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.