Comparison of lithium salts used in lithium ion battery electrolyte

Comparison of lithium salts used in lithium ion battery electrolyte

2022-02-15 10:07:45

Lithium-ion batteries are the most widely used rechargeable batteries because of their high energy density, high operating voltage, no memory function and long service life. With the rapid development of its application fields, people have put forward higher requirements on the energy density, rate performance, applicable temperature, cycle life and safety of lithium ion batteries. Electrolyte is an important component of lithium-ion batteries, and lithium salt, as a key component of liquid electrolyte, is an important factor in determining electrolyte performance, and is one of the breakthroughs in solving the problem of lithium-ion battery performance improvement. This article will compare LiPF6 with various new lithium salts.

Lithium salt is the supplier of lithium ions in the electrolyte. LiPF6 (lithium hexafluorophosphate) is the most common lithium salt at present. The development of new lithium salts LiBF4 (lithium tetrafluoroborate), LiBOB (lithium dioxalate borate), LiDFOB (lithium oxalate diborate), LiFSI (Lithium bisfluorosulfonimide), LiTFSI (lithium bistrifluoromethylsulfonimide), LiPF2O2 (lithium difluorophosphate) and LiDTI (lithium 4,5-dicyano-2-trifluoromethylimidazolium) has also received increasing attention from researchers. The advantages and disadvantages of the comparison are as follows:

Table 1. Comparison of advantages and disadvantages of various lithium salts.

Lithium salt	advantage	disadvantage
LiPF6	(1) has suitable solubility and high ionic conductivity in non-aqueous solvent; (2) forming a stable passivation film on the surface of the Al foil current collector; (3) A stable SEI film can be formed on the surface of the graphite electrode in synergy with the carbonate solvent.	The thermal stability is poor and the decomposition reaction is easy to occur.
LiBF4	LiBF4 has wide operating temperature range, good high temperature stability and excellent low temperature performance. It can enhance the film forming ability of electrolyte on the electrode and inhibit the corrosion of Al foil.	The ionic conductivity is low, which has great limitations. It is often associated with lithium salt with high conductivity.
LiBOB	LiBOB has higher conductivity, wider electrochemical window, good thermal stability, better cycle stability, passivation protection for positive Al foil current collector	low solubility, and almost insoluble in solvent of low dielectric constant.
LiDFOB	LiDFOB has good film formation, good low temperature performance, good compatibility with battery positive electrode, can form a passivation film on the surface of Al foil, and inhibit the oxidation of electrolyte.	High price
LiFSI	LiFSI has high conductivity, low water sensitivity and good thermal stability.	Corrosion potential of Al foil is 4.2V.
LiTFSI	LiTFSI has higher solubility and conductivity, thermal decomposition temperature exceeds 360 °C, and is difficult to hydrolyze.	When the voltage is higher than 3.7V, it will seriously corrode the Al current collector.
LiPF2O2	LiPF2O2 has good low temperature performance; as an additive, it is beneficial to reduce the interface impedance of the battery and effectively improve the cycle performance of the battery.	The solubility is low.
LiDTI	LiDTI has better thermodynamic stability; it can be stably existed at 4.5V and has a high lithium ion migration number, which can meet the charging and discharging requirements of commercial cathode materials.	High synthesis requirements

What are the characteristics of an ideal electrolyte lithium salt?

(1) Low dissociation energy and high solubility: low dissociation can ensure the electrolyte formed after the dissolution of lithium salt has high conductivity, thus achieving high rate of battery; high solubility ensures sufficient lithium ion in electrolyte transmission.

(2) Better stability: When the battery is operated under high voltage and high temperature, the lithium salt will not react with other components.

(3) Good SEI film forming properties to ensure that the electrolyte is not continuously consumed during subsequent cycles.

(4) Good passivation of Al current collector to prevent corrosion of Al foil at high voltage.

(5) Low cost, non-toxic and pollution-free.

The electrolyte has an important influence on battery cycle life, rate performance, applicable temperature and safety. Lithium salts, as providers of lithium ions in the electrolyte, significantly affect the performance of the electrolyte. LiPF6 is currently the most widely used lithium salt, but its thermal stability is poor and sensitive to water. Therefore, finding new lithium salts with excellent performance has become the focus of current research. Although new lithium salts such as LiBF4, LiBOB, LiTFSI, LiFSI, LiPF2O2, and LiTDI can avoid the disadvantages of LiPF6, they have problems in electrical conductivity, corrosion and price. Therefore, no lithium salt has been able to completely replace LiPF6, and researchers need further optimization and development.

Edited by Suzhou Yacoo Science Co., Ltd.

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