How to improve the safety performance index of lithium battery packs

       The safety of lithium battery packs has always been a concern of the majority of consumers. Although lithium battery safety cannot be cured, it is controllable and preventable. Facing it correctly and actively exploring some new safety technologies will help promote batteries. Technological advances, such as improving the thermal stability of the material interface, developing self-excited thermal protection technology for monomers, and system thermal expansion prevention technology, can effectively improve the safety of the battery system.

       One: Surface coating. The thermal decomposition of the positive electrode and the oxygen evolution caused by it are mainly due to the reaction between it and the interface (electrolyte), so we can cover the active surface of the positive electrode with a thermally stable protective layer. For example, after coating the surface of the high-nickel positive electrode with phosphoric acid film or lithium phosphate, the direct contact between the high-nickel material and the electrolyte can be reduced, thereby reducing the intensity of side reactions and heat generation. Common coating materials include phosphates, oxides, fluorides, and can also be some polymers.

       Two: Build a concentration gradient. The high nickel cathode is unsafe. In addition to its poor thermal stability, it is more important that nickel has a very strong oxidative decomposition effect on the electrolyte, and the heat release of the material itself is not that large, but after adding the electrolyte, Its heat production temperature and heat production are sharply increased, the reason is that the interface reaction of the electrolyte accounts for a large part. If we take high nickel as the core and use some low nickel content material as the shell, let it have a concentration gradient inside and outside, which will help reduce the reactivity of the material interface and improve the safety of the battery.

       Three: Improve the stability of SEI film. As mentioned above, thermal failure often starts from the decomposition of the negative SEI film. If we adopt some methods to increase the decomposition temperature of the SEI film and improve the thermal stability, it will play a vital role in the safety of lithium-ion batteries. Current research shows that some organic lipids, some organic phosphates, and even some fluorine-containing lithium salts can effectively improve the thermal stability of the negative SEI film and increase its decomposition temperature.

       Four: Establish monomer self-excited thermal protection. Its technical principle is to use temperature-sensitive materials to cut off the electron transmission or ion transmission on the electrodes at dangerous temperatures, and even shut down the battery reaction, thereby stopping heat production. For example, PTC material, as the temperature increases, the material will change from a good conductive state to an insulating state, cutting off the circuit. Using the PTC material as the coating of the polar fluid or as the conductive agent of the electrode or as the surface modification layer of the active material can effectively realize the self-heating protection of the single cell. Similarly, there is a microsphere modified diaphragm. When the temperature rises, the microsphere melts, closing the pores on the diaphragm and causing the battery reaction to shut down.

       At present, ensuring the safety of power batteries through the structural design of the battery system is also the focus of all battery companies and OEMs. At the same time, improving the test intensity of power batteries is also the key to further ensuring battery safety. At the Power Battery Technology Summit, Wang Fang, an expert from China Automotive Technology Research Center, also introduced the "Compulsory Standards for Safety Requirements for Power Batteries for Electric Vehicles" that the Ministry of Industry and Information Technology posted on the Internet for comments on January 10. Strengthen the test content.