Lithium-ion batteries are indispensable in the automotive sector as energy storage devices due to their energy and power density. With increasing energy contents of automotive battery systems, enabling greater ranges for the vehicles, safety requirements become an even more important aspect. However, present condition monitoring of lithium-ion batteries is limited to current, voltage and temperature measurements outside the cell. The integration of electronics and sensor technologies into the inside of the cell enables direct and unaltered measurements and extends battery monitoring to a new scale. However, it must be taken into account that current battery cell formats have little useful space available inside and the electrolyte is a chemically aggressive environment for electrical components.
In this talk, the integration of a novel capacitive measuring method into a battery cell is presented. By introducing a new type of sensor in a 2-D arrangement, we use the little available space with no visible negative effect on the total capacity of the assembled cell. However, this includes previous testing of different materials and coatings in the aggressive environment of the electrolyte. Furthermore, allowing no leakage of the same electrolyte, the integration of any kind of electronic into cell is a challenge and therefore the tightness of the battery cells has to be ensured at all times.
A coplanar interdigital arrangement of a capacitor enables to measure the interior of the cell and to sense changes in the material. The measured variation of the capacitance results from the direct change of the relative permittivity due to internal processes and their implicit temperature dependence. Since those temperature dependencies are critical for all known measurement setups a direct measurement of internal temperature offers a new insight into operating conditions of battery cells. This is particularly important, as cells used in automotive applications get larger, with gradients between the surface and the core of the cell becoming more relevant. A capacitive measurement of the cell interior using an interdigital capacitor structure enables a possible application for in-operando diagnosis.
As a proof-of-concept, a few coin cells were equipped with such a sensor and are still being tested. Further steps include analyzing the collected data and correlating significances in the sensor data, but already shows promising first results. Thus, an advanced characterization of battery cells can be achieved with the integration of such a sensor technology.