## 3 Results
### Material and Volume Distribution  
In contrast to the experimental data, the model allows detailed insights into the processes of the cathode space. The model provides predictions of the concentration distributions and the exchange current density over the simulation period. In Figure 7a, the concentration distribution of the active material \(NiCl_{2}\) across the segments is plotted. The distribution for different DoDs during a discharge cycle with 1/8C is shown. First, the model is initialized with a homogeneous concentration distribution over the entire cathode space (\(DoD=0\)). Afterwards, the reduction of \(NiCl_{2}\) can be observed beginning from segment #1, which is located at the separator. Even though the largest mass conversion is identified for segments close to the separator, it is evident that segments participate over the entire cathode area. As the active material is depleted in segments near the separator, the reaction contribution of segments deeper in the cathode space increases. Hence, for \(DoD=0.5\), \(NiCl_{2}\) is no longer available in the first segments and accordingly, a reaction front [32; 33] appears from segment #20 onwards. Finally, for \(DoD>0.7\), \(NiCl_{2}\) is available exclusively in the segments located close to the current collector and the conversion is reduced to a lower level. At this point, the reaction front has migrated deep into the cathode space, which increases the distance the ions have to travel. As described above, the ionic resistance increases with distance. This leads therefore to a massive drop in cell voltage until finally \(E_{OCV,Fe}\) is reached. From this point onward, a change in concentration of the second active material, \(FeCl_{2}\), can be observed (Figure 7b). First, the initialized, homogeneous distribution of the second active material stays constant for \(DoD<0.7\). For \(DoD>0.7\) and consequently a cell voltage below \(E_{OCV,Fe}\), the reduction of \(FeCl_{2}\) starts from the separator, similar to the \(NiCl_{2}\) reaction progression. As described by [PERSON] et  
Figure 6: (**a**) Resistance due to ion transport over DoD. (**b**) Electrical resistance in the solid metal matrix over segment number, for \(DoD=0.5\). Both graphs have been modelled for discharging with 1/8C. al. [33], in addition to the \(NiCl_{2}\) reaction front near the current collector, a second reaction front forms at the separator, which also migrates towards the current collector. Therewith, the distance of the ions to the second reaction front and thus the ionic resistance are reduced. The related reduction in overpotential leads to the formation of the second voltage plateau in the discharge curve (Figure 4a). The calculation of the concentration distribution is based on the exchange current density, which is also shown across the segments in Figure 7c. Here, the findings from Figure 7a,b are combined in one plot. One additional point may be highlighted here. At \(DoD=0.7\), the exchange current density increases significantly near the current collector. Due to the \(NiCl_{2}\) depletion, no current is provided by the segments closer to the separator and consequently the voltage losses increase dramatically. In the next step, for DoD = 0.8, the second reaction front related to \(FeCl_{2}\) reduction appears at the separator. Due to the lower losses, the \(FeCl_{2}\) reaction at the separator is preferred and the contribution of segments closer to the centre is strongly reduced. This clear shift in the exchange current density distribution corresponds to the change in the plateaus in the voltage curve (Figure 4). It is worth highlighting that in this case, both active compounds provide current at the respective reaction front. As shown in the theoretical part of the paper, the material composition, porosity and geometry have a significant effect on cell resistance. In cases where the resistances are too high, there is a risk that active material will not be converted, resulting in a loss in capacity.