1. Safety hazards of cathode materials
When lithium-ion batteries are used improperly, the internal temperature rises, causing the positive electrode material to degrade and the electrolyte to oxidize. These two reactions generate significant heat, further increasing the battery temperature. Different delithiation states have vastly different effects on the lattice transition of the active material, the degradation temperature, and the battery's thermal stability.
2. Safety hazards of negative electrode materials
Early lithium-metal anode materials were used, and these batteries were prone to lithium dendrite formation after repeated charge-discharge cycles. This dendrites could puncture the separator, leading to short circuits, leakage, and even explosions. Lithium-intercalated compounds effectively prevent lithium dendrite formation, significantly improving the safety of lithium-ion batteries. As temperature increases, the lithium-intercalated carbon anode first undergoes an exothermic reaction with the electrolyte. Under the same charge-discharge conditions, the exothermic reaction rate between the electrolyte and lithium-intercalated artificial graphite is much greater than the exothermic reaction rate with lithium-intercalated mesophase carbon microspheres, carbon fibers, coke, etc.
3. Safety hazards related to the diaphragm and electrolyte
The electrolyte in lithium-ion batteries is a mixture of lithium salt and organic solvent. Commercially available lithium salt is lithium hexafluorophosphate (LiPF6). This material is prone to thermal analysis at high temperatures and undergoes thermochemical reactions with trace amounts of water and organic solvents, reducing the thermal stability of the electrolyte. The organic solvent in the electrolyte is a carbonate, which has a low boiling and flash point and readily reacts with the lithium salt to release PF5 at high temperatures, making it easily oxidized.
4. Safety hazards in the manufacturing process
In the manufacturing process of lithium-ion batteries, processes such as electrode fabrication and battery assembly can all affect battery safety. Quality control in steps such as mixing positive and negative electrodes, coating, rolling, cutting or punching, assembly, electrolyte addition, sealing, and formation all impact battery performance and safety. The uniformity of the slurry determines the evenness of the distribution of active materials on the electrodes, thus affecting battery safety. If the slurry fineness is too large, significant expansion and contraction of the negative electrode material will occur during charging and discharging, potentially leading to lithium metal precipitation; if the slurry fineness is too small, it will result in excessive internal resistance. Insufficient coating temperature or drying time can leave solvent residue and partial dissolution of the binder, causing some active materials to easily peel off; excessively high temperatures may cause binder carbonization, leading to active material detachment and internal short circuits within the battery.
5. Safety hazards during battery use
Lithium-ion batteries should be used in a way that minimizes overcharging or over-discharging, especially for batteries with high single-cell capacity, as thermal disturbances may trigger a series of exothermic side reactions, leading to safety issues.
