Abstract
Lithium-ion batteries may experience thermal runaway when subjected to overcharging, external short-circuits, or overheating, resulting in smoke and heat generation. To prevent this, lithium-ion batteries usually have a protection circuit. However, the protection circuit cannot prevent internal short-circuits caused by contamination with foreign matter. As a result, lithium batteries have generated heat or ignition or eroded in notebook computers, airplanes, and electric vehicles. The test method to simulate this internal short-circuit is specified in IEC 62133 Ed. 2.0 as the forced internal short-circuit (FISC) test in the case of portable electronic devices.1 However, no internal short-circuit simulation test is specified for lithium-ion batteries used in vehicles, so studies for similar test methods are being conducted.
In the FISC test, internal short-circuits are caused by disassembling the battery, inserting an Ni piece into the electrodes, inserting the battery into a polyethylene bag, and then applying an external force to the portion where the Ni piece is inserted. However, this test involves disassembling the battery, so sometimes it is difficult to implement due to technology and safety concerns. Therefore, the Blunt rod test and ceramic nailing test, which do not need battery disassembly, are proposed as simple and safe alternatives of this test. In the Blunt rod test, an internal short-circuit is caused by pressing the battery from the outside. In the ceramic nailing test, the short-circuit is caused by pricking the nail. In this study, we examined the validity of these tests as a substitute for the FISC test.
For the tests, a total of five kinds of batteries were used, including three pouch types, one cylinder type, and one rectangular type. The main test conditions were an atmospheric temperature of 25±5 ºC, SOC of 100%, restricted or unrestricted battery, and a measurement duration of 1 hour after the voltage drop. The nailing rate was 0.05 mm/s, the voltage was measured at a sampling ratio of 1 ms, and pressurization or nailing was stopped when the battery voltage dropped by 2 mV or more and 5 mV or more. The FISC test was conducted in accordance with IEC 62133; the Blunt rod test, with nail diameters of 3 mm (r=3 mm) and 6 mm (r=6 mm); and the ceramic nailing test, with a tip angle of 45º , tip metal length of 0.3 mm and 1.0 mm, and a nail diameter of 3 mm.
As an example, part of the results of tests conducted by stopping the Indenter of pouch cell A when the voltage dropped by 2 mV or more with an unrestricted battery are depicted in Table 1. Here, the short-circuited layers were counted as one each time a hole was opened in the separator between the positive and negative electrodes. In this example, the Blunt rod test had a greater voltage drop and more short-circuited layers than in the FISC test, sometimes resulting in smoking. The voltage drop in the ceramic nailing test, on the other hand, had a voltage drop in the same range resulting in the same event as in the FISC test but more short-circuited layers. This difference could have been caused by the required load and the number of short-circuited layers being larger in the Blunt rod test because the short-circuit was caused by pressing the battery with a round nail, and the load required in the ceramic nailing test was smaller because the tip of the nail was sharp. This trend was also the same in four other kinds of batteries and under other test conditions. Therefore, under these test conditions, we consider that the ceramic nailing test is more valid than the Blunt rod test as a substitute for the FISC test based on comparisons of the number of short-circuited layers, volume of voltage drop, and observed event.
Acknowledgments
This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
References
1) IEC 62133 Ed. 2.0:2012 (b), "Secondary cells and batteries containing alkaline or other non-acid electrolytes - Safety requirements for portable sealed secondary cells, and for batteries made from them, for use in portable applications," 2012-12-06
Figure 1