What are the safety risks of lithium-ion batteries?
Generally speaking, safety problems of lithium-ion batteries manifest themselves as burning or even explosion. The root cause of these problems is the thermal runaway inside the battery. In addition, some external factors such as overcharge, fire, squeeze, Problems such as puncture and short circuit can also cause safety problems. Lithium-ion batteries will generate heat during charging and discharging. If the heat generated exceeds the battery’s heat dissipation capacity, the lithium-ion battery will overheat, and the battery material will decompose the SEI film, the electrolyte, the positive electrode, and the negative electrode. Destructive side reactions such as the reaction of the electrolyte and the reaction of the negative electrode and the binder.
1. The safety hazards of the cathode material
When the lithium-ion battery is used improperly, the internal temperature of the battery will increase , So that the positive electrode material will decompose the active material and oxidize the electrolyte. At the same time, these two reactions can generate a lot of heat, causing the battery temperature to rise further. Different delithiation states have very different effects on the lattice transformation of the active material, the decomposition temperature and the thermal stability of the battery.
2. The potential safety hazards of negative electrode materials
The negative electrode material used in the early days is lithium metal, and the assembled battery has repeatedly Lithium dendrites are easy to produce after charging and discharging, which can pierce the diaphragm and cause short circuit, leakage and even explosion of the battery. Lithium intercalation compounds can effectively avoid the generation of lithium dendrites and greatly improve the safety of lithium-ion batteries. As the temperature increases, the carbon negative electrode in the state of lithium intercalation first reacts exothermically with the electrolyte. Under the same charging and discharging conditions, the heat release rate of the reaction between the electrolyte and the lithium-intercalated artificial graphite is much greater than that of the lithium-intercalated mesophase carbon microspheres, carbon fibers, coke, etc.
3. The safety hazards of diaphragm and electrolyte
The electrolyte of lithium ion battery is composed of lithium salt and organic solvent Mixed solution, in which the commercial lithium salt is lithium hexafluorophosphate, which is prone to thermal decomposition at high temperatures and undergoes thermochemical reactions with trace amounts of water and organic solvents to reduce the thermal stability of the electrolyte. The organic solvents of the electrolyte are carbonates. Such solvents have a low boiling point and flash point, and are easy to react with lithium salt to release PF5 at high temperatures, and are easy to be oxidized.
4. Potential safety hazards in the manufacturing process
In the manufacturing process of lithium-ion batteries, electrode manufacturing, battery assembly, etc. The process will have an impact on the safety of the battery. The quality control of various processes such as positive and negative electrode mixing, coating, rolling, cutting or punching, assembling, filling of electrolyte, sealing, forming and other processes all affect the performance and safety of the battery. The uniformity of the slurry determines the uniformity of the active material distribution on the electrode, thereby affecting the safety of the battery. If the fineness of the slurry is too large, the negative electrode material will undergo relatively large changes in expansion and contraction during battery charging and discharging, and the precipitation of metallic lithium may occur; if the fineness of the slurry is too small, the internal resistance of the battery will be too large. If the coating heating temperature is too low or the drying time is insufficient, the solvent will remain, and the binder will be partially dissolved, causing some active materials to be easily peeled; too high temperature may cause the binder to be carbonized, and the active materials may fall off and cause internal short circuits in the battery.
5. Potential safety hazards during battery use
Lithium-ion batteries should minimize overcharging during use Or over-discharge, especially for batteries with high cell capacity, due to thermal disturbance may cause a series of exothermic side reactions, leading to safety problems.