## 3 Results
### Single Impedance Values over Capacity Decay  
In Figure 9, impedance spectra measured on \(TC1\) and \(TC2\) of _hard AK3.4_ are shown. They were measured with \(-1\)I\({}_{20}\) DC-current and after 15-135 FCE. The spectra of the whole cell are on the left side, and the spectra of the negative electrode are on the right side. Only impedance values of frequencies between 6 kHz and 80 mHz are presented, as they fulfill the TI criterion. This part of the spectrum corresponds to the ohmic resistance and the negative electrode. Impedance values of the positive electrode are only present in the tail of the whole cell spectrum as a superposition with impedance values of the negative electrode. Figure 8: Impedance spectra with charging DC-currents measured on \(TC1\) of _hard AK3.4_ after 120 FCE at the end of cycling phase and after recharging. Figure 9: Impedance spectra measured on \(TC1\) and \(TC2\) of _hard AK3.4_ with \(-1\)I\({}_{20}\) after 15–135 FCE after the cycling phase. The spectra of the whole cell and the negative electrode are presented. The impedance spectra of the negative electrode are measured using the reference electrode. Impedance values, which do not fulfill TI criterion, have been removed. For all spectra, an increase in \(Re\{\underline{Z}\}\) in the high-frequency part is visible. The increase of the first half-cycle is well recognizable in the spectra of the negative electrode. For both test cells, the radius of the first half-cycle indicates a steeper increase toward the end of the aging test. Although the test cells had similar impedance spectra at the beginning of the aging test, the first half-cycle in the spectrum of \(TC1\) is considerably larger than in the spectrum of \(TC2\). \(TC1\) ended the aging test with a smaller capacity than \(TC2\). Figure 10 shows single impedance values measured at all test cells after cycling and after charging phase (or at 80% SoC accordingly) over capacity loss \(C_{loss}\). The first values after cycling phase and after charging for _hard AK3.4_ are from the check-up after 15 FCE and not from the initial impedance measurements. In the upper graph, the frequencies at which the impedance contains nearly onlya real part are given. This frequency increases with increasing capacity loss for all test cells and independently of the pre-history. The frequencies are defined by the _EISmeter_ and have the discrete values of 336 Hz, 450 Hz, 600 Hz, 800 Hz, and 1067 Hz. Figure 10: Single impedance values of all test cells in _hard AK3.4_ and _soft AK3.4_ test recorded with \(-\)1I\({}_{20}\) after the cycling phase (**left**) and after charging (**right**). In the upper graph, the frequencies at which the imaginary part of impedance is near zero, are plotted over capacity loss \(C_{loss}\). In the middle graph, the corresponding impedance amplitudes are given. In the lower graph, impedance amplitudes measured at 0.1 Hz, 10 Hz, and 1 kHz are presented. The data of different test cells are distinguishable by the marks and the different frequencies in the lower graph are distinguishable by the line color. In the middle graph, the corresponding impedance amplitudes, at which the imaginary part is nearly zero, are plotted. The values with cycling pre-history are very similar during the aging test, while the values measured after charging have different trends depending on the aging test. The test cells in the _soft AK3.4_ test show a slower increase in the impedance than the test cells in the _hard AK3.4_ test. In the lower graph, impedance amplitudes at fixed frequencies of 0.1 Hz, 10 Hz, and 1 kHz are presented. The values of the same frequency change their location in the spectrum over the aging test, because the time constants of the processes change. This is shown in Section 3.3 for the charge-transfer process on the negative electrode. At 1 kHz, the impedance amplitudes measured after cycling have similar values. The maximum difference is 1.2 m\(\Omega\).