Electrolyte, as the lifeblood of lithium-ion batteries, bears the crucial responsibility of transporting lithium ions. Its quality directly affects the performance of lithium-ion batteries and, to some extent, their safety. This article will provide a simple analysis and summary of electrolytes from several aspects, including basic knowledge of electrolytes, the mechanism of electrolyte additives, and the development trend of electrolytes.
The general principles for selecting electrolytes are as follows:
(1) It has good electrochemical stability and does not react with positive electrode materials, negative electrode materials, separators, current collectors, binders, etc.;
(2) It has good ionic conductivity, high dielectric constant, low viscosity, and low resistance to ion migration;
(3) It remains liquid over a wide temperature range, typically -40℃ to 70℃, which is suitable for improving the high and low temperature characteristics of batteries;
(4) It can optimally promote the reversible reaction of the electrode, that is, it has high cycle efficiency;
(5) Environmentally friendly, preferably non-toxic or low toxic.
The physical properties of common solvents are shown in the table above. Choose a suitable solvent based on the selection principles of the electrolyte and the system in which it is located. The basic solvents include cyclic, chain, and carboxylic acid ester series.
The commonly used lithium salt is LiPF6, which is very sensitive to moisture. Once it comes into contact with moisture, it reacts, causing problems such as gas production, battery swelling, and severe cycle degradation. Within a temperature range of 20–60°C, the reaction rate constant k of LiPF6 with water in three mixed solvents is: EC + DMC < EC + dec < EC + dec + dmc (as shown in Table 1). The reaction rate of LiPF6 with water increases significantly with increasing temperature; the reaction rate constant at 40°C is 3–4 times that at 20°C, and at 60°C it increases to 8–12 times that at 20°C. Therefore, it is crucial to control the temperature and humidity of the environment when preparing the electrolyte. Currently, the electrolytes used in mass production generally have a moisture content controlled below 20 ppm.
Some commonly used additives are shown in the table above, which can improve a certain aspect of performance with a small amount.
(1) Film-forming additives: VC is widely used. Its main mechanism is that free radical polymerization occurs on the surface of carbon anode to generate polyalkyl lithium carbonate compounds, thereby effectively inhibiting the co-intercalation reaction of solvent molecules; PS, ES, DES, DMS and other substances, whose main mechanism is reduction decomposition to form SEI film. The main components are inorganic salts Li2S, Li2SO3 or Li2SO4 and organic salt ROSO2Li, which greatly enhance the stability of SEI film;
(2) Safety additives: Flame retardant additives reduce the exothermic value and self-heating rate of the electrolyte. These are mainly organic compounds containing phosphorus or phosphorus, such as organophosphorus compounds, organofluorine compounds, and fluoroalkyl phosphates. Overcharge additives primarily utilize redox reactions (ferrocene) and electropolymerization reactions (biphenyl, cyclohexylbenzene).
(3) Multifunctional additives: They have a comprehensive effect of removing water, conducting electricity, and forming films. Amide additives form hydrogen bonds with water and contain lone pairs of electrons, which help stabilize the SEI film.
A comparison of the performance of graphite anodes after cycling with added film-forming additives clearly shows that the surface of the anode material becomes much smoother after cycling with the addition of film-forming additives, while the anode without film-forming additives is much rougher and the cycling performance degrades faster.
After adding flame retardant additives, it is clear that the electrolyte is no longer flammable after adding a certain amount, providing a certain level of safety for high-energy-density batteries.
The impact of two novel additives on battery performance will be introduced below:
As the nickel content and the upper limit of charging voltage increase, the requirements of the positive electrode material for the electrolyte also become more stringent. High-nickel materials will generate NiO during cycling, which will then absorb water and produce gas, causing battery failure.
Some polyphosphates can significantly improve the performance of high-nickel materials.
LiPO2F2 can form films on the surfaces of positive and negative electrodes, significantly improving the performance of high-nickel and high-voltage materials, and is now widely used as a common additive.
With the increase in energy density, the widespread use of high-voltage, high-nickel cathode materials, and silicon-carbon anodes, more and more functional additives will be used.
According to the technical roadmap provided by the expert group, the current focus is on developing high-purity, high-stability electrolytes. Subsequent development will gradually move towards high-voltage, composite lithium salt, and all-solid-state electrolytes, based on material advancements. While the entry barrier to the Chinese electrolyte market is not high at present, there are hidden technological barriers. With the localization of key raw materials, the cost of electrolytes is further decreasing, and Japanese and South Korean companies are beginning to relocate their manufacturing plants to China. It is believed that in the near future, Chinese electrolytes will go global.
