Ionic transport in heterogeneous solid electrolytes: role of interfaces between poor ionic conductors

Next-generation battery technologies promise higher energy density, improved safety, and longer cycle life, but realizing these advantages critically depends on the availability of solid electrolytes with sufficiently high ionic conductivity. Many candidate materials are fundamentally constrained by extremely poor bulk ion transport, constituting a major bottleneck for practical implementation. Remarkably, numerous studies have demonstrated that composites composed of two or more poor ionic conductors can exhibit ionic conductivities that substantially exceed those of the individual phases, an effect that has been systematically reviewed and rationalized within the nanoionics framework. This counterintuitive enhancement is commonly attributed to internal interfaces, where space-charge layers, defect redistribution, and structural disorder locally promote ion transport. Despite extensive experimental evidence and established phenomenological descriptions, a quantitative and mechanistic understanding of how interfacial structure and connectivity govern macroscopic ionic transport remains incomplete[1].

This master’s thesis investigates ionic transport in heterogeneous solid electrolytes, with a particular emphasis on how interfaces between poor ionic conductors can dominate overall transport. LiF-based composite thin films are employed as a model system, chosen for their extremely low bulk ionic conductivity and their relevance to conversion-type battery electrodes, where interfacial effects are expected to play a decisive role. The work builds on established thin-film and microfabrication expertise to create measurement platforms in which lateral ionic transport can be probed over long interface lengths, making interfacial contributions directly accessible. Within this controlled geometry, electrochemical transport measurements are interpreted in relation to interface density and microstructure, allowing bulk and interfacial transport pathways to be meaningfully distinguished.

The results of this work are expected to clarify the physical mechanisms governing interfacial ionic transport, including space-charge layer formation and percolation effects, and to establish design principles for engineering interfaces that enhance conductivity. These insights will directly inform the development of high-performance solid electrolytes and conversion electrodes for next-generation battery technologies.