Effect of electrolyte components, salt concentration, and electrolyte additive, on electrolyte distribution over a charging LiMO₂ electrode

This paper discusses the electrolyte distribution over the surface of a 4.5 V LiCoO₂ electrode via linear sweep voltammetry using the Pt probe of a scanning electrochemical microscope. On a typical electrolyte composition for Li-ion batteries (LIBs), 1.0 M LiPF₆ EC/DEC (3/7, vol. %), the reduction peak of non-solvated free-ethylene carbonate (EC) increased, and the peak of diethyl carbonate (DEC) decreased as the Pt probe approaching the oxidizing LiCoO₂ substrate. When the salt concentration increased, the reduction current by the free-EC decreased as the Pt probe approaching the substrate. By adding 1,3,5-trifluorobenzene as an electrolyte additive, the separation of these two reduction peaks became unclear, and the variation of the peak current almost disappeared closer to the oxidizing electrode. Compared to their electrochemical performance for LIBs, the electrolyte distribution measured by the Pt probe over the electrode surface has a relationship with electrolyte decomposition at the 1st charging process, charge transfer resistance (R_ct) at the electrode interface, electrolyte impregnation into the coated electrode, and electrochemical performances of LIBs.