Invited Lecture
Atomic Transport in Battery Materials

The function of rechargeable batteries depends decisively on atomic or ionic transport in solid materials. In intercalation compounds, which are most commonly applied as active material of battery electrodes, atomic transport is best understood by interstitial diffusion. Thin film active materials are technological interesting for all-solid-state batteries or protective coatings of conventional bulk materials. But they also represent suitable model structures for kinetic studies. After a short introduction to the principles of battery function, electronic and ionic transport, electrochemical measurements as well as thin film deposition of relevant materials, the talk considers generic examples of kinetic studies.

Thin film intercalation compounds of the olivine structure LiFePO₄ and the spinel phase LiMn₂O₄ are deposited by ion-beam sputtering. The kinetics of Li intercalation is studied by cyclo-voltammetry under variation of the cycling rate over 4 to 5 orders of magnitude. The well-defined layer geometry allows a detailed quantitative analysis that directly compares both materials. It is shown that LiFePO₄ undergoes phase separation during intercalation, although the material is nano-confined and very high charging rates are applied. Adapted to a phase separating system, we present a modified Randles-Sevcik evaluation of diffusivity.

It is shown that both charging currents and overpotentials depend on the film thickness in a systematic way. The analysis of this dependence yields strong evidence that in both materials the grain boundaries are important to understand the electrochemical behavior. They provide fast paths of transport but also increase the electrochemical active area with increasing layer thickness. Evidence is derived that the grain boundaries in LiFePO₄ have the character of an ion-conductor of vanishing electronic conductivity.