Impedance spectroscopy modeling of lithium borate with silica: A dispersed ionic conductor system

Lithium borate/silica composites, 40 wt% SiO₂ with x·Li₂O+(1-x)·B₂O₃, x=0.33, 0.50, were explored with the goal of achieving Li-ion conductivity enhancements across batches with different compositions and processing steps. Two batches were made for each composition, namely micron and nanoscale batches, which differ in their processing and fabrication methods. Phase and microstructural characterization showed a composite which is consisted of a conductor-rich and an insulator-rich region. Previous dispersed ionic conductors, in which conductivity is enhanced by the insulator/conductor interaction, were modeled mostly by percolation models. However, these percolation models are not compatible with conventional impedance spectroscopy circuit models and complex non-linear regression analysis. Hence, new circuit models were created based upon a brick-layer construction that assumed dispersed ionic conductor phenomena to be present. The new circuit models, which are named the theoretical model (TM) and approximation model (AM), uniquely and accurately fitted the impedance spectroscopy data and correctly showed lower resistance and activation energy values for the interfacial conduction pathways. Meanwhile, a typical circuit model which has two parallel RC circuits (2P) showed an anomalous high-frequency M” tail in some samples which does not correspond to the expectations of a realistic and physical modulus data. The combined activation energy and interface activation energy from the new models were compared to literature values of lithium borate conductors and Li-ion dispersed ionic conductors. The comparison results suggested that conductivity of the lithium borate/silica composites can be further enhanced through compositional, microstructural, and phase control.