In general, lithium-ion batteries have safety problems that appear to be burning or even exploding. The root cause of these problems lies in the thermal runaway inside the battery. In addition, some external factors such as overcharge, fire, extrusion, puncture, and short circuit. Other issues can also lead to security issues. Lithium-ion batteries will generate heat during charging and discharging. If the generated heat exceeds the heat dissipation capacity of the battery, the lithium-ion battery will overheat, and the battery material will decompose the SEI film, decompose the electrolyte, decompose the positive electrode, and the negative electrode. A destructive side reaction such as a reaction of an electrolytic solution and a reaction between a negative electrode and a binder.
1 Safety hazard of cathode material
When the lithium ion battery is used improperly, the internal temperature of the battery is increased, so that the decomposition of the active material and the oxidation of the electrolyte occur in the positive electrode material. At the same time, these two reactions can generate a large amount of heat, causing a further increase in battery temperature. 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, liquid leakage and even explosion of the battery. The lithium intercalation compound can effectively avoid the generation of lithium dendrites and greatly improve the safety of the lithium ion battery. As the temperature increases, the carbon negative electrode in the lithium intercalation state first undergoes an exothermic reaction with the electrolyte. Under the same charge and discharge conditions, the heat release rate of the electrolyte reacted with lithium-incorporated artificial graphite is much higher than that of the intercalated carbon microspheres, carbon fibers, coke, etc. with lithium intercalation.
3 Safety risks of 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 Thermal stability of the electrolyte. The organic solvent of the electrolyte is carbonate. These solvents have a low boiling point and a low flash point, and are easily reacted with the lithium salt to release PF5 at a high temperature, and are easily 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. Such as the positive and negative mixture, coating, rolling, cutting or die cutting, assembly, filling of the amount of electrolyte, sealing, chemical and other processes of quality control, 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, thereby affecting the safety of the battery. The fineness of the slurry 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. When the coating heating temperature is too low or the drying time is insufficient, the solvent remains, and the binder is partially dissolved, causing some active materials to be easily peeled off; if the temperature is too high, the binder may be charred, and the active material may fall off and cause internal short circuit of the battery.
5 Safety hazards 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.