In the electric vehicle market, with the surge in demand for lithium-ion batteries, researchers are stepping up the development of new electrolytes. The new electrolyte must not only ensure the performance, life and safety of the battery, but also meet the requirements of high-energy anode materials (such as silicon) and cathode materials (NMC 811, etc.). High-purity electrolyte can reduce battery side reactions and avoid premature battery degradation, which is of great significance.
The new electrolyte additive LiTDI introduced by Arkema can not only extend battery life and speed up charging, but also solve the problem of material purity and stability for high-capacity battery materials necessary for electric vehicles.
LiTDI, lithium 4,5-dicyano-2- (trifluoromethyl) isopyrazole, was originally discovered by Warsaw University of Technology (WUT), Warsaw University of Technology, French National Center for Scientific Research (CRNS) and French University of Amiens (University of Amiens) Cooperative research on its synthesis and purification. When Dr. Gregory Schmidt was in the team of Professor Michel Armand of the University of Amiens in France, he tried to apply this lithium salt to lithium-ion batteries, and focused on two aspects of synthesis and electrolyte formulation. Research results show that additives made with this lithium salt can greatly improve electrolyte performance. The researchers integrated this molecule into Arkema ’s powerful battery platform and renewable energy solutions.
LiTDI has many advantages, and its molecular structure has been tailored to greatly improve electrochemical stability and promote ion dissociation. The isopyrazole ring promotes negative charge dissociation through resonance effects. Second, the electronegativity / weight ratio of the two nitrile groups is optimized, which can further promote the dissociation of positive charges. Finally, the F3 group attached to the isopyrazole ring is beneficial for maintaining electrochemical stability. Cyclic voltammetry studies have shown that the molecule has no reactivity below 4.6 ~ 4.7V. In addition, DSC research shows that such molecules have extremely high thermal stability, and will only degrade when the temperature is higher than 250 ℃, which is the best choice for lithium-ion batteries that work under high pressure and high temperature for a long time.
In the electrolyte, in addition to its inherent stability, LiTDI is also an important dehumidifier. Lithium ions and carbonitrile groups interact with water molecules to trap water molecules through hydrogen bonding, thereby effectively inhibiting the hydrolysis of LiPF5. LiPF5 is a decomposition product of LiPF6 on the negative electrode. It is a strong Lewis acid and the main cause of electrolyte solvent degradation. In addition, due to the degradation of LiPF6, the nitrile group interacts with the HF molecule, which can further reduce the parasitic reaction on the positive side. By reducing the impact of impurities on different electrolyte components, only 1% LiTDI can be added to improve electrolyte stability and extend battery life.
In addition, LiTDI helps to form a passivation layer on the aluminum current collector and can also have an important impact on battery performance. Researchers have tried to find LiPF6 alternatives suitable for high-voltage applications, but the corrosion phenomenon that occurs on the positive electrode current collector can increase the internal resistivity and reduce the positive electrode capacity. For batteries containing new electrolytes based on alternative salts (LiFSI, etc.), a stable aluminum protective layer formed of LiTDI helps increase battery life and supports the use of high-voltage electrodes (such as NMC 622).
It is worth mentioning that LiTDI helps to form a stable solid electrolyte interface film (SEI) and protects the negative electrode from degradation by organic solvents. LiTDI is combined with traditional SEI additives (such as FEC or VC) to help the growth of LiF mineral phases and promote the formation of polymer phases by defluorinating CF3 groups. The resulting SEI film is thinner and has strong cross-linking, which helps reduce resistivity and reduce initial capacity loss. This phenomenon occurs not only on the graphite negative electrode, but also on the silicon-based positive electrode. For battery life and battery internal resistance, SEI additives can play a more important role.
To sum up, in Li-ion batteries, LiTDI additives and traditional SEI additives work synergistically, the battery impedance is reduced, and the rapid charge and discharge performance is significantly improved. In addition, this salt can improve electrolyte purity and stability, enabling traditional electrolytes to cycle at high temperatures (> 45 ° C). Finally, LiTDI is a very good electrolyte additive, whether it uses graphite negative electrode or silicon-based positive electrode, it can significantly extend battery life.