Abstract
LiNi0.5Mn1.5O4 (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.
Highlights
To cite this article: Daniel Pritzl et al 2019 J
Physical origin of the high-frequency semi-circle in LNMO cathodes.—So far, the literature offers two different interpretations about the origin of the high-frequency semi-circle in cathode impedance spectra. This feature has been ascribed to a surface film resistance associated with the formation/growth of a charge-transfer or surface film (CEI) on LNMO cathodes;[14,15] on the other hand, it has been ascribed to an electronic contact resistance at the cathode electrode/aluminum current collector interface for LFP and LNMO cathodes.[18,23]
To clarify this discrepancy in interpretation, we first investigate the effect of the carbon black content of the LNMO cathodes on the high-frequency impedance response, using LNMO cathodes with a carbon black content of either 5% or 1.5% at a constant porosity of ≈30% (Type I and Type 2 cathodes, see Table I), which were cycled in a three-electrode cell with graphite as anode and equipped with a gold wire reference electrode (GWRE) to monitor the impedance of the LNMO cathode
Summary
Identifying Contact Resistances in High-Voltage Cathodes by Impedance Spectroscopy. To cite this article: Daniel Pritzl et al 2019 J. While graphite/LNMO cells have a reasonable cycle life at room temperature, they usually show severe capacity fading at elevated temperatures (> 40°C).[5,6] The reasons for the poor capacity retention are frequently related to: i) manganese dissolution from the LNMO cathode and its subsequent reduction on the surface of the graphite anode (often described as cross-talk phenomenon), which damages the solid-electrolyteinterphase (SEI) on the graphite anode and catalyzes further electrolyte decomposition,[3,6,7,8,9] thereby resulting in a loss of active lithium; (ii) electrochemical electrolyte oxidation at the high LNMO cathode potential,[10,11] which leads to the release of protic species (e.g., HF),[12] often seen as a key driver for transition metal dissolution and concomitant SEI damage;[6,13] and, (iii) an increase of the full-cell impedance over cycling.[6] Based on detailed impedance studies, the latter is often ascribed to an increase of the LNMO cathode impedance.[14,15] For example, Aurbach et al.[14] measured the impedance of the LNMO cathode versus a lithium wire reference electrode after cycling at elevated temperatures (60°C), and observed both a high-frequency semicircle (apex frequency ≈1 kHz) and a low frequency semi-circle (apex frequency ≈100 mHz). We found that the main resistance increase over the course of extended charge/discharge cycling of graphite/LNMO cells at 40°C comes from an increase of RContact rather than from an increase in the LNMO charge-transfer or surface film (CEI) resistance that was suggested previously.[14,15]
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