Electrolytes are an important component of lithium-ion batteries. They not only transport and conduct current at the positive and negative electrodes, but also largely determine the battery's working mechanism, affecting its specific energy, safety performance, rate charge and discharge performance, cycle life, and production cost.
The electrolyte plays a crucial role in lithium-ion batteries, acting as a conductor of electrons between the positive and negative electrodes. This ensures the high voltage and high specific energy of lithium-ion batteries. Electrolytes are generally prepared under specific conditions and in specific proportions from high-purity organic solvents, lithium electrolyte salts (lithium hexafluorophosphate, LiFL6), and necessary additives.
1. Organic solvents
Organic solvents form the main component of electrolytes, and the performance of the electrolyte is closely related to the performance of the solvent. Commonly used solvents in lithium-ion battery electrolytes include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Propylene carbonate (PC) and dimethyl ethylene glycol ether (DME), solvents commonly used in primary lithium batteries, are generally not used. PC, used in secondary batteries, has poor compatibility with the graphite anode of lithium-ion batteries. During charge and discharge, PC decomposes on the surface of the graphite anode, causing the graphite layer to peel off, resulting in decreased battery cycle performance. However, a stable SEI film can be established in EC or EC+DMC composite electrolytes. It is generally believed that a mixture of EC and a chain carbonate is an excellent electrolyte for lithium-ion batteries, such as EC+DMC or EC+DEC. For the same electrolyte lithium salt, such as LiPF6 or LiClO4, the PC+DME system consistently exhibits the worst charge-discharge performance for mesophase carbon microspheres (C-MCMB) (compared to the EC+DEC and EC+DMC systems). However, this is not absolute; when PC is used with relevant additives in lithium-ion batteries, it can improve the battery's low-temperature performance.
Organic solvents must be strictly controlled in quality before use, requiring a purity of 99.9% or higher and a moisture content below 10 x 10⁶. Solvent purity is closely related to stable voltage. The oxidation potential of organic solvents meeting purity standards is around 5V, and this oxidation potential is significant for research on preventing battery overcharging and ensuring battery safety. Strict control of the moisture content in organic solvents has a decisive impact on the preparation of qualified electrolytes.
Reducing the moisture content to below 10*106 can decrease the decomposition of LiPF6, slow down the decomposition of the SEI membrane, and prevent gas expansion.
The required moisture content can be achieved by using molecular sieve adsorption, atmospheric or vacuum distillation, or the introduction of inert gas.
2. Electrolyte lithium salt
LiPF6 is the most commonly used lithium electrolyte salt and represents the future direction of lithium salt development. Although LiClO4 and LiAsF6 are also used as electrolytes in laboratories, batteries using LiClO4 have poor high-temperature performance. Furthermore, LiClO4 itself is prone to explosion upon impact and is a strong oxidizing agent, making it unsuitable for large-scale industrial use of lithium-ion batteries.
LiPF6 is stable at the negative electrode, has a large discharge capacity, high conductivity, low internal resistance, and fast charge and discharge speed. However, it is extremely sensitive to moisture and HF acid, and is prone to reaction. It can only be operated in a dry atmosphere (such as in a glove box with ambient moisture less than 20 × 10⁻⁶). It is also not resistant to high temperatures, and decomposes at 80℃~100℃ to produce phosphorus pentafluoride and lithium fluoride, which are difficult to purify. Therefore, when preparing the electrolyte, the self-decomposition caused by the exothermic dissolution of LiPF6 and the thermal decomposition of the solvent should be controlled.
3. Additives
There are many types of additives, and different lithium-ion battery manufacturers have different requirements for the purpose and performance of their batteries, resulting in differences in the focus of the additives they choose. Generally speaking, the additives used have three main uses:
(1) Adding anisole to the electrolyte improves the performance of the SEI film. Adding anisole or its halogenated derivatives to the electrolyte of lithium-ion batteries can improve the cycle performance of the battery and reduce irreversible capacity loss. Huang Wenhuang studied its mechanism and found that the reaction between anisole and the reduction product of the solvent to generate LiOCH is beneficial for the formation of a highly efficient and stable SEI film on the electrode surface, thereby improving the cycle performance of the battery. The discharge plateau of a battery can measure the energy that the battery can release above 3.6V and reflects the high-current discharge characteristics of the battery to a certain extent. In actual operation, we found that adding anisole to the electrolyte can prolong the discharge plateau of the battery and increase the discharge capacity of the battery.
(2) Adding metal oxides to reduce trace amounts of water and HF acid in the electrolyte. As mentioned earlier, lithium-ion batteries have very strict requirements regarding the amount of water and acid in the electrolyte. Carbodiimide compounds can prevent LiPFs from hydrolyzing into acid. In addition, some metal oxides such as Al2O3, MgO, BaO, Li2Co3, and CaCO3 are used to remove HF. However, the acid removal rate is too slow compared to the hydrolysis of LiPFs, and it is difficult to filter out completely. In lithium-ion battery electrolytes, the total content of the three elements Li, P, and F is 96.3%, while the total content of other important impurity elements such as Fe, K, Na, Cl, and Al is 0.055%.
(3) Prevent overcharging and over-discharging.
Battery manufacturers have a pressing need for improved overcharge and discharge resistance in batteries. Traditional overcharge protection relies on internal battery circuitry; the current approach seeks to add additives to the electrolyte, such as sodium imidazole, biphenyl compounds, and carbazole compounds. These compounds are currently under research.
