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

Abstract

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 0.3 M LiPF6 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.