Lithium ion batteries stand out among chemical energy storage devices due to their high energy density, high power density, and long service life. They have been widely used in the field of portable electronic products due to their mature technology. With the support of national policies, the demand for lithium-ion batteries in the fields of electric vehicles and large-scale energy storage is also experiencing explosive growth.
Lithium ion batteries are generally safe, but there are reports of safety accidents that are presented to the public. Famous examples include battery fires on Boeing 737 and B 787 aircraft in recent years, as well as Tesla Model S fires. Until now, safety remains a key factor restricting the application of lithium-ion batteries in high-energy and high-power fields. Thermal runaway is not only the fundamental cause of safety issues, but also one of the shortcomings that constrain the performance of lithium-ion batteries.
The potential safety issues of lithium-ion batteries greatly affect consumer confidence. Although it is expected that BMS (Battery Management System) can accurately monitor safety conditions and predict the occurrence of certain faults, the situation of thermal runaway is complex and diverse, and it is difficult for a single technical system to ensure all safety conditions faced during its life cycle. Therefore, analyzing and studying the causes of thermal runaway is still necessary for a safe and reliable lithium-ion battery.
There have been many related studies on the chemical reactions involved in the occurrence of thermal runaway in thermal analysis, and this article will not elaborate further. This article takes the lifeline of power batteries as a clue to explain and analyze the factors and solutions that constrain the safety performance of a lithium-ion battery during its life cycle, in order to provide valuable basis for the study of safety issues.
1. Battery Cell material
The internal composition of lithium-ion batteries mainly consists of the positive electrode, electrolyte, separator, and negative electrode. Based on this, the electrode ears are welded, and the outer packaging is wrapped to form a complete battery cell. After the initial charging and discharging, formation, and capacitance separation steps of the battery cell, it can be used at the factory. The first step in this process is the selection of materials. The main factors affecting the safety of materials are their intrinsic orbital energy, crystal structure, and material properties
1.1. Positive electrode material
The main role of positive electrode active materials in batteries is to contribute specific capacity and specific energy, and their intrinsic electrode potential has a certain impact on safety. In recent years, lithium iron phosphate, a medium and low voltage material, has been widely used as the positive electrode material for power batteries in transportation vehicles (such as hybrid electric vehicles (HEVS) and electric vehicle EVS) and energy storage devices (such as uninterruptible power supplies (UPS) worldwide.
However, the safety advantage demonstrated by lithium iron phosphate in many materials is actually at the cost of sacrificing energy density, which restricts the endurance of its users (such as EVS, UPS). Although ternary materials exhibit excellent energy density, as ideal positive electrode materials for power batteries, their safety issues have not been fully addressed.
In order to study the thermal behavior of positive electrode materials, researchers have done a lot of work and found that the intrinsic electrode potential and crystal structure are the main factors affecting their safety. For example, the perfect match between the potential of the positive electrode material and the highest molecular orbital occupied by the electrolyte HOMO directly affects the stability of the electrolyte;
The starting temperature and heat release of reactions between different positive electrode materials and electrolytes may vary depending on whether multiple lithium ions can smoothly pass through the lattice simultaneously. By selecting material types and element doping, selecting materials that match the potential and electrolyte electrochemical window, have higher initial reaction temperatures, and lower reaction heat release, the safety performance of the battery cell can be improved from the perspective of positive electrode active materials.
1.2. Negative electrode materials
The impact of negative electrode active materials on safety performance mainly comes from their intrinsic orbital energy and electrolyte configuration relationship. During the fast charging process, the speed of lithium ions passing through the SEI film may be slower than the deposition speed of lithium on the negative electrode. The dendrites of lithium will continue to grow with the charging and discharging cycles, which may cause internal short circuits and ignite combustible electrolytes, leading to thermal runaway. This characteristic limits the safety of the negative electrode during the fast charging process.
In addition to the growth of lithium dendrites, the reaction between the negative electrode material and the electrolyte is also an important factor affecting safety performance. At around 100 ℃, exothermic peaks of lithium embedded graphite and electrolyte can be observed, which is also considered a decomposition reaction of SEI film. The reaction rate increases with the increase of the specific surface area of the negative electrode material.
After the decomposition of the SEI film, the lithium embedded in the negative electrode will continue to react with the electrolyte and binder to release heat, and the reaction heat increases with the increase of lithium insertion amount. By improving the thermal stability of SEI, reducing the specific surface area of negative electrode materials, and reducing the amount of lithium embedded, the performance of the battery cell can also be improved from the perspective of negative electrode materials.
1.3. Electrolytes and membranes
The impact of electrolytes and separators on safety is mainly due to their characteristics. Although the thermal stability of lithium salts is a fundamental factor affecting the thermal stability of electrolytes, their impact on battery safety performance is limited due to their relatively small decomposition reaction heat. The flammability and liquid state of widely used commercial electrolytes are important factors affecting safety.
In addition, using electrolytes with wider electrochemical windows (especially higher LUMO) and adding flame retardant materials to the electrolyte, such as modifying mixed ionic liquids and organic liquid electrolytes into non flammable electrolytes, are effective ways to improve safety. The mechanical strength (tensile and puncture strength), porosity, thickness uniformity, and rupture temperature of the diaphragm are important factors determining its safety.
The application of ceramic coatings in diaphragms can increase the mechanical strength of the original membrane, enabling the diaphragm to exhibit excellent performance in high temperature resistance, puncture resistance, and thickness reduction. The temperature at which the microporous structure is closed, whether too high or too low, can affect the performance of the battery cell. Therefore, it is necessary to comprehensively consider the composition of the membrane polymer and the optimal configuration of the porous structure, while ensuring that the rupture temperature is higher than the interruption temperature.
2. Battery Cell
In general, the manufacturing process of lithium-ion batteries includes steps such as mixing positive and negative electrode materials, coating, rolling, cutting, winding or stacking, electrode ear welding, liquid injection, sealing, formation, exhaust, and capacity separation. Each of these processes may cause an increase in battery internal resistance or short circuit, resulting in safety issues.
For example, an incorrect capacity ratio between positive and negative electrodes may cause internal short circuits, which are caused by the deposition of a large amount of metallic lithium on the negative electrode surface; Insufficient uniformity of the slurry may cause internal short circuits, which are caused by uneven distribution of active particles leading to significant changes in the volume of the negative electrode during charging and discharging, resulting in lithium precipitation, or by an increase in internal resistance caused by excessively fine slurry;
Poor quality control of coating may also cause peeling of active substances or internal short circuits. During the welding process, virtual welding (between the positive and negative electrode pieces and the ear, between the positive electrode piece and the cover, between the negative electrode piece and the shell, etc.), material dust, small or improperly padded diaphragm paper, holes in the diaphragm, and uncleaned burrs can all form safety hazards.
In addition, the quality of SEI film formation during the formation step directly determines the cycling and safety performance of the battery, affecting its lithium insertion stability and thermal stability. The factors that affect SEI film formation include the types of negative electrode carbon materials, electrolytes, and solvents, the control of current density, temperature, and pressure during formation. By selecting appropriate materials and adjusting the parameters of the formation process, the quality of SEI film formation can be improved, thereby enhancing the safety performance of the battery cell.
3. Thermal safety
3.1. BMS Battery Management System
Battery management systems (BMS) are highly expected in the use of power batteries. The management system needs to manage the battery and its consistency to achieve maximum energy storage, round-trip efficiency, and safety under different conditions (temperature, altitude, maximum rate, charge state, cycle life, etc.). BMS includes some common modules: data collector, communication unit, and battery status (SOC, SOH, SOP, etc.) evaluation model. With the development of power batteries, there are more and more stringent requirements for the management ability of BMS. By adding some safety modules, such as heat management and high-voltage monitoring, it is expected to improve the safety and reliability of power batteries during use
3.2. Thermal runaway
After thermal runaway of the battery, it can cause destructive consequences such as smoking, catching fire, and explosion, endangering the personal safety of the user. Even if the theoretically safest configuration method is chosen, it is not enough to give people peace of mind. No matter how reasonable the design and manufacturing of battery cells are, unexpected situations cannot be avoided during use. Only a reasonable battery integration design can enable the battery stack to stop losses in the event of battery cell problems
4. Abuse of batteries
Lithium ion batteries are flawless even in the integration process mentioned earlier, and it is difficult to avoid abuse in the actual operating conditions of users. The charging and discharging system (overcharging and discharging), environmental temperature (temperature chamber), other conditions (nail penetration, crush, internal short circuit), and the newly added environmental humidity testing conditions (seawater immersion) are all reasons for safety issues caused by abuse.
Overcharging can cause crystal field trapping of the positive electrode active material, hinder the lithium ion deintercalation channel, cause a sharp increase in internal resistance, generate a large amount of Joule heat, and reduce the lithium intercalation ability of the negative electrode active material, resulting in lithium branching and short circuits. Overheating of the ambient temperature can lead to chain reactions within lithium-ion batteries, including the melting of the separator, the reaction between the active material and the electrolyte, the decomposition of the positive electrode/SE film/solvent, and the reaction between the lithium embedded negative electrode and the binder. Acupuncture and compression both cause internal short circuits locally, leading to the accumulation of a large amount of heat in the short circuit area and causing thermal abuse.
5. Conclusion
The safety performance of power batteries determines the market and future of lithium-ion batteries in the power field. The factors that affect the safety performance of power batteries run through the entire life cycle of power batteries from battery cell selection to the end of use. Therefore, the reasons are complex and diverse, with rich levels. The intrinsic orbital energy, crystal structure, and properties of the material itself determine the intrinsic safety performance of a battery cell; The degree of excellence, automation, and formation conditions in each manufacturing process of battery cells determine their safety performance, which affects their thermal stability;
It is difficult to avoid manufacturing errors and abusive working conditions for batteries. In this reality, the design of BMS and safety in battery integration, including the design of contingency plans for thermal runaway of batteries, is particularly important. In short, the research on the safety issues of lithium-ion power batteries is a long and arduous task. Only by combining theory with practice and constantly innovating can they truly achieve their glory in the field of high-energy/high-power applications.
