A small amount of non-energy storage materials used in lithium-ion batteries can significantly improve certain performance of the battery, and these small amounts are called additives. The organic electrolyte additives have the outstanding characteristics of "low dosage (generally less than 5% by volume or mass ratio) and quick effect", which can significantly improve some macroscopic properties of lithium-ion batteries without increasing the cost of the battery []. Additives should generally have the following characteristics:
(1) One or several performances of the battery can be improved with less usage;
(2) It has no side effects on battery performance and does not cause side reactions with other materials constituting the battery;
(3) It has good compatibility with organic electrolyte, and it is best to be easily soluble in solvent;
(4) The price is relatively low, no toxicity or low toxicity; at present, the research of lithium ion battery electrolyte additives mainly focuses on the following aspects:
(1) Improve the stability of SEI film [7073);
(2) Improve the safety performance of the battery [4-70;
(3) Control the acid and water content in the electrolyte;
(4) Improve the conductivity of the electrolyte [8-7 to.
1. Additives to improve the stability of the SEI film of lithium-ion batteries
The SEl (Solid Electrolyte Interface) film, that is, the solid electrolyte phase interface film, is a passivation film formed on the surface of the negative electrode of the lithium ion battery to isolate the electrolyte from the carbon material/lithium negative electrode. The SEl film is formed during the initial cycling of the lithium-ion battery. Under a certain potential, at the negative electrode/electrolyte interface, organic solvent molecules, lithium salt anions, impurities and additives undergo reductive decomposition to form insoluble substances and deposit on the electrode surface.
Film-forming additives are divided into organic film-forming additives and inorganic film-forming additives.
Organic film-forming additives include sulfite additives, sub-alum additives and sulfonate additives.
Commonly used sulfite additives include vinyl sulfite (ES), propylene sulfite (PS), dimethyl sulfite (DMS), diethyl sulfite (DES), etc. 18]. The main components of the SEI film formed by the reduction and decomposition of sulfite additives on the surface of the carbon negative electrode are inorganic salts LizS, LizSO; or LiuSO, and organic salts ROSO2Lil81]. The specific composition is also related to the current density. Under high current density, inorganic lithium salt is first generated.
The organic lithium salt component only appears below 0.5V; at low current density, the organic lithium salt precipitates at 1.5V, and then no inorganic salt is formed. The film strength of different sulfite additives at the interface of carbon anode is ES>PS2DMS>DES.
Substance additives 28 include dimethyl subsulfate (DMSO), butyl subsine, ethyl methyl subsine (EMS), cyclopropyl subsulphate (TriMS), 1-methylcyclopropyl subsulphate (MTS), Ethyl sec-butyl sulfite (EsBS), ethyl isobutyl sulfite (EiPS) and 3,3,3-trifluoropropylmethyl sulfite (FPMS), etc.
2. Additives to improve the safety performance of lithium-ion batteries
Safety issues are an important prerequisite for the innovation of the lithium-ion battery market, especially applications in electric vehicles and other fields put forward higher and newer requirements for battery safety. Lithium-ion secondary batteries emit a lot of heat when they are overcharged and discharged, short-circuited, and working for a long time with large currents. This heat becomes a safety hazard for flammable electrolytes, which may cause catastrophic thermal breakdown (thermal runaway) or even battery explosion [8]. The addition of flame-retardant additives can turn the flammable organic electrolyte into a non-flammable or non-flammable electrolyte, reduce the battery's heat release value and battery self-heating rate, and also increase the thermal stability of the electrolyte itself to prevent the battery from overheating. Burning or explosion under.
3. Additives to control the acid and water content in the electrolyte of lithium-ion batteries
The trace amounts of water and HF in the organic electrolyte have a certain effect on the formation of the excellent SEl film, which can be seen from the reaction of solvents such as EC and PC at the electrode interface. But too high content of water and acid (HF) will not only lead to LiPF. Decomposition, and will destroy the SEI film [8]. When AlbO3, MgO, Bao, and lithium or calcium carbonates are added as additives to the electrolyte, they will react with a small amount of HF in the electrolyte, reduce the HF content, and prevent its damage to the electrode and decomposition of LiPF6 The catalysis of the electrolyte improves the stability of the electrolyte, thereby improving battery performance. However, these substances are slow to remove HF, so it is difficult to prevent HF from damaging the battery performance. Although some acid anhydride compounds can quickly remove HF, they will also produce other acidic substances that damage battery performance. Alkane diimine compounds can form weak hydrogen bonds with water molecules through hydrogen atoms in the molecule, thereby preventing water and LiPF. The reaction produces HF.
4. Conductive additives
The high conductivity of the electrolyte is an important guarantee for reducing the migration resistance of Lit and improving the charge and discharge performance of the battery. The role of the conductive additive is that the additive molecule and the electrolyte ion undergo a coordination reaction to promote the dissolution and ionization of the lithium salt, reduce the solvation radius of the solvated lithium ion, and prevent the solvent co-intercalation from damaging the electrode. According to its interaction with electrolyte ions in the electrolyte, it can be divided into cation interaction type (cation ligand), anion interaction type (anion ligand) and electrolyte ion interaction type (neutral ligand yl).
5. Additives to improve low temperature performance
Low-temperature performance is one of the important factors in broadening the use range of lithium-ion batteries, and it is also a must-have in current aerospace technology. N,N-Dimethyltrifluoroacetamide has low viscosity (1.09mPa-s, 25℃), high boiling point (135℃) and flash point (72℃). It has good film-forming ability on the surface of graphite. The positive electrode also has good oxidation stability, and the assembled battery has excellent cycle performance at low temperatures. Organic borides and fluorine-containing carbonates are also conducive to the improvement of battery low-temperature performance.
6. Multifunctional additives
Additives that have two or more functions at the same time are called multifunctional additives. Multifunctional additives are ideal additives for lithium-ion batteries. They can improve the performance of electrolytes in many ways and play a prominent role in improving the overall electrochemical performance of lithium-ion batteries. They are becoming the main direction of research and development of additives in the future.
In fact, some of the existing additives are themselves multifunctional additives. Such as 12-crown-4 ether/8] After adding PC solvent, while improving the conductivity of Li itself, the electrophilic effect of the crown ligand on the electrode surface makes the possibility of Li reacting with solvent molecules at the electrode interface greatly reduced. The preferential solvation effect of crown ether on Li inhibits the co-insertion of PC molecules, and the SEI membrane of the electrolysis interface is optimized, which reduces the first irreversible capacity loss of the electrode. In addition, fluorinated organic solvents, halogenated phosphate esters such as BTE and TTFP, etc. added to the electrolyte not only help to form an excellent SEl film, but also have a certain or even obvious flame retardant effect on the electrolyte, which improves many aspects of the battery. performance.
