In general, lithium-ion batteries have safety problems such as burning or even explosion. The root cause of these problems lies in the thermal runaway inside the battery. In addition, some external factors, such as overcharge, fire source, extrusion, puncture, and short circuit. Such problems can also lead to safety problems. Lithium-ion batteries will heat up during charging and discharging. If the heat generated exceeds the heat dissipation capacity of the battery, the lithium-ion battery will overheat and the SEI film will be decomposed. Decomposition of the electrolyte, decomposition of the cathode, reaction of the anode with the electrolyte and reaction of the anode with the binder.
1, the safety potential of the cathode material
When the lithium ion battery is used improperly, the internal temperature of the battery rises, and the active material decomposes and the electrolyte oxidizes. The two reactions can generate a large amount of heat, which causes the battery temperature to rise further. Different delithiation states have a great influence on the lattice transition of the active material, the decomposition temperature and the thermal stability of the battery.
2, the safety hazard of the anode material
The anode material used in the early stage is metallic lithium. The assembled battery is prone to lithium dendrite after repeated charge and discharge, and then pierces the diaphragm, causing short circuit of the battery, leakage and even explosion. The lithium intercalation compound can effectively avoid the generation of lithium dendrites. , greatly improve the safety of lithium-ion batteries. With the increase of temperature, the carbon anode in the lithium-intercalation state first reacts with the electrolyte. Under the same charge and discharge conditions, the electrolyte reacts with the lithium-inlaid artificial graphite. The heat rate is much higher than the reaction heat release rate of mesophase carbon microspheres, carbon fiber, coke, etc. with lithium intercalation.
3, the safety hazard of the diaphragm and electrolyte
The electrolyte of the lithium ion battery is a mixed solution of a lithium salt and an organic solvent, wherein the commercial lithium salt is lithium hexafluorophosphate, the material is susceptible to thermal decomposition at a high temperature, and is subjected to a thermochemical reaction with a trace amount of water and an organic solvent to reduce The thermal stability of the electrolyte. The electrolyte organic solvent is carbonate. The boiling point of this solvent is low, the flash point is low, and it is easy to react with the lithium salt to release PF5 at high temperature, and it is easy to be oxidized.
4, safety hazards in the manufacturing process
Lithium-ion batteries in the manufacturing process, electrode manufacturing, battery assembly and other processes will have an impact on the safety of the battery. For example, positive and negative mixture, coating, rolling, cutting or die-cutting, assembly, filling with electrolyte Quality control of quantity, sealing, chemical conversion, etc., all affect the performance and safety of the battery. The uniformity of the slurry determines the uniformity of the distribution of the active material on the electrode, thus affecting the safety of the battery. The fineness is too large. When the battery is charged and discharged, there will be a large change in the expansion and contraction of the negative electrode material, and precipitation of metallic lithium may occur. If the fineness of the slurry is too small, the internal resistance of the battery may be too large. The coating heating temperature is too low or baked. Insufficient drying time will cause the solvent to remain, the binder will be partially dissolved, and some active materials will be easily peeled off; if the temperature is too high, the binder will be charred, and the active material will fall off and cause internal short circuit of the battery.
5, the safety hazard during battery use
Lithium-ion batteries should be used to minimize over-charging or over-discharging during use. Especially for batteries with high monomer capacity, thermal disturbances may cause a series of exothermic side reactions, leading to safety problems.
