Constructing Unique Cathode Interface by Manipulating Functional Groups of Electrolyte Additive for Graphite/LiNi0.6Co0.2Mn0.2O2 Cells at High Voltage.

A novel electrolyte additive, 1-(2-cyanoethyl) pyrrole (CEP), has been investigated to improve the electrochemical performance of graphite/LiNi0.6Co0.2Mn0.2O2 cells cycling up to 4.5 V vs Li/Li+. The 4.5 V cycling results present that after 50 cycles, up to 4.5 V capacity retention of the graphite/LiNi0.6Co0.2Mn0.2O2 cell is improved significantly from 27.4 to 81.5% when adding 1% CEP to baseline electrolyte (1 M LiPF6 in EC/EMC 1:2, by weight). Ex situ characterization results support the mechanism of CEP for enhancing the electrochemical performance. On one hand, the significant enhancement is ascribed to a formed superior cathode interfacial film by preferential oxidation of CEP on the cathode electrode surface suppressing electrolyte decomposition at high voltage. On the other hand, the duo Lewis base functional groups can effectively capture dissociation product PF5 from LiPF6 with the presence of an unavoidable trace amount of water or aprotic impurities in the electrolyte. Thus this mitigates the hydrofluoric acid (HF) generation that leads to the reduction of transition-metal dissolution in the electrolyte upon cycling at high voltage. The theoretical modeling suggests that CEP has a mechanism of stabilizing electrolyte via combination of -C≡N: functional group and H2O. The work presented here also shows nuclear magnetic resonance spectra analysis to prove the capability of CEP reducing HF generation and X-ray photoelectron spectroscopy analysis to observe cathode surface composition.