Because of its low cost, high performance, high power, green environment and many other advantages, lithium-ion batteries have become a typical representative of a new type of energy, widely used in 3C digital products, mobile power and electric tools and other fields. In recent years, due to the intensification of environmental pollution as well as national policy guidance, the demand for lithium-ion batteries in the electric vehicle-based electric transportation market has been increasing. In the process of developing high-power lithium-ion battery systems, battery safety issues have attracted widespread attention, and the problems that exist need to be further addressed.
The temperature change of the battery system is determined by two factors: the appearance and emission of heat. The appearance of heat in lithium-ion batteries is important due to thermal decomposition and reactions between battery materials. Reduce the heat of the battery system and improve the system's resistance to high temperatures, the battery system is safe. Unlike small portable devices such as cell phones, laptops battery capacity is generally less than 2Ah, the power type lithium-ion battery used in electric vehicles is generally greater than 10Ah, its local temperature in normal operation is often higher than 55 ℃, the internal temperature will reach more than 300 ℃ [2], in high temperature or large multiplier charge and discharge conditions, the exothermic high energy electrodes and the rise in temperature of combustible organic solvents will cause A series of side reactions will occur, eventually leading to thermal runaway and battery combustion or explosion [3]. In addition to its own chemical reaction factors leading to thermal runaway, some human factors such as overheating, overcharging, short circuit caused by mechanical shock can also lead to thermal instability of lithium-ion batteries and thus cause safety accidents. Therefore, it is of great practical significance to study and improve the high temperature performance of lithium-ion batteries.
1 thermal runaway cause analysis
The thermal runaway of lithium-ion batteries is important because of the internal temperature rise of the battery. Currently the most widely used electrolyte system in commercial lithium-ion batteries is LiPF6 mixed carbonate solution, such solvents are highly volatile, low flash point, and very easy to burn. When internal short circuit caused by ramming or deformation, large multiplier charging and discharging and overcharging, a lot of heat will occur, causing the battery temperature to rise. When a certain temperature is reached, it leads to a series of decomposition reactions that disrupt the thermal balance of the battery. When the heat released from these chemical reactions cannot be evacuated in time, it will intensify the reaction and trigger a series of self-heating side reactions. The battery temperature rises sharply, that is, thermal runaway, eventually leading to battery combustion, and in serious cases, even an explosion.
In general, the causes of thermal runaway in lithium-ion batteries are focused on the thermal instability of the electrolyte and the thermal instability of the electrolyte and the coexistence system of the positive and negative electrodes.
At present, from a broad perspective, safe lithium-ion batteries are important from the external management and internal design to take measures to control the internal temperature, voltage and air pressure to achieve safety purposes.
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