(Invited) Investigation of Thermal Propagation Test Methods of Lithium-Ion Batteries for Vehicles

Abstract

Introduction The thermal propagation test is conducted to evaluate the influence of ignition or rupture of the battery pack on its surroundings in case of the thermal runaway due to an internal short circuit etc. of a single cell in the battery pack. One of the issues to be considered regarding the thermal propagation test is the unclearness of the method of appropriately inducing thermal runaway of a single cell in the battery pack. Even if the same packs are tested, the final evaluation result may be affected by different situations in the initiation cell (heat generation only, smoke, fire, etc.) depending on different methods of induction. In addition, applicable initiation methods may differ according to the type of cell. Accordingly, it is necessary to investigate appropriate initiation methods in order to establish the thermal propagation test method that provides fair evaluations. Therefore, the applicability of thermal runaway generation methods such as overcharge, heater heating, and nail penetration was investigated by using several kinds of cells. Experimental At first, overcharge, nail penetration, and heater heating tests were conducted using a single cell to investigate the reproducibility of such methods and the applicability to each type of cell. Three types of single cell were tested: pouch type, prismatic type, and cylindrical type. For the overcharge test, it was charged at a rate of 1C or 2C from the 100% state of charge. For the nail penetration test, all electrode layers in the cell were penetrated at a penetration speed of 0.1 mm/s by a steel nail with a diameter of 3 mm and a tip angle of 60°. For the heater heating test, it was heated at a heating rate of approximately 100°C/min using a ceramic heater for the pouch and prismatic cells, and a wire heater for the cylindrical cells. Furthermore, a battery pack composed of pouch cells was evaluated by the method that showed good applicability in the single cell test, in order to confirm whether a single initiation cell thermal runaway can occur in the battery pack. Results and discussion In the single cell tests, applicability of the each initiation method was evaluated with regard to the following three points: 1) good reproducibility of the occurrence event, 2) cell temperature exceeding the upper limit of operating temperature (about 60 to 80°C) in the battery pack which indicates thermal runaway occurring, and 3) upper limit of the calorific value during thermal runaway almost equivalent to the original value of a single cell. Evaluation item of 3) was adopted because of the possibility that the sound battery pack may fail if thermal runaway of the initiation cell causes excessive heat generation. As a result of evaluating the applicability of the initiation methods based on the data obtained from the test, only the heater heating method proved to be applicable to any of the cell types in this study. A battery pack composed of pouch cells was tested using a heater heating method which showed good applicability in the single cell test. Fig. 1 presents the test results. The initiation cell reached thermal runaway appropriately 35 seconds after the start of heating. The cell voltage was measured for two cells connected in parallel. Since the cell temperature rapidly increased immediately after the voltage decreased by approximately 0.6 V, it was confirmed that the thermal runaway of the first one cell occurred at this point. From the above, it was confirmed that thermal runaway of a single initiation cell can occur even in a battery pack composed of pouch cells. AcknowledgmentsThis paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1