Lithium ion batteries have become one of the main power sources for portable electronic products such as mobile communications and laptops due to their high specific energy and high voltage. However, under abusive conditions such as heating, overcharging, over discharge short circuit, vibration, compression, etc., lithium-ion batteries may experience incidents such as fire, explosion, and even personal injury, resulting in a large number of lithium-ion batteries being recalled.
Therefore, how to improve the safety performance of lithium-ion batteries has become a key issue in their development. Currently, many countries or testing institutions have developed relevant safety testing methods for lithium-ion batteries. Lithium ion batteries must pass safety tests to reduce their usage risks. The international standards related to the safety of lithium-ion batteries mainly include IEC 62133, IEC 62281, UL 1642, UL 2054, UN 38.3, etc.
In the above standards, the heavy object impact test is a project that simulates the internal short circuit of lithium-ion batteries. This article conducted heavy object impact tests on different models of lithium-ion batteries according to the requirements of the standard. The test results were compared and analyzed by dismantling the battery after the experiment.
1.1 Test equipment and environment
The testing instrument is the DGBELL explosion-proof impact test chamber. The ambient temperature during the experiment was always maintained at (20 ± 5) ℃.
1.2 Test samples
The lithium-ion battery samples used in this experiment are as follows: 18650 cylindrical lithium-ion battery; Square aluminum shell battery; Square lithium-ion polymer battery
1.3 Test methods
Place lithium-ion batteries of different shapes on a flat surface, place a 15.8 mm diameter iron rod horizontally at the center of the battery, and let a 9.1 kg hammer fall from a height of (610 ± 15) m onto the battery. For square batteries, they should also be rotated 90 ° along the long axis to withstand heavy impacts on both the wide and narrow surfaces. The temperature on the surface of the battery during the experiment is monitored by a thermocouple attached to the surface of the battery.
2.1 18650 lithium-ion batteries
After being hit by a heavy object, a deep dent is left on the surface of the battery, which is mainly caused by the compression of the iron rod placed on the surface of the battery after being hit by a heavy hammer. Rust appeared on the surface of the positive pole of the battery, mainly due to the opening of the internal pressure relief valve during the impact test, causing some electrolyte to flow out. The electrolyte produced corrosive substances upon encountering air, resulting in the corrosion of the positive pole cap.
After disassembling the battery casing, cracks appeared on the surface of the battery cells. This is mainly attributed to the extension and deformation of the electrodes caused by the iron rod squeezing the battery. The battery cell was unfolded, and both the electrode and the opening film broke at the location where the iron rod was squeezed. The positive and negative electrode coating materials have experienced detachment.
2.2 Square battery
(1) Aluminum shell battery
For square lithium-ion batteries, there are generally two packaging forms: aluminum shell and aluminum-plastic.
The battery also showed dents due to the compression of the iron rod and became very thin in the middle. After disassembling the aluminum shell, it can be seen that the internal battery cells have been divided into two halves. By unfolding the battery cell, it can be seen that the coating of the battery material has peeled off, and the electrodes are wrinkled. This is due to the impact of the iron rod on the interior of the battery, which generates squeezing force in the plane direction. Additionally, the high hardness of the aluminum shell hinders the extension of the battery cell in the horizontal direction, resulting in wrinkling of the electrodes.
After the narrow surface test of the square battery, the narrow surface direction of the battery has been completely squeezed and deformed. After disassembling the battery casing, it was found that the internal battery cells had serious fractures. Upon unfolding the electrode, it was discovered that the active material had fallen off and the diaphragm had ruptured.
(2) Lithium ion polymer batteries
For lithium-ion polymer batteries packaged in aluminum-plastic. The results of the battery test are similar to those of aluminum shell packaged batteries. However, due to the thermal sealing of the aluminum-plastic shell, the internal battery cells are exposed to external forces after the battery test. Secondly, due to the thin thickness of the aluminum-plastic packaging battery, it was divided into two parts under external impact.
2.3 Discussion of Test Results
From the above test results, it can be seen that the heavy object impact test can simulate the internal short circuit of lithium-ion batteries. In the heavy object impact test, the battery was subjected to external forces, causing deformation of the battery casing. And it caused deformation of the battery cell, causing tension on the electrodes and separator. Under the action of this stress, the electrode material will experience detachment, and the diaphragm will rupture due to its thinness, resulting in direct electronic conduction between the positive and negative electrode materials, or contact between the copper (aluminum) collector and the positive (negative) electrode material (i.e. internal short circuit), resulting in locally large discharge current and Ohmic heat.
Due to internal short circuits occurring in multiple locations, the heat generated causes other side reactions, such as electrode decomposition and electrolyte decomposition. During the experiment, there was a significant increase in battery temperature. For the 18650 lithium-ion battery model, an internal short circuit generated a significant amount of gas, causing an increase in internal pressure. After reaching a certain value, the battery pressure relief valve opened to release acidic gas, ultimately avoiding the occurrence of safety accidents. For square lithium-ion batteries, their thickness is relatively small. In the event of external impact, the battery may rupture or split in half. When an internal short circuit occurs, the internal components of the battery are directly exposed to the air and react.
Through heavy object impact tests on different types of lithium-ion individual batteries, it can be seen that this test method can effectively simulate the situation of internal short circuits in batteries. The disassembly analysis of different types of batteries found that there were electrode breakage and diaphragm rupture inside the batteries, resulting in internal short circuits inside the batteries. The analysis and understanding of this experimental phenomenon will be more helpful for battery manufacturers to understand the testing standards of batteries, thereby improving the product quality of lithium-ion batteries and enhancing their safety.