Lithium ion battery is considered to be one of the most important energy storage technologies. Because of its low cost, high performance, high power, green environment and many other advantages, lithium-ion battery has become a typical representative of new energy, which is widely used in 3C digital products, mobile power supply and electric vehicles. With the continuous promotion of lithium-ion batteries, people pay more and more attention to the safety of lithium-ion batteries. Due to the technical reasons or improper use of the batteries, lithium-ion batteries may explode and cause fire and other safety accidents. In this paper, the thermal simulation analysis of 18650 lithium-ion battery is carried out. The user subroutine dflux is used to set the heat source of the battery.

Because of its low cost, high performance, high power, green environment and many other advantages, lithium-ion battery has become a typical representative of new energy, which is widely used in 3C digital products, mobile power supply and electric vehicles.

With the continuous promotion of lithium-ion batteries, people pay more and more attention to the safety of lithium-ion batteries. Due to the technical reasons or improper use of the batteries, lithium-ion batteries may explode and cause fire and other safety accidents. Especially in recent years, the demand for lithium-ion batteries in the market of electric vehicles is increasing. In the process of developing high-power lithium-ion battery system, battery safety has attracted extensive attention, and the existing problems need to be further solved.

Thermal runaway process of lithium ion battery

In recent years, the cases of fires caused by thermal runaway of batteries are all caused by the fact that the heat generation rate of batteries is much higher than the heat dissipation rate, and a large amount of heat is accumulated and not sent out in time. In essence, "thermal runaway" is a process of energy positive feedback cycle: the rising temperature will lead to the heating of the system, and the temperature will rise after the heating of the system, which in turn makes the system hotter.

Figure 1 - thermal runaway process of Li ion battery

Stage 1: thermal runaway inside the battery

When the temperature rises to 90-120 ℃, the SEI film begins to decompose and release heat, and the temperature rises. However, when the temperature reaches 120-130 ℃, the SEI film of the protective layer is destroyed, the negative electrode reacts with solvent and binder, the temperature rises, and the diaphragm melts and closes. When the temperature continues to rise above 150 ℃, the internal electrolyte begins to decompose and continue to release heat to further heat the battery.

Stage 2: Battery bulging stage

When the temperature of the battery reaches above 200 ℃, the cathode material decomposes and releases a large amount of heat and gas. The lithium intercalated anode began to react with the electrolyte at 250-350 ℃.

Stage 3: battery thermal runaway, explosion failure stage

During the reaction process, the electrolyte reacts with the oxygen produced by the positive electrode, which makes the battery thermal runaway.

Causes of thermal runaway of lithium ion battery

In fact, the general probability of short circuit in the battery in electronic products is one in ten million, that is to say, the safety of single battery used in daily life is relatively high. But in electric vehicles, the battery pack of an electric vehicle needs thousands of batteries, so the probability of thermal runaway will rise from 1 in 10 million to 1 in 1000. And once the battery of electric vehicle is in danger, the consequences will be very serious, so it is particularly important to study the causes of thermal runaway of battery.