Lithium ion battery has the advantages of high specific energy, high specific power, high voltage platform, small self discharge, long cycle life, low environmental pollution and no memory effect. It has been widely used in mobile phones, computers, electric vehicles, military, space technology and other related fields. However, lithium ion batteries bring some potential safety hazards while bringing benefits to mankind. For example, lithium ion batteries may cause fire, explosion and other hazards under abuse conditions (such as overcharge, over-discharge, short circuit, extrusion, acupuncture, high temperature, etc.).
In recent years, accidents related to the safety of lithium ion batteries continue to occur, which poses a great threat to people’s personal and property safety. The safety performance of lithium battery is still an urgent problem to be solved. From the perspective of lithium battery itself, it is the carrier of energy itself, which has unsafe factors. Different capacity production processes, and whether the use method is standardized will greatly affect the safety performance of the battery.
At present, there are many researches on the influence of positive and negative electrode materials, electrolyte, diaphragm and other factors on the safety performance of lithium batteries at home and abroad, but there is little research on the influence of cell structure and battery shell material on the safety performance of lithium batteries. In this paper, the influence of cell structure and battery shell material on the safety performance of lithium battery was studied in depth.
The three kinds of batteries used in this experiment are 18650 type steel shell battery, winding type soft package battery and laminated type soft package battery with a capacity of 2 Ah. The difference is that the battery cell structure and packaging shell material structure are divided into winding type and laminated type. The cells of the steel case battery and the wound flexible battery are of the wound structure, and the cells of the laminated flexible battery are of the laminated structure. The materials include aluminum plastic composite film and nickel plated steel.
Safety Performance Test
In this study, three tests of external short circuit, acupuncture and overcharge were used to characterize the safety performance of the battery. The above three safety tests were carried out in accordance with international standard. Under room temperature conditions, the nail penetration test is carried out with the nail penetration test chamber. The overcharge performance is tested with the test cabinet.
The test results show that the temperature of laminated flexible battery, wound flexible battery and 18650 steel shell battery rises rapidly in a short time when external short circuit occurs, and the maximum temperature is 64, 82 and 102 ℃ respectively. Due to the protection of the dry diaphragm, although the battery is short circuited externally, the two poles of the battery still show a certain voltage early. When the battery voltage rises to zero, the surface temperatures of the three kinds of batteries reach their maximum temperatures respectively. When the short-circuit current drops to zero, the battery discharge stops and the battery surface temperature gradually drops to room temperature. There is no fire or explosion after the short circuit of the three kinds of batteries.
The safety valve of the steel shell battery is opened and there is liquid leakage. No obvious phenomenon is found for the other two kinds of batteries. The reason for the above phenomenon is that at the moment when the copper wire is connected, the positive and negative electrodes of the battery form a closed circuit through the copper wire, the voltage decreases rapidly, the current increases instantaneously, and the short circuit current can reach more than 60 A. The heat generated by the large current through the internal resistance of the battery and the heat accumulation. The surface temperature of the battery increases rapidly.
From the short circuit experiment, it can be concluded that the temperature rise of the three kinds of batteries is different. The maximum temperature rise of the steel shell battery is the smallest. In addition to taking extreme conditions such as fire and explosion as the judgment criteria, the surface temperature change of the lunar pool is also the intuitive data of the experimental results, which can characterize the advantages and disadvantages of the short circuit test. The higher the battery temperature is, the worse its safety performance is. The above results show that the battery safety of the three production processes is from high to low, respectively, laminated flexible battery, wound flexible battery and 18650 steel shell battery.
It can be seen from the test that the temperature of the wound flexible battery and 18650 steel shell battery rises to 190 ℃ and 239 ℃ respectively in a short time, and the steel shell battery catches fire and burns; A large amount of smoke was emitted from the coiled flexible battery, but there was no fire. Then the surface temperature of the battery decreases gradually. However, the laminated flexible battery did not smoke or burn, and the maximum temperature was only 82 ℃.
The mechanism of the above reaction is that after the battery is punctured, the steel needle and the positive and negative pole pieces of the battery form a closed circuit, and an instant internal short circuit occurs. The voltage at both ends of the battery drops rapidly to zero, and the current increases sharply, generating huge heat. The surface temperature rises rapidly, which leads to the further expansion of the melting short circuit area of the diaphragm, causing a vicious cycle.
The difference between the results of wound flexible battery and 18650 steel shell battery lies in the different packaging shell materials. The former uses aluminum plastic film for packaging and the latter uses nickel plated steel for packaging. The reason why the bursting pressure of the former is much lower than that of the latter is that the aluminum plastic composite film of the soft packed lithium ion battery has certain ductility and low mechanical strength. In the case of internal short circuit, the battery is easy to bulge and exhaust, reducing the explosion risk. The steel shell battery is a closed structure, which will produce great explosive power in case of explosion.
The difference between the results of coiled flexible battery and laminated flexible battery lies in the difference of their cell structures. The coiled cell consists of one positive sheet, one negative sheet and two diaphragms, which are wound by the winding machine to form an internal resistance of about 50 m Ω. The cells of the laminated flexible battery are alternately stacked with the positive and negative electrode plates and the diaphragm, wherein the diaphragm is Z-shaped. The laminated battery is equivalent to multiple batteries in parallel, with an internal resistance of about 10 mΩ.
In addition, as the number of punctured layers of the winding type electric core is more than that of the laminated electric core, the contact area of the short circuit is larger, and the heat generated is more. Therefore, the surface temperature of the coiled flexible battery is 108 ℃ higher than that of the laminated flexible battery. To sum up, the laminated flexible battery has the highest safety performance.
The research results show that 18650 type steel case battery has serious ignition and combustion phenomenon in the acupuncture experiment. During the 3 C,5 V overcharge test, the wound type flexible battery has bulge and ignition and combustion phenomenon, while the laminated flexible battery only has bulge phenomenon. The steel case battery has no obvious potential safety hazard due to the protection of its own safety valve. Therefore, the cell structure and battery shell material are important factors affecting the safety performance of lithium ion batteries.