Electrochemical kinetic studies of Li-ion in O2-structured Li_2/3(Ni_1/3Mn_2/3)O₂ and Li_(2/3)+x(Ni_1/3Mn_2/3)O₂ by EIS and GITT

Abstract

The kinetics of Li-ion intercalation into O2 structure layered nickel-manganese oxides, Li_2/3(Ni_1/3Mn_2/3)O₂[O2 (Li)] and Li_(2/3)+x(Ni_1/3Mn_2/3)O₂, x = 1/3 [O2 (Li + x)] were determined by electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) in conjunction with other electrochemical techniques. Modeling the EIS data with equivalent circuit approach enabled the determination of charge transfer, bulk, and surface film resistances. The formation and nature of surface film is shown to be the signature for the cell performance which in turn affects the kinetics of electrode processes. The improved cycling performance of O2 (Li + x) is shown to be due to the better electrode kinetics and the formation of stable surface film. The Li-ion diffusion coefficient (D_Li) was determined at different cell potentials by GITT on O2 (Li) and O2 (Li + x) and analyzing the Warburg region of the impedance plots of O2 (Li + x). The D_Livalues are in the range 1.0×10⁻¹¹ to 10⁻¹⁰ cm²/s for both the compounds in the entire composition (voltage) range. The D_Li (GITT) values are lower by a factor of two as compared to those obtained from EIS in the entire voltage range for O2 (Li + x). The irreversible phase change from the T2 to O2 structure observed during the first charging in these compounds, is reflected as minima in the D_Li vs. voltage plots in the vicinity of the cyclic voltammetric peaks.