Why do lithium-ion battery safety issues occur frequently?

Recently, the new energy vehicle safety and recall theme summit co-sponsored by the Quality Development Bureau of the State Administration for Market Regulation and the Society of Automotive Engineers of my country was held in Boao. During the meeting, Qiu Xinping, a professor at Tsinghua University, expressed some of his views on the safety of lithium-ion batteries and put forward proposals for battery safety prevention. Why do lithium-ion battery safety issues occur frequently? Qiu Xinping told 'Product Safety and Recall' that this is determined by the characteristics of the lithium-ion battery itself. Compared with batteries of other systems, such as lead-acid batteries, nickel-aluminum batteries, etc., lithium-ion batteries have innovatively adopted some new reaction methods. It is important to show the following two aspects: First, the positive and negative electrodes of lithium-ion batteries adopt the intercalation reaction method, which is different from the general chemical reaction such as the reaction of lead to lead sulfate in lead-acid batteries. The surface of the material is the interface during the intercalation reaction. The structure is relatively stable, so the long-life system of the lithium-ion battery is guaranteed. Secondly, the lithium ion battery has the characteristics of high energy density. The high energy density is mainly due to two aspects: one is the use of high-capacity electrode materials; the other is the use of a wide electrochemical window electrolyte (the important thing here is the non-aqueous system Electrolyte or solid electrolyte), which greatly increases the voltage of the battery. The current average voltage of lithium-ion batteries can reach more than 3.7V, and the highest voltage can reach more than 4.4V. With this design, compared with general aqueous electrolytes, there is no clamping reaction in lithium-ion batteries. Qiu Xinping pointed out that, for example, if a lead-acid battery is overcharged, the excess electricity can be analyzed for water after the overcharge. After the analysis is completed, the excess electricity is converted into heat and released by a hydrogen-oxygen composite matrix. Lithium-ion batteries do not. This mechanism, so once overcharged, it will bring a lot of safety problems. Causes of thermal runaway There are two types of battery heat generation that is an inevitable product of the battery's working process. If the heat release of the battery, that is, the thermal diffusion rate is faster than the heat generation rate, the battery temperature will not rise and will not reach the thermal runaway temperature. But sometimes, the heat dissipation of the battery is not very good. Qiu Xinping divided the causes of thermal runaway into two categories. The first category is the external factors of the battery. If the ambient temperature of the battery is too high, the battery heat dissipation is not good, or the internal winding is not good, resulting in poor heat dissipation of the battery. In addition, there is another situation that the company should pay special attention to, that is, the battery spine can sometimes introduce heat. The metal spine of the battery has copper and aluminum. The thermal conductivity is relatively good. There are some components with high heat sources in the circuit. , Be sure to stay away from the battery spine to allow sufficient heat to play. Qiu Xinping added. The second category is the internal factors of the battery. One is micro short circuit. There are two cases of micro short circuit. One is short circuit in battery technology, including burrs; the other is short circuit during use. These will cause micro short circuit and cause the local temperature of the battery to be higher than the thermal runaway temperature. Thus, the phenomenon of thermal runaway occurs. The second is that the battery is overcharged, which will reduce the thermal runaway temperature of the material after overcharge. For example, the capacity of the battery has decayed in the middle and late stages of use. At this time, the battery itself is already in an overcharged state, and the thermal runaway temperature will naturally drop. Qiu Xinping pointed out. The third is the amount of electrolyte. If the amount is too much, the explosion will be very harmful, and the flame will spray very high and far, so the amount of electrolyte must be strictly controlled.