Lithium salt
There are many types of lithium salts, but there are few lithium salts for commercial lithium-ion batteries. The ideal lithium salt needs to have the following properties:
(1) has a small degree of association, easy to dissolve in organic solvents, to protect the electrolyte high ionic conductivity; (2) The anions have anti-oxidation and anti-reducing properties, and the reduced products are conducive to the formation of stable and low-impedance SEI films; (3) Good chemical stability, no harmful side reactions with electrode materials, electrolyte, diaphragm, etc.; (4) Simple preparation process, low cost, no pollution
Different kinds of lithium salts are introduced

LiPF6
LiPF 6 is the most widely used lithium salt. The single property of LiPF 6 is not outstanding, but it has relatively excellent comprehensive properties in the carbonate ester mixed solvent electrolyte. LiPF 6 has the following outstanding advantages: (1) suitable solubility and high ionic conductivity in non-aqueous solvents; (2) A stable passivation film can be formed on the surface of the Al foil collector fluid; (3) A stable SEI film was formed on the surface of the graphite electrode in collaboration with carbonate solvent. However, the thermal stability of LiPF 6 is poor, and decomposition reaction is easy to occur. The side reaction products will destroy the SEI film on the electrode surface, dissolve the active component of the positive electrode, and lead to the decay of the circulation capacity.
LiBF4
LiBF 4 is a commonly used lithium salt additive. Compared with LiPF 6, LiBF 4 has a wider operating temperature range, better stability at high temperatures and better performance at low temperatures.
LiBOB
LiBOB has high conductivity, wide electrochemical window and good thermal stability. Its advantage lies in the film forming performance, which can directly participate in the formation of SEI film.
LiDFOB
Structurally, LiDFOB is composed of half molecules of LiBOB and LiBF4 respectively, which combines the advantages of LiBOB's good film formation and LiBF4's good low temperature performance. Compared with LiBOB, LiDFOB has higher solubility in linear carbonate solvent and higher electrolyte conductivity. Its high and low temperature performance is better than LiPF 6, and it has good compatibility with the positive electrode of the battery, and can form a passivation film on the surface of Al foil and inhibit electrolytic liquid oxygenation.
LiTFSI
The CF 3SO2 - group in LiTFSI structure has a strong electron absorption effect, which intensifies the delocalization of negative charge, reduces the ion association pairing, and makes the salt have a high solubility. LiTFSI has high electrical conductivity, high thermal decomposition temperature and is not easy to hydrolysis. However, when the voltage is higher than 3.7V, the Al collector will be seriously corroded.
LiFSI
The fluorine atom in LiFSI molecule has strong electron absorption, can delocalize the negative charge on N, the ion association pairing effect is weak, Li + is easy to dissociate, so the conductivity is high.
LiPO2F2
LiPO 2F2 has good low temperature performance, but also can improve the high temperature performance of the electrolyte. As an additive, LiPO 2F2 can form SEI film rich in Li xPO yFz and LiF on the surface of the negative electrode, which is conducive to reducing the interface impedance of the battery and improving the cycle performance of the battery. However, LiPO 2F 2 also has the disadvantage of low solubility.
Organic solvent
The main component of the liquid electrolyte is an organic solvent, which dissolves lithium salts and provides carriers for lithium ions. The ideal organic solvent for lithium-ion battery electrolyte needs to meet the following conditions:
(1) High dielectric constant, strong ability to dissolve lithium salt;
(2) Low melting point, high boiling point, in a wide temperature range to maintain liquid; (3) Small viscosity, easy to transport lithium ions;
(4) Good chemical stability, does not destroy the structure of positive and negative electrodes or dissolve positive and negative electrode materials; (5) High lightning, good safety, low cost, no pollution.