Talk about the safety of lithium-ion batteries
The internal safety of lithium-ion batteries is caused by thermal runaway inside the battery and the accumulation of heat, which causes the internal temperature of the battery to continuously rise. The external performance is the intense energy release phenomenon such as combustion and explosion.
The battery is a high-density carrier of energy. Intrinsically, there are insecure factors. The higher the energy density, the greater the impact of the intense release of energy and the more prominent the safety problem. High-energy carriers such as gasoline, natural gas, and acetylene all have the same problems, and the number of safety accidents that occur every year is numerous.
Different electrochemical systems, different capacities, process parameters, use environments, and degree of use have a greater impact on the safety of lithium-ion batteries.
Since the battery stores energy, during the energy release process, when the heat generation and accumulation speed of the battery is greater than the heat dissipation speed, the internal temperature of the battery continues to rise. Lithium-ion battery consists of a highly active positive electrode material and an organic electrolyte. It is very prone to severe chemical side reactions under heated conditions. This reaction will generate a lot of heat, and even cause "thermal runaway", which is a dangerous battery. The main cause of the accident.
The thermal runaway inside the lithium-ion battery indicates that some of the chemical reactions inside the battery are not what we expected before, and they are in an uncontrollable and disorderly state, resulting in a rapid and intense release of energy. .
So, let's see what chemical reactions are there, accompanied by a lot of heat generation, which leads to thermal runaway.
1. SEI membrane decomposition, electrolyte exothermic side reaction
The solid electrolyte membrane is formed during the initial cycle of the lithium ion battery. We do not want the SEI membrane to be too thick or desirable. A reasonable SEI film exists to protect the negative electrode active material from reacting with the electrolyte.
However, when the internal temperature of the battery reaches about 130 ° C, the SEI film will be decomposed, causing the negative electrode to be completely exposed, and the electrolyte is largely liberated on the surface of the electrode, causing the internal temperature of the battery to rise rapidly.
This is the first exothermic side reaction inside the lithium battery and the starting point for a series of thermal runaway problems.
2. Thermal decomposition of electrolyte
Due to the exothermic side reaction of the electrolyte in the negative electrode, the internal temperature of the battery is continuously increased, which further causes thermal decomposition of LiPF6 and the solvent in the electrolyte.
This side reaction occurs at a temperature range of approximately 130 ° C to 250 ° C, which is accompanied by a large amount of heat generation, which further pushes up the temperature inside the battery.
3. Thermal decomposition of the cathode material
As the internal temperature of the battery further rises, the active material of the positive electrode is decomposed, and this reaction generally occurs between 180 ° C and 500 ° C with a large amount of heat and oxygen.
Different positive electrode materials, the heat generated by the decomposition of the active material is different, and the oxygen content released is also different. Lithium iron phosphate cathode material has the most thermal stability in all cathode materials due to less heat generated during decomposition. When nickel-cobalt-manganese ternary materials are decomposed, more heat is generated, accompanied by a large amount of oxygen release, which is prone to combustion or explosion, so the safety is relatively low.
4. Reaction of binder with negative active substance
The reaction temperature of the negative active material LixC6 and the PVDF binder starts from about 240 ° C, the peak appears at 290 ° C, and the reaction exotherm reaches 1500 J / g.
It can be seen from the above analysis that the thermal runaway of the lithium ion battery is not instantaneous, but a gradual process. This process, generally caused by overcharge, large rate charge and discharge, internal short circuit, external short circuit, vibration, collision, drop, impact, etc., causes a large amount of heat inside the battery in a short period of time, and continuously accumulates, pushing the temperature of the battery continuously rise.
Once the temperature rises to the threshold temperature of the internal chain reaction (about 130 ° C), the interior of the lithium-ion battery will spontaneously produce a series of exothermic side reactions, and further increase the heat accumulation and temperature rise inside the battery. A large amount of flammable gas will be precipitated. When the temperature rises to the flash point and ignition point of internal solvents and flammable gases, it will cause safety accidents such as burning and explosion.
The lithium-ion battery that has just been manufactured has passed the safety test certification and does not represent the safety of the lithium-ion battery during its life cycle. According to our previous analysis, in the long-term use process, lithium metal deposition on the surface of the negative electrode occurs, the electrolyte is decomposed and volatilized, the positive and negative active materials are detached, the internal structure of the battery is deformed, metal impurities are mixed in the material, and others. Many unintended changes, which can cause an internal short circuit in the battery, which in turn generates a lot of heat. In addition, external abuse, such as overcharge, extrusion, metal puncture, collision, drop, impact, etc., will also cause the battery to generate a large amount of heat in a short period of time, which becomes the cause of thermal runaway.
In the use of lithium-ion batteries, there is no absolute safety, only relative security. We must try to avoid abuse and reduce the probability of occurrence of hazardous events. At the same time, we must start with the main components such as positive and negative materials, electrolytes and separators, and choose materials with excellent chemical stability and thermal stability. Flame retardant properties, in the presence of internal and external thermal runaway incentives, reduce the heat of internal side reactions, or have a high ignition temperature to avoid thermal runaway. In the battery structure and housing design, the structural stability should be fully considered to achieve sufficient mechanical strength to withstand external stresses and ensure that there is no obvious deformation inside. In addition, the heat dissipation performance is also important to consider. If the heat can be dissipated in time, the internal temperature will not continue to rise, and thermal runaway will not occur.
The safety design of lithium-ion batteries is a systematic theory. It is not comprehensive to measure the safety of lithium-ion batteries by simply decomposing the heat of the positive electrode materials. From a system perspective, lithium iron phosphate batteries are not necessarily safer than ternary materials because there are many factors that ultimately affect thermal runaway, and the heat generated by the decomposition of the cathode material is only one factor.