Safety issues: At high voltages or high temperatures (battery abuse), "thermal runaway" occurs, causing the battery to catch fire or explode.
Main influencing factors: the role of anode material and electrolyte: graphite material has high degree of crystallinity, has a highly oriented graphite layer structure, is sensitive to electrolyte, has poor compatibility with solvent, and has elevated temperature. The carbon negative electrode can undergo an exothermic reaction with the electrolyte to decompose the electrolyte. Thermal decomposition of the positive electrode material and oxidation of the electrolyte: Under overcharge and high temperature, the positive electrode active material decomposes and oxidizes to the electrolyte, and both reactions generate a large amount of heat. Thermal decomposition of electrolyte: Lithium-ion batteries generally use solvents such as PC, EC, EMC, DMC, etc., which are organic flammable substances, which will oxidize and decompose at high temperatures; at a certain voltage, the solvent also decomposes; solvent The water content also has an effect, and the water content is increased to promote the decomposition of the SEI film. Diaphragm: the melting point of polyethylene ~125 ° C, polypropylene ~ 155 ° C, when the temperature exceeds the melting point, the diaphragm melts, the battery internal short circuit, generates a lot of heat.
The safety of lithium-ion batteries is closely related to the charge and discharge system. Under abuse conditions (such as overcharge), the internal temperature of the battery will increase due to excessive polarization. After the diaphragm is closed at 120 °C, the temperature will continue to rise due to the hysteresis effect of heat transfer, positive and negative materials and electrolysis. The liquid decomposes more rapidly, causing the battery to burn and explode; when the battery is over-discharged to 1~2V, the copper foil as the negative current collector will begin to dissolve and precipitate on the positive electrode. When less than 1V, the surface of the positive electrode begins to appear. Dendrites, causing a short circuit inside the battery.