Safety First Safety First - This is the mission of Dr. Carlos Ziebert, Head of the Center. The overall safety assessment is a prerequisite for battery technology upgrades and market acceptance, as an uncontrollable increase in temperature throughout the system (so-called thermal runaway) can cause the battery to ignite or even explode, causing negative public rejection or even rejection. As energy density increases, the safety of lithium-ion batteries becomes more and more important. Therefore, calorimetry is the basic technique for obtaining quantitative data on thermal and safe behavior - you need to know how many watts the cell will produce under each condition. This information can be used to adjust battery management, thermal management and safety systems.
Internal Pressure Measurement Method Internal pressure is an important parameter because its increase can be used as an early warning signal for thermal events, which may later lead to thermal runaway. A new calorimetric method for measuring internal cell pressure has been established on 18650 cells and has recently been transferred to bag cells. The pressure line connected to the pressure sensor is inserted directly into the battery, as shown in the X-ray tomography image of the 18650 battery in Figure 1a) and the left side of the pouch battery in Figure 1b. The battery was then resealed with epoxy and placed in a calorimeter chamber with heaters and thermocouples located at the lid, bottom and side walls. Finally, the internal pressure [1] was measured during the so-called Heat-Wait-Seek (HWS) test. This test begins with heat by heating the cell mode at a temperature step of 5K. At the end of each step, the wait mode is activated to achieve thermal equilibrium. The system then enters the search mode, looking for the temperature rate and ending in two possible modes - the heat release mode or the heat mode. If the measured temperature rate is greater than the initial sensitivity (usually 0.02 K / min), the system enters the exothermic mode. This mode represents a quasi-adiabatic condition, which means that the temperature of the calorimeter immediately follows the surface temperature of the battery; that is, the cells are no longer in heat exchange with the surrounding environment. As a result, it heats up more and more until thermal runaway occurs or the chemical is completely consumed by the exothermic reaction. On the other hand, if the temperature rate is small, the system returns to the heating mode.
The resulting graph clearly shows the increase in internal pressure. In the case of a 18650 battery, when a critical pressure of 12.5 bar is reached, this leads to the opening of the safety valve (s. Fig. 2a)) [2]. For pouch batteries, the weld is opened at a pressure of about 2.6 bar and a temperature of about 110 °C. This method is also applicable to large prismatic Car Batteries. The next step is to develop a pressure sensor that can be integrated into the battery management system (BMS) as an additional safety feature.
