2.2. Test analysis
The loadedisplacement curves and the cell voltage
measurements are provided in Figs. 2e4. It is noted that the
cells could endure significant deformation prior to the occurrence
of an internal short circuit. A good repeatability of the
loadedisplacement curves and of the punch displacement, at
which the cell voltage drops significantly, is observed. In some
cases the cell voltage did not immediately decrease to zero, but
remained at a reduced level. In these cases, the unloaded punch has
been applied again and driven further until the voltage signal
decreased to zero, Fig. 2: Exp1. In one of the bending tests, undesired
movement of the clamped bearings has been observed, Fig. 4:
Exp3.
For all tests, the temperature sensors recorded an increasing
temperature from about 25 C to approximately 50 C after the
tests, proving the occurrence of an internal short circuit. Additionally,
the infrared camera recorded high temperature spots at
the beginning of the short circuit (cell voltage drop), Fig. 14 in
Section 3.5, revealing the location of the short circuit. This crucial
information is used as a quality check of the numerical short circuit
criterion developed in Section 3.5.
Interestingly, a sudden drop in the force level is observed in
some of the loadedisplacement curves, just before initiation of
the internal short circuit. The effect is most pronounced for the
load case “indentation”, Fig. 3. The steel housing did not show
any sign of fracture. Hence, it is assumed that the load drop is
caused by structural fracture within the jelly roll. A computer
tomography (CT) analysis of the cell subjected to bending loading
indeed revealed a macroscopic fracture of the jelly roll, Fig. 5. It is
interesting to recapitulate that the onset of the short circuit is
not just due to fracture phenomena of the separator on a microor
meso-scale. Macroscopic through-thickness fracture of the
jelly roll enables relative movement of the “unprotected” anode
and cathode material at the fracture surface, which then easily
can lead to direct physical contact of the anode and cathode, or
to bridging contact via the steel housing, initiating the short
circuit.
For the other load cases, where the zone of internal short
circuit is subject to compressive/shear loading, no such clear
statement can be made regarding the existence of a macroscopic
fracture. It is however likely that existing cracks become closed
due to the compressive state of loading, and hence are difficult to
detect.
In the next section, a numerical short circuit detection criterion
is developed, based on the assumption that fracture within the jelly
roll initiates the short circuit.