Correlated Ionic Motion in Solid-State Electrolytes

Speaker: Vanessa Ward, Durham University
Series: Lennard-Jones Centre

A comprehensive understanding of ion transport is necessary for predicting conductivity in solid electrolytes. It is also key to the rational design of new materials. Ion transport is a dynamic process spanning a wide range of timescales, from the atomic level to the device scale. Here, we focus on how transport mechanisms at an atomistic level affect macroscopic transport coefficients. There are a number of different ways in which motion can be correlated. It is possible to derive a mathematical framework for the correlations in both liquids and solids. Firstly, the motion of an individual ion may not follow a random walk and there is self correlation. Self-correlated events include an ion returning to a previous position, in a back-and-forth motion. Ions may become trapped due to local structure, particularly if there are limited directions of travel available. They will undergo significant movement, but in a back-and-forth motion that does not contribute to overall transport. In contrast, distinct correlations involve multiple ions. For example, one ion may vacate a position and the newly created vacancy allows another ion to move. These moves could occur concurrently or sequentially. It is also worth noting that moves that appear connected may actually be dynamically independent, occurring only as a consequence of the structural change caused by a previous event. An induced rather than a synchronous collective mechanism. We compare different methods for identifying distinct correlations including k-means clustering and directed graphs. Characterising these correlations is an important step for designing next-generation batteries.