Redox enhanced slow ion kinetics in oxide ceramics

Abstract

AbstractThere is a growing interest in field‐assisted fast ceramic processing and long‐term service of electrochemical devices under harsh operation conditions. These extreme redox conditions can make some conventionally thought slow ions highly mobile in electrochemical materials and devices and greatly accelerate the microstructural evolution. Experimental observations for enhanced slow ion diffusion that controls mass transport include accelerated grain growth in zirconia and ceria under atmospheric and electrochemical reduction above 1000°C and lattice cavitation of high‐voltage Li‐rich layered cathodes of lithium‐ion batteries at room temperature. These observations cannot be explained by altered defect concentrations under defect chemistry arguments. Instead, they support hugely enhanced slow ion mobility. My colleagues and I recently proposed a new mechanism resulted from strong electron–phonon interactions and dynamic electronic relaxations at the saddle point of an ion hopping event, which significantly lowers the ion migration barrier. In this perspective article, the theory and its experimental and computational supports shall be discussed. Such an electronic effect holds even in nominally ionic ceramic materials with wide bandgaps and is expected to be general in many transition metal oxides. It rationalizes a variety of intriguing experimental observations and offers an electronic perspective on the redox enhancement of slow ion kinetics in structural, functional, and energy ceramics.