(Invited) The Role of Electrons in Nominally Ionic Ceramics: From Fast Ion Conductors to Mixed Ion Conductors

Abstract

This presentation will describe several observations that can only be explained if the role of electrons, however few in number, is considered. We will start with acceptor stabilized cubic zirconia such as YSZ, not because it is the most important fast ion conductor but because it is the simplest. The simplicity lies in its constant oxygen/oxygen-vacancy concentration over a very wide range of pO2, which is especially so at lower temperatures. Over this range, oxygen conductivity is constant and completely overwhelms electronic conductivity. Therefore, to drive a constant ion current in YSZ at the steady state, there must be a constant electric field meaning no discontinuity in the electric potential. From the reaction O2=2O2-+4e, the chemical potentials of O2-and electron/holes are correlated to that of O2, as are their electrochemical potentials. (For O2, the electrochemical potential is the same as the chemical potential.) Without any discontinuity in the electric potential in YSZ, any discontinuity in the electrochemical potential of O2-must be reflected in a discontinuity in the chemical potential of O2; likewise, any discontinuity in the electrochemical potential of electron/hole must also be reflected in a discontinuity in the chemical potential of O2. Generally, such discontinuity in electrochemical potential comes from a kinetic bottleneck. Although the above argument was made for electron, O2-and O2in YSZ, the same argument applies to fast cation conductors. In the latter case, Li+or Na+, for example, will replace O2-, and the discontinuity is now manifest in the chemical potential of Lior Na instead of O2. This has important consequences as it may lead to overshoots of chemical potentials of O2, Li or Na, which explains the following observations of inordinate phase formation at the kinetic bottlenecks in the ionic channel (i) oxygen bubbles in YSZ electrolyte in electrolyzers, (ii) Na islands in beta-alumina electrolyte in sodium batteries, and (iii) Li islands in Li7La3Zr2O12electrolyte in all-solid lithium ion batteries. The most common bottlenecks of this kind are transverse grain boundaries that lie normal to the current direction, which are indeed the locations where inordinate phases were found in (i-iii). Kinetic bottlenecks also exist in the electronic channel because electrode polarization often turns the interior distribution of electrons and holes into one of a p-n junction, with the junction being an electronic bottleneck having the extremely low intrinsic conductivity of a rather wide band-gap semiconductor. A sharp oxygen-potential transition thus results at these bottlenecks, causing a drastic change in the redox condition going from one side to the other, which can lead to a dramatic change in microstructure. Led by such observations, we have discovered that the mobility of the slowest ion can be elevated by many orders of magnitude by such extreme redox conditions. First-principles calculation revealed the origin of the enhanced mobility: electrons tend to localize at the saddle point of a migrating ion—electrons are attracted to it because of (a) its locally soft environment that enhances electron-phonon interactions, and (b) its broken symmetry that avails low-lying electronic states. This was found for cations in zirconia, ceria and barium titanate. Remarkably, the finding also applies to anion diffusion as is the case in [Li,(Ni,Co,Mn)]O2, used as a cathode in lithium ion batteries. Qualitatively similar phenomena occurs in mixed ion conductors resulting in other interesting effects. For example, the p-n junction being a conductivity minimum becomes a hot spot under Joule heating. The attendant oxygen-potential transition being akin to a first-order phase transition leads to an interface instability and finger-like growth. In the talk on “In Situ Filamentary Resistance-Switching Experiments Using Bulk Oxide Single Crystals and Polycrystals” (Symposium G05, Oxide Memristors II), I will describe how these effects can lead to a metal-insulator transition responsible for the formation and breakage of a conducting filament in the so-called resistance-switching memory. Thus, there is indeed a very important role played by electrons in many nominally ionic ceramics, from fast ion conductors to mixed ion conductors.