In recent years, the global energy crisis and environmental pollution have become increasingly serious. As a clean and pollution-free energy storage, lithium-ion batteries are gradually applied to new energy vehicles, solar energy conversion energy storage equipment, mobile communication equipment and other fields. At present, new energy vehicles with lithium-ion batteries appear in more and more cities, but some potential safety hazards are gradually exposed. According to the fire investigation report of new energy vehicles, the vast majority of fires are caused by the battery heat out of control. Improper use, accidental damage or defects of lithium-ion batteries may lead to explosion and fire. Among many reasons that trigger the thermal runaway of the battery, acupuncture is an irreversible destructive behavior that causes the battery damage. When the battery is pierced by sharp objects or subjected to a large impact force, it will cause mechanical damage to the battery, break the internal structure of the battery and expose the internal materials directly. At the same time, it is easy to cause a short circuit between the positive and negative poles inside the battery, which will generate a large amount of heat and make the temperature rise rapidly, causing thermal runaway hazards.
Previous research work mainly focused on the thermal model of battery acupuncture and acupuncture experiment, but in the aspect of acupuncture damage, the existing research on the thermal runaway model of acupuncture did not consider the randomness of the thermal runaway triggered by acupuncture, and the subjects of acupuncture experiment were mostly flat panel batteries, but the anti-acupuncture characteristics of flat panel batteries were good. Based on the existing research work, the cylindrical lithium iron phosphate battery is selected as the object, and the changes of battery shape, battery voltage and battery surface temperature in the event of thermal runaway are emphatically studied, providing reference value for the safe use and acupuncture prevention of lithium iron phosphate battery.
1.Occurrence Mechanism
In the cylindrical lithium iron phosphate battery, each electrode unit is composed of positive and negative electrodes, bipolar current collector, diaphragm, etc. The electrode unit is immersed in the electrolyte and sealed in the battery shell, and the battery pressure relief valve is located near the positive pole lug of the battery. When the battery is punctured by a bayonet, multiple electrode units are punctured, and all the punctured electrode units participate in the discharge. Due to the error of the punctured position of the battery and the slight deviation of the battery manufacturing process, the prick and the damaged electrode unit will produce different random contact interfaces, and the random interface will often affect the discharge effect of the electrode unit and the size of the contact interface resistance.
When the battery is punctured, the particles inside the battery will migrate. The positive and negative current collectors inside the battery are equivalent to a short circuit state, and a large current is generated instantaneously from the positive current collector to the negative current collector through the bayonet. Li is removed from the lithium structure embedded in the negative electrode and enters the electrolyte, and migrates through the diaphragm to the positive electrode. When the battery is punctured, the random contact interface will affect the internal discharge of the battery.
If the contact between the needle and multiple electrode units is good, there will be multiple electrode units participating in the short-circuit discharge, and the heat will be more. If the needle only contacts with a few electrode units, the number of electrode units participating in the discharge is relatively small, the heat generated will also be relatively small, and the thermal runaway reaction of the battery is relatively mild. During the process of internal short circuit of the battery, more heat will be generated during the violent discharge process, which will cause the temperature of the battery to rise.
In general, the specific situation of the battery suffering from the internal short circuit of the needle is complex and changeable, and it is difficult to measure the precise changes of the parameters such as the short circuit current, the internal resistance of the battery, and the electrode unit participating in the reaction. Therefore, this paper will study and analyze the external macro battery voltage changes and the surface temperature of the battery.
2.Nail Penetration Test Chamber
The main body of the acupuncture experimental platform is based on the battery extrusion acupuncture testing machine produced DGBELL. In the testing chamber, the needle speed can be set to 20 mm/s through the operation panel, and the needle is selected φ 5 mm tungsten steel needle, the needle stroke is 200 mm (the battery is completely punctured). At the same time, the battery voltage and battery surface temperature will be measured online, and the collected data will be saved in the upper computer for subsequent processing after passing the data acquisition card.
The experimental battery is 32650 lithium iron phosphate battery. The battery is fixed by a special clamp, which can prevent the battery from radial deviation during the experiment. Place the battery in the puncture test chamber with explosion-proof function, peel off the plastic skin at a distance of 15 mm and 45 mm from the negative end of the battery, and place the K-type patch thermocouple measuring point on the surface of the battery shell. The needle position is 30 mm from the negative electrode. After the battery is punctured, the needle stays inside the battery for 600 seconds.
Considering the randomness of the contact interface between the needle and the damaged electrode unit after the needle penetrates into the battery in the experiment, six 32650 lithium iron phosphate batteries with full charge (SOC=1) were selected for the experiment, and the experimental results of six groups of needle experiments were compared and analyzed. The ambient temperature of each experiment is controlled at (20+2) ℃. In order to prevent the previous group of experiments from affecting the next group of experiments, the interval between the two adjacent groups of experiments is 24 hours, and each experiment needs to be replaced with a new tungsten steel needle.