In the literature, the impedance spectrum (in Nyquist representation) of the LNMO electrode (measured versus a reference electrode) is commonly described by a high-frequency semi-circle and one at lower frequencies . The impedance spectra are interpreted as a surface film resistance (RSF) which is attributed to the semi-circle at high frequencies and a charge transfer resistance (RCT) for the semi-circle at low frequencies [3, 4]. The aim of this study is to examine the true origin of the semi-circle at high frequencies and its implications for the failure mechanism of high-voltage cells.
This was examined by assembling graphite/LNMO and graphite/LFP Swagelok T-cells equipped with a gold-wire reference electrode (GWRE)  and using 1 M LiPF6 in EC/EMC (3/7 by volume) as electrolyte. The cycling procedure was carried out at 40 °C. The GWRE allows the measurement of the half-cell impedances (anode vs. RE and cathode vs. RE) during cycling, so that the impedance evolution of each electrode can be monitored independently.
By means of temperature dependent impedance measurements (0°C, 10°C and 40°C) we will show that the semi-circle at high frequencies (100 kHz – 100 Hz) corresponds to a resistance at the interface between the LNMO electrode and the current collector (contact resistance, RContact) rather than to a surface film resistance (RSF). This is based on the finding that the semi-circle at high frequencies shows negligible temperature dependence, whereas the semi-circle at low frequencies (100 Hz – 1 Hz) strongly depends on temperature. A low temperature dependence is typically reported for a contact resistance in Li-Ion batteries .
Furthermore, we will prove that the high-frequency semi-circle corresponds to a contact resistance by: (i) compressing the T-cell after 50 cycles, and (ii) coating the LNMO electrode on a glassy carbon support. In the first case (i), the high frequency semi-circle is no longer observable, and in case (ii), the increase of RContact is lower by a factor of ten compared to a LNMO electrode coated on an aluminum substrate.
As a next step, the potential dependence of the increasing RContact will be investigated. In Figure 1, graphite/LFP cells are charged to different upper-cut-off potentials (4.0 V, 4.5 V and 5.0 V cell voltage). It can clearly be seen that between 4.5 and 5.0 V the LFP impedance shows an additional semi-circle at high frequencies (RContact). This leads us to the conclusion that the increase of RContact is a potential dependent rather than a material dependent property and thus might also play a role in all high-voltage materials (e.g., in overlithiated NCM’s with upper-cut off potentials of »4.7 V vs. Li/Li+). Lastly, we will show XPS spectra of cycled current collectors and identify the origin of RContact.
Figure 1: (a) Two formation cycles (C/10) are carried out in a graphite/LFP (on carbon-coated aluminum foil) cell with a GWRE (two glassfiber separators, 60 µL of 1 M LiPF6 in EC/EMC, 3/7). Subsequently, the cell is charged to 4.5 V and 5.0 V cell voltage at C/2. (b) The Impedance is recorded at 50% SOC (indicated by black symbols in a) after each upper potential step (at 40°C from 100 kHz to 0.1 Hz with a perturbation of 0.5 mA).
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