The heat flow gained from calorimetric measurements provides a good impression of the
condition of a battery cell. By taking a closer look, a distinction between endothermic and
exothermic reactions can be made. While the irreversible parts of the heat flow are mainly
related to the internal resistance and the associated ohmic losses, the reversible parts can
be traced back to changes in the entropy. Both of them change with increased aging.
To investigate the changes of the characteristic heat flow of NMC/Graphite cells, various
aged 21700 cells were examined postmortem. For this purpose, CR2032 coin cells were
produced by removing the electrode materials from the aged 21700 cells. These cells were
then aged under different usage loads (temperature, charging C-rate, DoD) and
compared to a fresh reference cell.
The heat flow was measured with a multi-module calorimeter (NETZSCH; Germany),
which is equipped with a coin-cell module using the DSC-like-method (Differential
Scanning Calorimetry). This setup enables the possibility of an in-situ measurement of
the heat generation, which leads to results that can be compared to DTV (Differential
Thermal Voltammetry) graphs.
The aim of this work is to show the relationships between aging, heat flow and thus the
thermal behavior of the cells. The ageing-specific change in heat flow results in an equally
specific change in the temperature of the cell. This state variable is directly measurable in
electric vehicles. The dependencies found can help to estimate the state of health during
usage for example in a DTV (differential thermal voltammetry) method.