Laplace transform impedance analysis in the two-phase coexistence reaction of spinel Li1 + xMn2O4 positive electrode

We demonstrate the Laplace transform (LT) impedance technique for measuring electrochemical lithiation and delithiation in the two-phase system of spinel Li1 + xMn2O4 (0 ≤ x ≤ 1). Square constant charge or discharge current pulses with various current densities are applied to the equilibrated Li1.25Mn2O4 system, and the response overpotential is recorded with various sampling rates. The LT of the current and overpotential as a function of time gives frequency-dependent impedance spectra. The results show asymmetric impedance between charge and discharge. In particular, inductive loop resistance, which may stem from the nucleation and growth mechanisms, is visible for mainly the anodic (charging) process. The LT impedance is fitted by a complex non-linear least squares technique. The resulting separated resistances decrease with current density in the lower frequency region, indicating non-ohmic Butler–Volmer-type behavior.