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.
At present, most commercial lithium-ion batteries use carbon materials as the negative electrode. During the charging and discharging process, lithium is embedded and extracted in the carbon particles, thereby reducing the possibility of lithium dendrite formation and improving the safety of the battery, but this is not Indicates that the carbon negative electrode has no safety issues. The factors affecting the safety performance of lithium-ion batteries are manifested in the following aspects.
(1) The lithium-intercalated negative electrode reacts with the electrolyte
As the temperature rises, the carbon negative electrode in the lithium-intercalated state will first react with the electrolyte exothermicly and generate flammable gas. Therefore, the mismatch between the organic solvent and the carbon negative electrode may cause the lithium-ion power battery to burn.
(2) Binder in the negative electrode
A typical negative electrode contains a binder with a mass fraction of 8% to 12%. As the degree of lithium intercalation in the negative electrode increases, the heat of reaction with the binder increases. XRD analysis reveals that the main product of the reaction is LiF. It has been reported that the temperature at the beginning of the reaction between LiC and PVDF is 200°C.
(3) Size of negative electrode particles
If the size of the negative electrode active material particles is too small, the resistance of the negative electrode will be too large. If the particles are too large, they will expand and contract severely during the charge and discharge process, leading to the failure of the negative electrode. At present, the main solution is to mix large particles and small particles in a certain proportion, so as to achieve the purpose of reducing electrode impedance and increasing capacity while improving cycle performance.
(4) The quality of the SEI film on the negative electrode surface
A good SEI film can reduce the irreversible capacity of lithium-ion batteries, improve cycle performance, increase lithium insertion stability and thermal stability, and to a certain extent help reduce the safety hazards of lithium-ion batteries. Current studies have shown that carbon materials that have undergone surface oxidation, reduction, or doping and the use of spherical or fibrous carbon materials all contribute to the improvement of SE1 film quality.
In addition, the ratio of the positive and negative active materials in the full battery is related to the service life and safety performance of the battery, especially the overcharge performance. If the capacity of the positive electrode is too large, lithium metal will deposit on the surface of the negative electrode. If the capacity of the negative electrode is too large, the battery will lose capacity. In order to ensure the safety of the battery, the general principle is to consider the cycle characteristics of the positive and negative electrodes and the ability of the negative electrode to accept lithium when overcharged, and give a certain design redundancy.