1. Improvement of the thermal stability of the electrolyte
A prerequisite for the successful commercialization of lithium secondary batteries is the safety of the batteries under all circumstances. Lithium batteries have the risk of thermal runaway, smoke, explosion, and burning at high temperatures. As shown in Figure 1, the three main factors for thermal runaway are oxygen in the cathode material, organic liquid electrolyte, and heat generated by the battery.
When the electrolyte is heated above its flash point, serious safety problems can arise due to overheating. Thermal runaway occurs at high temperature mainly from the chemical reaction of oxygen released from the cathode material with the organic electrolyte used as fuel. To solve this problem, when the temperature exceeds a certain value, the positive temperature coefficient device (PTC) will generate a high resistance, on the other hand, when overcharged, the protection circuit module (PCM) blocks the current. Other equipment includes safety valves and diaphragms. It is important to note that PCM is expensive relative to other battery components, and will be more expensive in larger batteries.
Although protective devices are expensive, they must be used under current technical conditions. However, some alternatives have been proposed. For example, we consider choosing a suitable solvent for the synthesis of thermally stable salts. As shown in Figure 2, different lithium salts lead to different levels of chemical activity, which in turn affect heat generation and self-generated heat rates. Another approach is to introduce functional or flame retardant additives. Functional additives can generate and maintain an SEI film on the electrode surface during the first cycle, which is used to delay thermal runaway.
As shown in Figure 3, hexamethoxycyclotriponitrile (HTMP) is a flame retardant that can suppress thermodynamic activity and self-generated heat, thus avoiding thermal runaway. In conclusion, additives used to improve the thermodynamic stability of liquid electrolytes need to meet various requirements, such as high solubility, large potential window, high ionic conductivity, and low viscosity. The reaction between electrodes is closely related to battery safety. By using flame retardants to ensure the thermodynamic stability of the liquid electrolyte, lithium-ion batteries can enter a new market such as electric vehicles, not just mobile devices such as mobile phones, tablets, digital cameras, etc.
2. Development Trend of Liquid Electrolyte
① Organic solvent
The new organic solvent being developed should meet the following requirements: it should promote the reversibility of the electrochemical reaction of the graphite anode, obtain higher ionic conductivity at lower temperatures to improve battery performance, and have better flame retardancy at high temperatures sex. Such as PC, EC chemically modified solvent is more conducive to the reversible chemical reaction of graphite anode. The PC solvent itself is not very reversible for the chemical reaction of the graphite anode. By using a fluorine-containing solvent, the decomposition reaction is slowed down, and the intercalation and deintercalation of lithium is promoted. It can also be used with partially chlorinated EC to inhibit the reduction and decomposition reactions between graphite and PC. Linear ester solvents such as methyl formate (MF) and isopropyl acetate are also considered for use because of their low melting points and low viscosity at low temperatures. Because of their low solubility for lithium salts alone, they are often used in combination with EC. For non-flammable solvents, phosphate esters, fluoroesters and fluoroethers are suitable. Combining asymmetric phosphate ethers such as EDMP or BuDMP with EC/DEC has been reported to produce flame retardancy. EFE is a representative of a fluoroether and a flame retardant with good charge-discharge reversibility. Although it poses some problems in terms of high rate capability, adding excess EFE can prepare safer electrolytes. Further development of new organic solvents is required to meet various requirements such as safety, low price, and high performance.
② Lithium salt
Among the lithium salts of Li-ion batteries, LiPF6 has poor thermal stability and is prone to hydrolysis when exposed to moisture, while LiBF4 has low ionic conductivity and generates SEI films on the electrode surface. To address these deficiencies while meeting the necessary requirements, by using molecular design, researchers are developing new lithium salts with different structures. A typical case is cations containing weakly conjugated fluorine such as perfluoroalkanesulfonates or imines. These cations are replaced by electron acceptors such as fluorine or CF3, etc., thus resulting in reduced intermolecular interactions and electrostatic interactions with lithium ions. However, their use in lithium secondary batteries is limited due to problems such as low stability, high price, corrosion of conductive agents, and the like. Other salts studied are borates and chelates. There are also attempts to combine insoluble salts such as LiF with ligand ions in organic solvents or to use known salts for better addition.
③ Ionic liquid
As concerns about the safety of lithium secondary batteries have intensified, there has been increasing interest in non-flammable and flame-retardant ionic liquid electrolytes. Most studies have focused on improving the performance of batteries at room temperature by promoting reduction stability through new cations or additives, selection of new anode materials, and viscosity reduction. Recently, the reversible capacity of graphite anode of ionic liquid composed of EMI-FSI and LiTFSI is as high as 360 mAh/g. In addition to this, studies have aimed to solve the problem of high viscosity of ionic liquids by reducing the current density and reducing the thickness of the electrode.
