- Overview of Liquid Electrolytes
In lithium-ion batteries, the liquid electrolyte acts as an ion conductor, transporting lithium ions back and forth between the positive and negative electrodes during charging and discharging. Since the electrode of the lithium-ion battery is a porous composite electrode, it is composed of active materials [carbon in the negative electrode and lithium transition metal (Co, Ni, Mn) oxides in the positive electrode, respectively], a conductive agent (carbon black) and a polymer binder. The liquid electrolyte must be able to penetrate into the porous electrode and allow unobstructed transport of lithium ions at the liquid-solid interface. Most of the lithium-ion batteries in the market use non-aqueous electrolytes in which lithium salts are dissolved in aprotic organic solvents. Gel-type electrolytes used in polymer batteries are generally considered to be solidified from liquid electrolytes and high molecular weight polymers. Therefore, liquid and gel electrolytes are required to have the same function to some extent.
Many literatures describe liquid electrolytes in lithium or lithium-ion batteries, describing the different properties of aprotic solvents, lithium salts, and other mixtures. The researchers also commented on the above substances from the perspective of solution chemistry. Recently, however, research on liquid electrolytes has mainly focused on electrolyte additives, which can play other roles in Li-ion batteries in addition to their basic functions as ionic conductors.
- Electrolytes with specific functions
The electrolytes of commercial lithium-ion batteries are mostly non-aqueous, and LiPF6 is dissolved in a mixed solvent of cyclic carbonate and chain carbonate to form a solution with a concentration of about 1 mol/L. Optional cyclic carbonates include ethylene carbonate (DEC), propylene carbonate (PC); chain carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) . The chemical structures of these carbonates are shown in Figure 1. Another liquid electrolyte, 1.5mol/L LiBF4/γ-butyrolactone (GBL) + ethylene carbonate, has recently been designed for high-safety thin-plate batteries on the market. Although researchers are working hard to develop new materials, the applications of many solvents and lithium salts are still limited. Electrolytes in which a small amount of additives are added to the above basic electrolytes are called “functional electrolytes”.
Now a variety of new additives have been developed, each with its own specific role. Adding additives to the electrolyte in the optimal proportion can make the electrolyte obtain a series of special functions, which can be called “electrolyte with specific functions”. According to the working principle, additives can be classified as follows: negative electrode film-forming additives; positive electrode protection additives; overcharge protection additives; wettability additives; flame retardant additives; other additives.
Although it is academically incorrect to use the terms “anode” and “cathode” to refer to the negative and positive electrodes in a battery, respectively, when charging, it is customary to use them that way. For the sake of convenience, the additives that affect the negative electrode are called type A (type 1), the additives that affect the positive electrode are called type C (type 2), and the additives that affect the bulk solution are called type B (types 3 to 5). It is also sometimes difficult to clearly define additives and co-solvents (or salts), especially when the additives are aprotic organic reagents (or lithium salts).
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