UCLA

A nontraditional double potential step (DPS) methodology along with two more commonly used approaches for examining interface kinetics in battery materials are presented and applied to the well-studied systems of lithium cobalt oxide, LixCoO2, and LiPON solid electrolyte. Values for the heterogeneous rate constants of LixCoO2 at the nonaqueous PC/LiClO4 and solid electrolyte interfaces are determined and equivalencies between electrochemical impedance spectroscopy (EIS), the method of Nicholson, and the double potential step measurements of interfacial kinetics are elucidated, while a qualitative mechanistic understanding of the heterogeneous rate constant is discussed. The anodic and cathodic heterogeneous rate constants for the PC/LiClO4 – LixCoO2 interface were 1.1�10-6 and 4.8�10-5 cm s-1 respectively, while the corresponding values for the sputtered LiPON - LixCoO2 solid-solid interface were 1.2�10-7 and 6.7�10-8 cm s-1, respectively. These provide an effective comparison respective to carbon graphite, ko = 3.0�10-7, LixMn2O4 where ko = 5.5�10-8, and LixTiO2 (Anatase) where ko = 2.6�10-10. We further use the method of Nicholson and EIS to assess the quality of and changes in kinetics between a common room temperature ionic liquid, BMIM-TFSI, and its quasi-solid porous ionogel counterpart, while indicating the shortcomings of the earlier explored DPS method for high-rate interface kinetics. Variations in kinetic parameters are attributed to the effects of confinement within the porous ionogel network at 11 and 8 nm average pore size, producing a reduction in the diffusion coefficient of 70 percent, and a reduction in the heterogeneous interfacial kinetics of 56 percent.