Investigating conductivity variations in Al stabilized LLZO by combining microelectrode impedance spectroscopy with spatially resolved chemical analysis.

Institute of Chemical Technologies and Vienna, Austria. E-mail: stefan.smetaczek@ Department of Chemistry and Physics of M Austria Institute for Chemistry and Technology of M Graz, Austria Christian Doppler Laboratory for Lithium Technology of Materials, Graz University of Austrian Institute of Technology, Vienna, A † Electronic supplementary informa 10.1039/c9ta00356h Cite this: J. Mater. Chem. A, 2019, 7, 6818

The oxygen stoichiometry of mixed conducting oxides depends on the oxygen chemical potential and thus on the oxygen partial pressure in the gas phase. Also voltages may change the local oxygen stoichiometry and the amount to which such changes take place is quantified by the chemical capacitance of the sample. Impedance spectroscopy can be used to probe this chemical capacitance. Impedance measurements on different oxides ((La,Sr)FeO3-δ = LSF, Sr(Ti,Fe)O3-δ = STF, and Pb(Zr,Ti)O3 = PZT) are presented, and demonstrate how the chemical capacitance may affect impedance spectra in different types of electrochemical cells. A quantitative analysis of the spectra is based on generalized equivalent circuits developed for mixed conducting oxides by J. Jamnik and J. Maier. It is discussed how defect chemical information can be deduced from the chemical capacitance.

Oxygen ion conduction in Nd3+‐doped Pb(ZrxTi1−x)O3 (PZT) was investigated by impedance spectroscopy and 18O‐tracer diffusion with subsequent secondary ion mass spectrometry (SIMS) analysis. Ion blocking electrodes lead to a second relaxation feature in impedance spectra at temperatures above 600°C. This allowed analysis of ionic and electronic partial conductivities. Between 600°C and 700°C those are in the same order of magnitude (10−5–10−4 S/cm) though very differently activated (2.4 eV vs. 1.2 eV for ions and electron holes, respectively). Oxygen tracer experiments showed that ion transport mainly takes place along grain boundaries with partly very high local ionic conductivities. Numerical analysis of the tracer profiles, including a near‐surface space charge zone, revealed bulk and grain‐boundary diffusion coefficients. Calculation of an effective ionic conductivity from these diffusion coefficients showed good agreement with conductivity values determined from impedance measurements. Based on these data oxygen vacancy concentrations in grain boundary and bulk could be estimated. Annealing at high temperatures caused a decrease in the grain‐boundary ionic conductivity and onset of additional defect chemical processes near the surface, most probably due to cation diffusion.

The oxygen partial pressure (pO2) dependent conductivity of Nd3+ doped PZT was investigated at different temperatures in order to get information on the nature of the conducting charge carriers and on the chemical oxygen diffusion coefficient. From a positive slope close to 0.25 in log (conductivity) vs. log (pO2) plots it is concluded that PZT with donor-type Nd3+ shows predominant hole conduction already at moderately high oxygen partial pressures, while under reducing conditions electron conduction prevails. This unexpected occurrence of hole conduction in a nominally donor doped system is interpreted in terms of the only partially controllable loss of lead oxide during preparation, which results in the formation of lead vacancies. Those act as acceptors at the temperatures employed in this study (560–615 °C) and turn the nominally donor doped material into a slightly acceptor doped one. From the conductivity relaxation after partial pressure changes, chemical oxygen diffusion coefficients could be extracted and values of about 6.5·10−9 cm2/s at 560 °C and 2·10−8 cm2/s at 611 °C are found in air.