Positive Electrode Passivation by LiDFOB Electrolyte Additive in High-Capacity Lithium-Ion Cells

The effect of LiF2BC2O4 (LiDFOB) electrolyte additive on the capacity and impedance characteristics of cells with Li1.2Ni0.15Mn0.55Co0.1O2-based positive, graphite-based negative and LiPF6-based electrolyte is evaluated using a combination of electrochemical cycling and surface analysis techniques. The impedance rise in these cells occurs primarily at the positive electrode whereas the capacity loss results from lithium trapping at the negative electrode. The LiDFOB serves as a bifunctional additive and reduces both cell capacity loss and impedance rise by reacting at both electrodes; probable reaction mechanisms have been highlighted in this article. X-ray photoelectron spectroscopy (XPS) data show that the LiDFOB additive forms a thicker passivation layer that inhibits electrolyte oxidation and reduces dissolution of Mn, Ni, Co ions from the positive electrode. Secondary ion mass spectroscopy (SIMS) data show that a thinner but more robust SEI forms at the negative electrode, which helps maintain cell capacity. The synergistic effects of LiDFOB at both electrodes results in a marked improvement in cell electrochemical performance.