By Zheng Honghe, College of Energy, Soochow University
As new energy vehicle market and stored energy market are developing at a fast pace, the demand for high-performance lithium-ion battery is booming, spurring up demand for liquid electrolyte. In 2018, the shipment of electrolyte for lithium ion battery in China exceeded 170 kt, an increase of more than 50% from a year ago. In 2025, China's demand for lithium-ion battery electrolyte will rise up to 1 million tons.
Status quo of electrolyte technology
The improvement in performance of electrolyte is still one of the key scientific and technical problems for lithium ion batteries to possess high performance, high safety and long life. Electrolyte producers should make efforts to meet the fundamental requirements of high voltage, high and low temperature performance, high safety and long life in the future. Now, they are applying three specific technical routes.
1. Development and application of fluorinated solvent technology
The main technical route of fluorinated solvent is to develop some fluorinated ether and ester solvents as solvent or additive of electrolyte. Fluorinated organic solvents not only have flame retardant property, but also have excellent film-forming property on the electrode surface.
Hitachi has introduced 1,1,2,2-tetrafluoro-2- (1,1,2,2-tetrafluoroethoxy)-ethane (HCF2CF2OCF2CF2H, referred to as D2) as the co solvent of electrolyte, which is conducive to prolonging the cycle life of batteries. HCF2CF2OCH2CF2CF2H (HFE) not only has flame retardant property, but also can significantly improve the cycle performance of ternary batteries. Fluoroethylene carbonate, methyl-2,2,2-trifluoroethyl carbonate and ethyl-2,2,2-trifluoroethyl carbonate all have high antioxidant property and have stabilizing effect in high voltage lithium-ion batteries. However, when excessive H atoms are replaced by F atoms or the amount of fluorinated solvent is too large, the solubility of LiPF6 will decrease obviously and the internal resistance of the battery will increase greatly. Therefore, it should be well considered before applying this technology.
2. High salt and mixed salt technology
High salt system refers to the electrolyte system with lithium salt concentration over 2M. It is highlighted that it can change the solvation environment and solvation structure of ions so as to significantly improve the rate performance and service life of batteries. Therefore, it is well performed in lithium-ion battery and lithium sulfur battery. However, the practical application of the high salt system is not wide as a result of high viscosity of the system, poor wettability of electrolyte to electrode materials, and expensive cost.
Lithium salt additives, including LiFSI, LiTFSI, LiPO2F2, LiBOB, LiODFB and LiODFP, protect electrode through the oxidation or reduction of anions on the surface of electrode. These inorganic solid additives are getting mature in industrial synthesis and application with the excellent advantages as below: they are easy to use; with better properties than organic combustible additives, they are conducive to the safety of batteries; they can not only passivate and protect the negative surface, but also protect the surface of positive electrode. For example, LiPO2F2 and LiODFB can remarkably improve the positive and negative electrodes of a battery, so they have been widely used in the whole battery in recent years.
LiODFB can significantly improve the cycle performance of NCA/graphite battery at high temperature of 60℃. Different from LiBOB, LiODFB battery shows lower internal resistance to film formation, so it can meet power batteries’ need for fast charging. Of course, these additives have disadvantages, mainly divided into the following three points: the price is relatively high, especially because the cost is significantly higher than the main salt LiPF6; excessive use of them can easily cause the internal resistance to increase, especially LiBOB; some salts, such as LiFSI, have certain corrosivity to aluminum foil, especially under the condition of high concentration and high potential, so it is necessary to add some other additives to inhibit corrosion.
3. Electrolyte additives
As the core raw material of lithium ion battery electrolyte, electrolyte additives are crucial to determine the electrolyte performance of high specific energy lithium ion battery. In the commonly used electrolyte ingredient for lithium ion batteries, lithium salts account for about 15% of the electrolyte weight, solvents about 80% and additives about 5%. At present, the cost of additives take up about 20-30% of the total cost of electrolyte.
The domestic lithium-ion battery electrolyte market value was close to RMB10 billion, and the additives market was close to RMB3 billion in 2018. The rapid growth in electric vehicles is driving up the development of lithium-ion battery technology. In the context, the variety of electrolyte additives is increasing. The mainstream products include organic lithium salt additives, fluorinated organic solvent additives, sulfonate additives, phosphate or phosphite additives. In addition, there are boric acid ester additives, nitrile additives, sulfone additives and ionic liquid additives.
Three development trends
The key to electrolyte technology is formula, and the development and application of electrolyte additives are the main cause of differentiation of products. The formula of electrolytes and the technology of additives in early period were mostly monopolized by foreign enterprises such as Mitsubishi, Ube and Daikin. Today, domestic electrolyte products have been replacing overseas ones with obvious advantages. A number of domestic suppliers, led by Capchem, Tinci Materials, HSC, Huarong and DFD, have been able to stably provide products in batches to overseas battery giants such as LG, Samsung, Panasonic and Sony.
The competition of high-end electrolytes, especially those with high safety, high voltage, wide temperature range and high stability, will be highly concerned. The main direction for future development will be:
1. High safety: with the emergence of solid electrolytes, solving the safety problem is the only way out for liquid electrolytes.
2. Simplification: the composition of electrolytes is more and more complex in line with the emergence of various electrolyte additives in recent years, so it is necessary for electrolytes to be used as the bridge and medium again – a basic function for them.
3. The fourth-generation development: the development of lithium ion battery has gone through three generations, and the fourth-generation electrolyte system is looming.
With the development and application of high-capacity positive and negative materials for lithium ion batteries, electrolytes need to be developed to have higher performance, especially in terms of reliability, durability and safety for battery operation, and thus it is necessary for the improvement in technical innovation of China's electrolyte industry. In addition, the categories, properties and application methods of innovative additives are very important for the future development of high-performance additives and lithium ion batteries.
