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Lithium-ion battery electrolyte additives

by:dcfpower     2021-03-16

The small amount of non-energy storage materials used in lithium-ion batteries can significantly improve certain performance of the battery. 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[]. Additives should generally have the following characteristics:

(1) Less dosage can improve one or several performances of the battery;

(2) No side effects on battery performance, no It has side reactions with other materials constituting the battery;

(3) It has good compatibility with organic electrolytes, and it is best to be easily soluble in solvents;

(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 the SEI membrane [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 membrane of lithium ion batteries

SEl (Solid Electrolyte Interface) membrane, that is, the solid electrolyte phase interface membrane, is formed on the surface of the negative electrode of lithium ion batteries. The formed layer of passivation film separates the electrolyte from the carbon material/lithium negative electrode. The SEl film is formed during the initial cycling of the lithium battery pack. 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.

Organic lithium salt appears below 0.5V; at low current density, organic lithium salt appears at 1.5V After precipitation, no more inorganic salts are formed. The film formation strength of different sulfite additives at the interface of carbon anode is ES>PS2DMS>DES.

Sub-additives 28 include dimethyl subsulfate (DMSO), butyl sub-base, ethyl methyl sub-base (EMS), cyclopropyl sub-base (TriMS), 1-methylcyclopropyl Kiyape (MTS), ethyl sec-butyl sulphite (EsBS), ethyl isobutyl sulphite (EiPS) and 3,3,3-trifluoropropylmethyl sulphite (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 battery pack market, especially for applications in electric vehicles and other fields. Sexuality puts forward higher and newer requirements. Lithium-ion secondary batteries emit a lot of heat under the conditions of excessive charge and discharge, short circuit and high current working for a long time. This heat becomes a safety hazard for flammable electrolyte, which may cause catastrophic thermal breakdown (thermal runaway) or even battery explosion [8]. The addition of flame-retardant additives can turn flammable organic electrolytes into flame-retardant or non-flammable electrolytes, reduce battery heat release value and battery self-heating rate, and 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 water and HF in the organic electrolyte contribute to the formation of the excellent SEl film. It has a certain effect, 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 content of HF, and prevent it from destroying the electrode and decomposing 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 to coordinate the additive molecules with the electrolyte ions, promote the dissolution and ionization of the lithium salt, reduce the solvation radius of the solvated lithium ions, and prevent the co-intercalation of the solvent 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.


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