What is liquid electrolyte?

Electrolytes are divided into liquid electrolytes (including traditional non-aqueous solvent electrolytes and ionic liquid electrolytes that have emerged in recent years) and solid electrolytes (including inorganic solid electrolytes and polymer electrolytes)

Figure 1 Liquid electrolyte and solid electrolyte

The electrolyte is an important part of the battery, and it is responsible for the transmission of ions between the positive and negative electrodes through the battery. It has an important impact on the battery's capacity, operating temperature range, cycle performance and safety performance. According to the morphological characteristics of electrolytes, electrolytes can be divided into two categories: liquid and solid.

Electrolytes used in lithium-ion batteries should generally meet the following basic requirements:
①High ionic conductivity, generally 1×10^-3~2×10^-2S/cm;
②High thermal and chemical stability, no decomposition in a wide temperature range;
③Wide electrochemical window, keep the stability of electrochemical performance in a wide voltage range;
④ It has good compatibility with other parts of the battery, such as electrode materials, electrode current collectors and separators;
⑤Safe, non-toxic and non-polluting.

The following will describe the liquid electrolyte and understand the basics of liquid electrolyte.

In traditional batteries, the electrolyte system with water as the solvent is usually used, but since the theoretical decomposition voltage of water is 1.23V, considering the overpotential of hydrogen or oxygen, the battery voltage of the electrolyte system with water as the solvent is only 2V. About (such as lead-acid batteries); in lithium-ion batteries, the battery's working voltage is usually as high as 3~4V. The traditional aqueous solution system is no longer suitable for the requirements of the battery, so a non-aqueous electrolyte system must be used as the electrolyte of the lithium ion battery. Organic solvents and electrolyte salts that do not decompose under high voltage are the key to the research and development of liquid electrolytes for lithium-ion batteries.

Non-aqueous organic solvents are the main components of the electrolyte. Many performance parameters of the solvent are closely related to the performance of the electrolyte, such as the viscosity, dielectric constant, melting point, boiling point, flash point, and redox potential of the solvent. Operating temperature range, electrolyte lithium salt solubility, electrode electrochemical performance and battery safety performance have important effects. A good solvent is an important guarantee for low internal resistance, long life and high safety of lithium-ion batteries. The non-aqueous organic solvents used in lithium ion batteries mainly include carbonates, ethers and carboxylates.

Carbonate (PC) mainly includes two types of cyclic carbonate and chain carbonate. Carbonate solvents have good chemical and electrochemical stability and a wide electrochemical window, which are widely used in lithium ion batteries. . In commercialized lithium-ion batteries, carbonates are basically used as electrolyte solvents. The mixed solvent composed of propylene carbonate and dimethyl ether (DME) is still the representative solvent for primary lithium batteries. Due to its low melting point (-49.2°C), boiling point (241.7°C) and flash point (132°C), the electrolyte containing PC has good low-temperature performance and safety performance, but PC has anisotropic, layered The compatibility of various graphite-like carbon materials of the structure is poor, and an effective solid electrolyte interface (SEI) film cannot be formed on the surface of the graphite-like electrode. During the discharge process, it is intercalated with the solvated lithium ions into the graphite layer and undergoes violent reduction. Decomposition produces a large amount of propylene, which leads to the peeling of graphite flakes, which destroys the graphite electrode structure and greatly reduces the cycle life of the battery. Therefore, in the current lithium-ion battery system, PC is generally not used as an electrolyte component. Ethylene carbonate (EC) is currently the main solvent component in most organic electrolytes. The dielectric constant of EC is very high. The main decomposition product ROCO₂Li can form an effective, dense and stable SEI film on the surface of graphite. EC has good compatibility with graphite anode materials, which greatly improves the cycle life of the battery, but the melting point of EC High (36°C), high viscosity, and poor low temperature performance of the electrolyte with EC as a single solvent, so EC alone is generally not used as a solvent. On the contrary, chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and other solvents have lower viscosity and lower medium. The electric constant, lower boiling point and flash point cannot form an effective SEI film on the surface of graphite electrodes or lithium electrodes, and generally cannot be used alone as a solvent in lithium-ion batteries. In general, a mixture of EC and low-viscosity chain carbonate is used as a solvent for the electrolyte of lithium-ion batteries. The electrolyte has good conductivity when the temperature is not too low (for example, above -20°C). Generally speaking, low-viscosity solvents have a low boiling point, although a large amount of low-viscosity chain carbonate is beneficial to improve the low-temperature performance of the electrolyte.

Although the two-component liquid-solid phase diagrams of EC, PC, DMC, EMC, DEC, DMEC, and iBC are different, all the combinations can form a simple eutectic system. The melting point of DEC is -74.3°C, and DEC is more effective than EMC and DMC in reducing the liquefaction temperature of the binary system composed of EC. The liquid range of the binary system with similar melting point and similar molecular structure is easy to expand to low temperature. EMC has a low melting point (-55°C) and can improve the low temperature performance of the battery as a co-solvent. For example, Li/LiCoO₂ or graphite/LiCoO₂ button batteries use 1mol/L LiPF6 1:1:1 EMC-DMC-EC electrolyte, which can work at -40°C. During the first charging of the battery, EMC will decompose to produce DMC and DEC. In the mixed solvent of DMC and DEC, transesterification will also occur to produce EMC:
2EMC↔DMC+DEC

The structures of the above-mentioned various carbonates are shown in Figure 2.

Figure 2 The molecular structure of various non-aqueous organic solvents for lithium-ion batteries

Ether organic solvents mainly include cyclic ethers and chain ethers. There are cyclic tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,3-dioxolane (DOL) and 4-methyl-1,3-dioxolane (4-MeDOL) Wait. Mixed solvents composed of THF, DOL, and PC are used in primary lithium batteries. Due to their poor electrochemical stability, ring-opening polymerization is prone to occur and cannot be used in lithium ion batteries. 2-MeTHF has a low boiling point (79°C) and a low flash point (-11°C). It is easily oxidized to form peroxides and is hygroscopic, but it can form a stable SEI film on the lithium electrode, such as LiPF6-EC -Adding 2-MeTHF to DMC can effectively inhibit the formation of dendrites and improve the cycle efficiency of lithium electrodes. Chain ethers mainly include dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), 1,2-dimethoxypropane (DMP) and diglyme (DG), etc. . With the growth of the carbon chain, the oxidation resistance of the solvent increases, but at the same time the viscosity of the solvent also increases, which is not good for improving the conductivity of the organic electrolyte. Commonly used chain ethers are DME, which has a strong ability to integrate lithium ions, and can form a stable LiPF₆-DME complex with LiPF₆. The lithium salt has a higher solubility and a smaller solvation ion radius in it. The electrolyte has high conductivity. The structure of the composite formed by lithium salt and DME is shown in Figure 3, but DME is easily oxidized and decomposed by reduction, and it is difficult to form a stable SEI film in contact with lithium ions. DG is a solvent with good oxidation stability among ether solvents. It has a relatively large molecular weight and relatively small viscosity. It has strong complexing and coordinating ability for lithium ions and can effectively dissociate lithium salts. It has good compatibility with carbon negative electrodes and has a thermal stability of at least 200°C, but the low temperature performance of the electrolyte system is poor.

Figure 3 Schematic diagram of the structure of 2DME-Li⁺

Carboxylic acid esters also include cyclic carboxylic acid esters and chain carboxylic acid esters. The most important organic solvent in cyclic carboxylic acid esters is γ-butyrolactone (γ-BL). The dielectric constant of γ-BL is lower than that of PC, and its solution conductivity is also lower than that of PC. It has been used in primary lithium batteries. Decomposition in water and greater toxicity are its main disadvantages. Chain carboxylic acid esters mainly include methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), methyl propionate (MP) and ethyl propionate (EP). Chain carboxylic acid esters generally have a relatively low melting point. Adding an appropriate amount of chain carboxylic acid esters to the organic electrolyte will improve the low-temperature performance of the battery. The battery using EC-DMC-MA as the electrolyte can release 94% of the room temperature capacity at -20°C, but the cycleability is poor. The batteries using EC-DEC-EP and EC-EMC-EP as electrolytes can release 63% and 89% of the room temperature capacity at -20°C, and the initial capacity and cycleability at room temperature and 50°C are good.

The replacement of hydrogen atoms in organic solvent molecules by other groups (such as alkyl or halogen atoms) will increase the asymmetry of the solvent molecules, thereby increasing the dielectric constant of the organic solvent and increasing the conductivity of the electrolyte. For the same type of organic solvent, as the molecular weight increases, its boiling point, flash point, and oxidation resistance will be improved, so that the electrochemical stability of the solvent and the safety of the battery will be correspondingly improved. For example, the halogenated substances of organic solvents have lower viscosity and high stability. They are generally not easy to decompose and burn, which will make the battery have better safety. Trifluoromethyl ethylene carbonate (CF₃-EC) has very good physical and chemical stability, and also has a high dielectric constant, is not easy to burn, and can be used as a flame retardant in lithium-ion batteries. For example, chloroethylene carbonate (Cl-EC) and fluoroethylene carbonate (F-EC) can form a stable SEI film on the surface of the carbon negative electrode to inhibit the co-intercalation of solvents and reduce the loss of irreversible capacity.

In most cases, a new type of electrolyte solvent containing acidic boron atoms is connected to the heterocyclic ring through the reaction of boric acid or oxide with ethylene glycol. The ethylene glycol borate is called BEG. This type of electrolyte solvent has great corrosion resistance for dissolving salts and stabilizing alkali metals. In some cases, it can also stabilize other solvents, especially olefin carbonates, to prevent anode decomposition. The 1,3-propanediol borate (BEG-1) containing two linked borate groups gives an electrochemical stability window of more than 5.8 in a mixed solvent obtained by mixing one part of BEG-1 and two parts of EC V (comparison: when the EC alone has a width of 4.5V on lithium metal, it remains bright after several days of soaking at 100°C)