Abstract
Vinylene Carbonate (VC) is an effective electrolyte additive to produce a stable solid electrolyte interphase (SEI) on graphite anodes, increasing the capacity retention of lithium-ion cells. However, in combination with LiNi0.5Mn1.5O4 (LNMO) cathodes, VC drastically decreases cell performance. In this study we use on-line electrochemical mass spectrometry (OEMS) and electrochemical impedance spectroscopy (EIS) with a micro-reference electrode to understand the oxidative (in-)stability of VC and its effect on the interfacial resistances of both anode and cathode. We herein compare different VC concentrations corresponding to VC to graphite surface area ratios typically used in commercial-scale cells. At low VC concentrations (0.09 wt%, corresponding to 1 wt% in commercial-scale cells), an impedance increase exclusively on the anode and an improved capacity retention is observed, whereas higher VC concentrations (0.17 wt – 2 wt%, corresponding to 2 wt - 23 wt% in commercial-scale cells) show an increase in both cathode and anode impedance as well as worse cycling performance and overcharge capacity during the first cycle. By considering the onset potentials for VC reduction and oxidation in graphite/LNMO cells, we demonstrate that low amounts of VC can be reduced before VC oxidation occurs, which is sufficient to effectively passivate the graphite anode.
Highlights
Chair for Technical Electrochemistry, Department of Chemistry and Catalysis Research Center, Technical University of Munich, Munich, Germany
In order to investigate the onset potential for Vinylene carbonate (VC) oxidation and its products, we performed on-line electrochemical mass spectrometry on electrolytes based on only VC or ethylene carbonate (EC) mixed with 1 M LiPF6
The small current starting at ∼3.7 V vs. Li/Li+ for VC is most likely related to the oxidation of the BHT stabilizer, as its integration between 3.67 V and 4.0 V yields a charge of 10.2 mAs compared to the 11.9 mAs theoretically needed for the 1-electron oxidation of the 200 ppm BHT contained in VC
Summary
The electrolyte/active material mass ratio in the latter is ∼12-fold lower compared to commonly used lab-scale cells, so that a concentration of 2 wt% VC in the study from Burns et al.[10] based on 18650 cells would corresponds to ∼0.17 wt% VC in a typical lab-scale cell. This scaling factor between commonly used lab-scale cells and commercial-scale cells needs to be considered when using lab-scale cells to evaluate the effect of additives. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract)
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