Identifying contact resistances in high-voltage cathodes by impedance spectroscopy

LiNi_0.5Mn_1.5O₄ (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 (RCont.) between the aluminum current collector and the cathode electrode rather than a surface film resistance (RCEI). 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 RCont. 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.