Dramatic Effects of Low Salt Concentrations on Li-Ion Cells Containing EC-Free Electrolytes

Different concentrations of LiPF6 (0.3 M-2 M) in ethyl methyl carbonate (EMC) electrolyte and ethylene carbonate (EC)-based electrolyte were studied in LiNi0.4Mn0.4Co0.2O2 (NMC442)/graphite pouch cells. Fresh cells containing 0.3 M LiPF6 in EMC electrolyte showed extremely large charge transfer resistance while those with 0.3MLiPF(6) in EC/EMC electrolyte did not. Impedance spectra taken on symmetric cells and ionic conductivity measurements suggest this difference is due to difficulty in dissociating and desolvating Li+ ions from the EMC-based electrolyte to intercalate into both the electrodes. After elevated temperature storage experiments at 4.5 V, cells with 0.3 M LiPF6 in EC/EMC showed a large increase in positive electrode charge transfer impedance, presumably caused by electrolyte oxidation. With salt concentrations greater than 1 M, charge transfer resistance was much smaller in EMC-based electrolytes and was stable during storage for both electrolyte types. Conductivity and cycle testing measurements suggest that 1.5 M LiPF6 should be used in EC-free EMC-based electrolytes to optimize cell performance. (C) The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.