Identifying Contact Resistances in High-Voltage Cathodes by Impedance Spectroscopy

LiNi 0.5 Mn 1.5 O 4 (LNMO) cathodes cycled versus a graphite anode at elevated temperatures usually show severe capacity fading upon extended charge/discharge cycling. In the literature, the impedance increase at the cathode is often related to the formation of a so-called cathode/electrolyte interphase (CEI) and is presented as one of the possible failure mechanisms. In this study, we show that the main reason for the increasing cathode impedance is a contact resistance ( R Cont. ) between the aluminum current collector and the cathode electrode rather than a surface film resistance ( R CEI ). First evidence is presented by temperature-dependent impedance measurements and external compression of the electrode stack in the cell, which suggest an electronic nature of the commonly observed high-frequency semi-circle in a Nyquist plot. Further, by coating the LNMO cathode onto a glassy carbon disk, we demonstrate that the impedance increase arises from the interface between the cathode electrode and the aluminum current collector. Finally, we examine whether R Cont. correlates with the release of protic species (e.g., HF) formed upon electrolyte oxidation. This is done by cycling graphite/LFP cells in the absence/presence of deliberately added HF, showing that a contact resistance upon cycling only develops upon HF addition.