Lithium ion batteries have been widely used in portable devices, scientific equipment, space transportation vehicles, and satellite systems due to their high battery voltage and specific energy, wide operating temperature range, long storage life, no environmental pollution, and no memory effect. However, if lithium-ion batteries experience internal short circuits, external short circuits, overcharging, or are used in high-temperature environments, the Joule heat and reaction heat inside the battery will sharply increase, leading to catastrophic hazardous events such as explosions, fires, and thermal runaway.
In order to test the safety performance of lithium-ion batteries, relevant organizations such as Writers Laboratories Japan Battery Association (JBA) and Chinese National Standards (GB) have successively developed lithium-ion battery safety testing standards. The commonly used safety testing items currently include four aspects: thermal performance, mechanical performance, electrical performance, and extreme environmental adaptability testing items.
The nail penetration test is used to evaluate the internal short circuit of lithium-ion batteries caused by lithium deposition, manufacturing defects, or other reasons, or the situation of needle like objects piercing lithium-ion batteries.
At present, there are problems with unclear mechanisms and poor repeatability in the safety accidents caused by nail penetration. Improving battery design is an important direction to enhance the safety of lithium-ion batteries by studying and analyzing the mechanism and influencing factors of thermal runaway during the battery nail penetration process. This article will analyze and summarize the internal short circuit situation under nail penetration conditions, introduce the existing research methods of nail penetration experiments, analyze their respective advantages, and finally propose methods to improve the nail penetration model for the next step of establishing an accurate lithium-ion battery nail penetration model as a reference.
1 Lithium ion battery nail penetration safety test
The nail penetration test for lithium-ion batteries is conducted using φ 5~ φ 8 mm high-temperature resistant steel needle (with a needle tip angle of 60 ° and a smooth surface without rust, oxide layer, and oil stains) should be inserted at a speed of (25 ± 5) mm/s from the direction perpendicular to the battery electrode plate, and the penetration position should be close to the geometric center of the nail penetration surface (the steel needle stays in the battery). The short circuit inside the battery should be artificially triggered and observed for a period of time. The nail penetration test is shown in Figure 1. If the battery does not catch fire, smoke or explode, it will pass the nail penetration test. Otherwise, it will not pass. The nail penetration experiment mainly studies the effects of nail penetration rate, nail penetration position, state of charge, battery capacity, etc. on battery safety.
During the nail penetration process of lithium-ion batteries, internal short circuits may be caused in four different situations
(1) Internal short circuit occurs between the positive and negative current collectors (aluminum foil and copper foil)
(2) Occurred between aluminum foil and negative electrode
(3) Occurred between the positive and negative electrodes
(4) It occurs between the copper foil and the positive electrode gate.
On the other hand, during the nail penetration process of lithium-ion batteries, more than one type of internal short circuit is often triggered, and the situation of internal short circuit will also evolve over time. This is the underlying reason for the unclear mechanism of internal short circuit and poor repeatability during the nail penetration testing process of lithium-ion batteries.
According to current understanding, the basic process of internal short circuit caused by lithium-ion batteries during the nail penetration process is as follows: Firstly, the Joule heat generated by the internal short circuit causes a rapid increase in the local temperature of the battery. After the temperature reaches a certain value, it causes the decomposition of the SEI membrane (80-120 ℃) and the melting of the membrane (165 ℃). The decomposition of SEI membranes and the melting of membranes generate more heat, which promotes electrolyte decomposition (130-300 ℃) and negative electrode reduction reaction (100-400 ℃), while positive electrode oxidation reaction (160-400 ℃) ultimately leads to uncontrolled heating.
The parameters that need to be tested in the safety testing of nail penetration include
(1) Temperature changes at different positions of lithium-ion batteries during nail penetration process
(2) Voltage changes in lithium-ion batteries during nail penetration process
(3) The self heating rate, initial thermal runaway temperature, reaction level, and Arrhenius coefficient of lithium-ion batteries during the piercing process.
These parameters are used to analyze the possible reactions that may occur during the nail penetration process of lithium-ion batteries, as well as the occurrence of thermal runaway. At present, research groups at home and abroad have analyzed the process and its impact on battery safety from different perspectives through nail penetration experiments.
2 Actual test results
Conducted a nail penetration test on a 18650 lithium-ion battery with a capacity of 22 Ah and found that as the nail penetration rate increased, the probability of the lithium-ion battery passing the safety test increased. After studying the effect of needle speed on the safety of lithium-ion batteries in China, it is believed that needle speed has a relatively small impact on the needle safety of cylindrical batteries; And it has a significant impact on the safety of soft pack power batteries. Specifically, the higher the needle penetration speed, the greater the possibility of thermal runaway of the battery. Some people believe that when the needle penetration speed is slower, the local heat generation of the battery is higher.
It can be seen from this that the conclusions of the above three works are not consistent. There are many reasons that can cause this situation. Firstly, the winding and stacking structures are different, with the winding type battery having tighter contact between the layers. Secondly, when the nail penetration speed is low, on the one hand, the extensibility of the diaphragm protects the battery and prevents the occurrence of internal short circuits. On the one hand, after an internal short circuit occurs, the duration of local high current increases. In addition, different thicknesses of copper foil, aluminum foil, positive and negative electrodes, and separators can also lead to different test results at different needle piercing speeds.
Conducted needle piercing tests on fully charged lithium-ion batteries using cubic steel needles with dimensions of 40 mm x 1.5 mm x 1.5 mm from different positions of the battery. They found that the position in the middle of the green edge of the battery, far away from the pole ear direction, caused the greatest temperature rise and had the worst safety. They believe that the main reason for this phenomenon is the poor thermal conductivity of the battery edge separator, which limits the thermal dissipation of lithium-ion batteries.
Conducted nail penetration tests on 18650 lithium-ion batteries with a nominal capacity of 22 Ah under different states of charge (SOC). It was found that as SOC decreases, the probability of lithium-ion batteries passing safety tests increases. This is because the higher the state of charge, the higher the initial voltage of the battery. This further increases the internal short-circuit current and prolongs the short-circuit time. As a result, the safety of lithium-ion battery nail penetration testing becomes worse.
Conducted a nail penetration analysis on fully charged 604-1104 m Ah lithium-ion batteries and found that the higher the battery capacity, the worse the safety of nail penetration testing for lithium-ion batteries.
In addition, conducted nail penetration testing analysis on polymer lithium-ion batteries using ceramic separators. They collected the temperature of multiple batteries with different SOC in the nail penetration area and battery surface, the voltage changes of the batteries, and the burr state of lithium-ion batteries after nail penetration. They analyzed the mechanism of nail penetration based on this. It is believed that the process of needle piercing through the battery causes aluminum burrs and copper burrs to connect, forming an internal short circuit between aluminum foil and copper foil.
The temperature of the local short-circuit zone increases with the generation of Joule heat. If the temperature reaches the melting temperature of aluminum, the aluminum burrs will melt and burn, causing a circuit breaker with the copper burrs. They can be summarized into three models: Model A, where the aluminum burrs melt and the copper burrs no longer come into contact. The aluminum burrs in Model B did not melt and formed an internal short circuit in contact with the copper burrs. The aluminum burrs in Model C do not completely melt, and after a period of time, they come into contact with the copper burrs again, forming an internal short circuit. They believe that changing the combustion and melting of aluminum burrs is a new direction for battery safety design.
Not only recorded the temperature near the battery needle, electrode temperature, and battery voltage in the nail penetration experiment, but also recorded the pressure changes on the battery surface and found a clear correspondence between the pressure peak on the battery surface and the temperature peak on the battery.
They believe that in addition to battery voltage and temperature, battery safety can also be described by increasing other measurement quantities such as pressure. In the above experiment, people can analyze the nail penetration of lithium-ion batteries more realistically through experimental methods. However, the type of short circuit that occurs inside the battery during the nail penetration test is only speculation and does not have good theoretical support. The nail penetration model of lithium-ion batteries is another method to analyze the nail penetration mechanism of lithium-ion batteries and improve their safety performance.
3 Conclusion
This article summarizes the experimental analysis of the needle punching process in lithium-ion batteries. Among them, modeling and simulation of battery nail penetration process is an important tool for studying the mechanism of battery nail penetration, improving battery structure design, and enhancing battery safety performance. At present, the experimental method cannot clearly indicate the safety issue of battery nail penetration. However, the existing nail penetration models are not sufficient to accurately describe the nail penetration process and the internal short circuit caused by lithium-ion batteries. Therefore, further improving and perfecting the nail penetration model of lithium-ion batteries is an important means to enhance the safety analysis of internal short circuits in batteries.