With the rapid development of electric vehicles, lithium-ion batteries have been widely used, and the safety issues of lithium-ion batteries are the foundation of the development of electric vehicles. Due to the high energy density of lithium-ion batteries and the flammability of their internal electrolytes, lithium-ion batteries frequently experience thermal runaway during use, leading to electric vehicle fires and self ignition accidents, endangering the property and life safety of passengers. Therefore, it is crucial to have a deep understanding of and address the potential overheating issues that may occur during battery operation, as well as to study the causes of thermal runaway in lithium-ion batteries, in order to improve their safety, stability, and lifespan.
1 Causes of thermal runaway in lithium-ion batteries
Thermal runaway is a common feature of lithium-ion battery safety accidents, and the problem of battery thermal runaway is the core issue of current electrochemical energy storage system safety. Thermal runaway refers to the phenomenon of overheating caused by a chain reaction of heat release inside the battery, resulting in a rapid change in the rate of temperature rise. The causes of thermal runaway in batteries are summarized in the following figure, mainly including mechanical abuse, electrical abuse, and thermal abuse. The thermal runaway caused by the three types of triggers is accompanied by the gas production phenomenon of the battery.
(1) Mechanical abuse
Under the action of mechanical external force, the battery separator ruptures and causes an internal short circuit, leading to rapid energy release and causing deformation of the battery material and structural damage.
(2) Electricity abuse
Due to overcharging or overdischarging, the electrode active material and electrolyte partially decompose, and the products react and cause heat accumulation, or external short circuits lead to rapid discharge of the battery, generating a large amount of Joule heat, mainly including internal short circuits, external short circuits, improper charging, overdischarging, and other behaviors.
(3) Hot abuse
High temperature environments or intense heat generation can cause battery heat accumulation, and when the battery temperature rises to a certain level, it can lead to thermal runaway. The main cause of failure is local overheating of the battery, and abnormal increase in battery resistance can exacerbate its heat generation during operation. Excessive heat generation and poor heat dissipation conditions can easily lead to thermal abuse.
In addition, the causes of thermal runaway in lithium-ion batteries can also be divided into material and production process factors of the battery body, as well as causes that occur during the battery application process. There are various causes of thermal runaway in battery applications, such as internal and external short circuits, overcharging and discharging, high temperature environments, high rate charging and discharging, aging, and compression deformation. These causes of thermal runaway are not independent of each other, and their relationship can lead to cascading consequences.
2 Thermal runaway mechanism of lithium-ion batteries
The thermal runaway of lithium-ion batteries can be roughly divided into three stages: heating stage, injection and combustion stage, and extinguishing stage.
(1) Heating stage
As the radiation heating time increases, the surface temperature of lithium-ion batteries rises, and the chemical reactions inside the battery accumulate a large amount of gas and heat. Due to the steel shell of the lithium-ion shell, there is almost no volume change such as expansion. When the internal pressure of the battery reaches the pressure resistance limit of the safety valve, it ruptures and releases flammable gases, causing the battery to change from a “closed system” to an “open system”, accelerating side reactions such as lithium metal and electrolyte, and thus accelerating temperature rise.
(2) Injection and combustion stages
As the side reactions of the battery continue, the temperature of the battery continues to rise, and the accumulated energy and gas increase. When the injection pressure is reached, a large number of aerosol droplets from the decomposition of the electrolyte will be ejected and form white smoke. And the larger the SOC, the more intense the internal chemical reactions of the battery, and the non flammable substances inside will be ejected together with the aerosol droplets to form white sparks. This type of white smoke can be ignited at the moment of release, causing safety accidents such as combustion and explosion.
(3) Extinguishing phase
As the battery burns, combustible gases are continuously consumed, and the combustion flame will gradually weaken until it is eventually extinguished.
When thermal runaway occurs in lithium-ion batteries, a large amount of heat is generated due to side reactions, causing rapid decomposition of internal materials and intensified reactions in various parts, resulting in a continuous increase in battery temperature and thermal runaway reactions of various component materials inside the battery
3 Simulation analysis process
The simulation analysis of thermal runaway in lithium-ion batteries is conducted through numerical simulation to simulate and analyze the thermal behavior of the battery under different conditions. The specific steps and methods of the simulation analysis process may vary depending on the simulation tool used and the specific problem. Before conducting simulation analysis, ensure a thorough understanding of the physical characteristics and actual usage conditions of the battery to improve the accuracy and reliability of the simulation.
4 Preventive measures against thermal runaway
There are various factors that affect the safety of lithium-ion batteries. Currently, the main ways to improve the safety of lithium-ion batteries are through modifying the positive electrode material, adding flame retardants to the electrolyte, and designing thermal management for the battery.
(1) Modification of battery positive electrode material
The main exothermic reactions that positive electrode materials participate in during thermal runaway include the decomposition of positive electrode materials and the release of O2, which are the main causes of power battery fires and explosions; The main exothermic reactions that negative electrode materials participate in include SEI decomposition, exothermic reactions between lithium embedded in the negative electrode and electrolyte and binder, where the temperature corresponding to SEI decomposition is considered the characteristic temperature at which thermal runaway begins.
In the future, further breakthroughs are needed in the coating and modification of positive electrode materials, compatibility between the same electrolyte and electrodes, and improving the thermal conductivity of battery cells.
(2) Adding flame retardants to the electrolyte
Functional additives can be added or new electrolyte salts can be developed to suppress the decomposition and combustion of electrolytes. Functional additives for electrolytes can be divided into flame retardant additives and overcharge protection additives. Flame retardant additives can be composed of organic phosphorus compounds, nitrogen-containing compounds, carbonates, silanes, etc., which can improve their thermal stability by suppressing temperature; Overcharge additives can be classified into redox type and electropolymerization based on their mechanism of action, achieving high thermal safety by limiting the battery voltage within a controllable range.
(3) Battery thermal management design
The thermal management system for power batteries mainly solves the problems of thermal runaway, inability to discharge deeply, and inability to discharge with high current when the temperature is too high or too low. It is the key to ensuring the safe and efficient use of power batteries. At present, the field of new energy vehicles is in a critical stage of industry transformation, and technologies in related fields are learning from and integrating with various new technologies, such as big data technology, artificial intelligence technology, and cloud computing technology.
In summary, thermal runaway of lithium-ion batteries is a key factor restricting their further development, and improving their safety has become particularly urgent for the development of electric vehicles. Thermal simulation of lithium-ion batteries can improve the performance and lifespan of electric vehicle battery systems, which is of great significance for enhancing the safety of electric vehicles. By establishing a heat generation model and simulation calculation for lithium-ion batteries, different configurations of battery thermal management systems can be tested, optimizing the design of battery thermal management systems, shortening the design cycle, and saving design costs.