Guide for the Selection of Lithium Salts in the Electrolyte of Lithium-Ion Batteries
Introduction
The power battery is a key component of an electric vehicle, and its performance directly determines key parameters such as the cruising range and environmental adaptability of the electric vehicle. The current mainstream power battery is a lithium-ion battery, which has the advantages of high energy density, small size, no memory effect, and long cycle life, but it still has the problem of insufficient cruising range. The electrode material determines the energy density of the battery, and the electrolyte in the electrode material is called the "blood" of the battery. In lithium-ion batteries, the main function is to transport lithium ions and form a solid electrolyte film at the interface between the positive and negative electrodes.
Lithium -ion battery electrolyte is mainly composed of lithium salts, solvents and additives. Lithium salt is an important part of lithium-ion battery electrolyte, which determines the power density, energy density, cycle and safety performance of the battery to a large extent [1].
Lithium Salt Selection Criteria
There are many types of lithium salts, which often need to be selected according to specific system requirements. In general, high-quality lithium salts have the following requirements:
(1) Ensure high ionic conductivity of the electrolyte. Has a small degree of association and is easy to dissolve in organic solvents;
(2) A stable and low-resistance SEI film can be formed. It is beneficial to improve the cycle performance of the battery;
(3) Good chemical stability. No harmful side reactions with electrode materials, electrolytes, diaphragms, etc.;
(4) Low cost. The preparation process is simple, non-toxic and pollution-free.
Common Lithium Salts and Their Scope of Application
Figure 1: Common lithium salts and their structural formulas
Lithium Hexafluorophosphate: LiPF6
LiPF6 is the most widely used lithium salt. The single property of LiPF6 is not the most prominent, but it has relatively optimal comprehensive performance in carbonate mixed solvent electrolyte. LiPF6 has the following outstanding advantages: (1) It has suitable solubility and high ionic conductivity in non-aqueous solvents; (2) It can form a stable passivation film on the surface of Al foil current collector; (3) Synergistic The carbonate solvent forms a stable SEI film on the surface of the graphite electrode. However, LiPF6 has poor thermal stability and is prone to decomposition reactions. The side reaction products will destroy the SEI film on the electrode surface, dissolve the active components of the positive electrode, and cause the cycle capacity to decay.
Lithium Tetrafluoroborate: LiBF4
LiBF4 is a common lithium salt additive. Compared with LiPF6, LiBF4 has a wider operating temperature range, better stability at high temperature and better performance at low temperature.
Lithium Dioxalate Borate: LiBOB
LiBOB has high electrical conductivity, wide electrochemical window, and good thermal stability. Its biggest advantage is that it has good film-forming performance and can directly participate in the formation of SEI film.
Lithium Difluorooxalate Borate: LiDFOB
Structurally, LiDFOB is composed of half molecules of LiBOB and LiBF 4 , which combines the advantages of good film-forming properties of LiBOB and good low-temperature performance of LiBF 4 . Compared with LiBOB , LiDFOB has higher solubility in linear carbonate solvents and higher electrolyte conductivity. Its high-temperature and low-temperature performance are better than LiPF 6 and have good compatibility with the positive electrode of the battery. It can form a passivation film on the surface of Al foil and inhibit the oxidation of the electrolyte.
Lithium Bistrifluoromethanesulfonyl imide: LiTFSI
CF3SO2–group in the LiTFSI structure has a strong electron-withdrawing effect, which intensifies the delocalization of negative charges, reduces ion association and pairing, and makes the salt have a higher solubility. LiTFSI has high electrical conductivity, high thermal decomposition temperature and not easy to be hydrolyzed. However, when the voltage is higher than 3.7V, the Al current collector will be severely corroded.
Lithium Bisfluorosulfonyl Imide: LiFSI
The fluorine atom in the LiFSI molecule has a strong electron-withdrawing property, which can delocalize the negative charge on the N, the ion association and pairing effect is weak, and Li + is easy to dissociate, so the conductivity is high.
Lithium Difluorophosphate: LiPO2F2
LiPO2F2 has better low-temperature performance, and can also improve the high-temperature performance of the electrolyte. As an additive, LiPO2F2 can form an SEI film rich in LixPOyFz and LiF on the surface of the negative electrode, which is beneficial to reduce the battery interface impedance and improve the cycle performance of the battery. But LiPO2 F2 also has the disadvantage of low solubility.
Next Generation Lithium Salt Materials
Current efforts in lithium salts, including the development of new lithium salts and combinations of two or more lithium salts, are aimed at improving battery performance. Electrolyte types offer more possibilities, as some lithium salts have been thoroughly studied. Further understanding of the compatibility and properties of lithium salts, solvents and other additives is necessary.
In their latest paper, Cui et al. elaborate on the development of mixed-salt electrolytes (Fig. 2a) classified by LIBs and other batteries (e.g., LMBs, Li-S batteries, solid-state lithium batteries), while also considering the electrolyte solvent. Research progress [2]. Studies have shown that the mixed salt electrolyte of lithium -ion batteries can improve the thermal stability (safety) of the battery, inhibit the corrosion of the aluminum foil of the positive electrode current collector, improve the wide temperature range (or high temperature, low temperature) performance of the battery, and improve the corrosion resistance of the battery. Good results have been achieved in many aspect, such as the formation of a good interfacial layer on the electrodes on both sides, the protection of the lithium metal anode, and the realization of high ionic conductivity (Figure 2 b).
Figure 2: (a) Development process of mixed lithium salt electrolyte; (b) Functional classification of mixed lithium salt
References:
[1] Zhang S S. A review on electrolyte additives for lithium-ion batteries[J]. Journal of Power Sources, 2006, 162(2): 1379-1394. https://doi.org/10.1016/j.jpowsour.2006.07.074
[2] Xu G, Shangguan X, Dong S, et al . mit gemischten Lithiumsalzen für Lithium‐Batterien [J]. Angewandte Chemie, 2020, 132(9): 3426-3442.
http://dx.doi.org/10.1002/ange.201906494