% (GC detection)
Water content ≤10ppm
Provide MSDS and DSC thermal analysis data
Cost control strategy:
Give priority to domestic additives (such as Zhejiang Yongtai’s FEC)
Develop customized compounding solutions to reduce the use of expensive additives
With the development of solid electrolytes, liquid electrolyte additive technology is expected to continue to dominate the market for at least 10 years. It is recommended that battery manufacturers establish a dedicated additive evaluation laboratory and accelerate the verification process of new additives through high-throughput screening platforms (such as the use of robotic automated testing systems). At the same time, it is necessary to pay close attention to the restrictions on fluorinated additives under the EU REACH regulations and plan the development of green additives in advance.