Charge and mass transport properties of LiI-P(EO)_n-Al₂O₃-based composite polymer electrolytes

We recently demonstrated that LiI-P(EO)_n-Al₂O₃-based composite polymer electrolytes (CPE) appear to be good candidates for use in lithium batteries, providing high conductivity and good electrochemical and thermal stability. The main goals of the present investigation were to study and analyze with the use of linear-sweep-voltametry LSV and AC techniques, charge and mass transport properties of a broad spectrum of CPEs (80 ≥ n ≥ 1.5). The following parameters were determined at 120°C, conductivity (σ), Warburg impedance (Z_d(0), apparent t₊, apparent diffusion coefficient (D) and limiting-current density for Li deposition. Several assumptions were made: (1) The salt is fully dissociated in the electrolyte, (2) Only the salt cation M⁺ is being reduced, (3) The reaction M⁺ + e → M(s) is reversible, (4) There are no adsorption phenomena. It was found that these assumptions are not completely valid at least for concentrated CPEs. An additional arc appears in the Nyquist plot at a low frequency range, indicating the formation of ion pairs or aggregates. However, there were similar dependencies of D and σ on n, each going through a maximum at n = 20. The limiting current density, measured for 100 μm thick CPEs increased monotonically from 0.3 to 2.3 mA/cm² with n varying from 80 to 9. The calculated values of the limiting-current density (with the use of D values determined by AC) and experimental linear-sweep-voltammetry data are in a good agreement.