Lithium-ion batteries have been widely used in mobile phones, notebook computers and other small portable electronic devices. Because they use lithium-ion batteries with small capacity (below 1~2A.h), most of them use single-cell batteries, and their batteries are safe. The problem is not too prominent. Even so, cell phone battery explosions and fire incidents occasionally occur. When lithium-ion batteries with a single battery capacity of 10A.h or even 100A.h are used as power sources for electric bicycles, electric vehicles, hybrid electric vehicles, and power tools, safety issues have attracted global attention. For lithium-ion batteries for mobile phones, the basic requirement is that the probability of a safety accident should be less than one in a million, which is also the lowest standard acceptable to the public. For large-capacity lithium-ion batteries, especially large-capacity power lithium-ion batteries used in automobiles, the safety problem is particularly prominent, and it has always been a research hotspot. There are many reasons for battery safety problems, mainly concentrated in overcharging, internal and external short circuits, and hidden safety hazards of backward batteries during the use of battery packs. The main factors affecting the safety of lithium-ion batteries are the battery’s electrode material, electrolyte, manufacturing process and use conditions. With the advancement of material science and manufacturing technology, the use of electrode materials with higher thermal stability, selection of electrolytes containing flame retardants or overcharge protection agents, well-designed heat dissipation structures, battery protection circuits and management systems are all conducive to improvement. The safety performance of lithium-ion batteries, so the safety problems of large-capacity power lithium-ion batteries are expected to be solved.
The safety of positive electrode materials mainly includes thermal stability and overcharge safety. In the oxidized state, the positive electrode active material undergoes exothermic decomposition and releases oxygen, and the oxygen reacts exothermically with the electrolyte, or the positive electrode active material directly reacts with the electrolyte.
Oxidation temperature refers to the temperature at which the material undergoes redox exothermic reaction, and is also an important indicator of the material’s oxidizing ability. The higher the temperature, the weaker its oxidizing ability.
Lithium cobalt oxide and lithium nickel cobalt manganese oxide are very active and have strong oxidizing properties. Due to the high voltage of lithium ion batteries and the use of non-aqueous organic electrolytes, these organic electrolytes are reductive and will undergo redox reactions with the positive electrode material and release heat. The stronger the oxidation ability of the positive electrode material, the more violent the reaction will be. , The easier it is to cause safety accidents. Lithium manganate and lithium iron phosphate have higher oxidation exothermic temperature, and their oxidability is weak, or the thermal stability is much better than lithium cobaltate and lithium nickel cobalt manganate, so they have better safety. From the above comprehensive performance, it can be seen that considering safety, lithium cobalt oxide and lithium nickel cobalt manganate are extremely unsuitable for use in the field of power-type lithium-ion batteries; lithium manganate and lithium iron phosphate are more suitable as positive electrode materials for power lithium batteries.