Metal Fluoride Open Frameworks for Next-generation K-ion Battery Cathodes

Recent environmental concern has raised our awareness of a low-carbon future, demanding immediate action towards sustainable energy solutions. Latest geopolitical events and rising energy prices have further stimulated a global sense of energy independence and security, calling for drastic expansion and deployment of homegrown renewable energy, such as wind, solar, tidal power etc, all of which require efficient grid storage systems for integration. It is in this context that batteries, particularly lithium-ion batteries (LIBs), come to the spotlight.

Whilst LIBs have been dominating the market for mobility applications, such as portable electronics and electric vehicles, they also make up 90% of the current global grid storage market. However, Li's scarcity in Earth's crust with an uneven geographical distribution means that the Li-ion technology is not a sustainable net-zero solution in terms of affordability and ability to secure energy independence. In contrast, originating from the same alkali-metal family, K-ion chemistry shows several key advantages in its economic viability, strategic relevance, and cycling stability, placing K-ion batteries (KIBs) among the prioritised new battery technologies for future development, especially for stationary applications.

This project aims to solve the bottleneck problem in the current KIB development on cathodes and will design a library of new metal fluoride open framework (MeFOF) materials that can meet the criteria for cathode application in both cost and durability. The design strategy of these materials stems from the structure diversity and chemical flexibility of the alkali-metal-fluoride chemistry and is corroborated by the recent progress in their mechanochemical synthesis. Their crystal structures are supported by corner-shared metal-fluoride-octahedra linkages and exhibit a variety of open-channel motifs with desirable cavity sizes to host large K ions. These unique motifs not only constitute a library of advantageous topologies for facile K ion transport, but also compose a robust and flexible scaffold for reversible K ion accommodation.

In this work, an emphasis will be laid on elucidating the impact of the materials' cavity sizes, structural flexibility, and chemical substitution on their electrochemical performances. Through an investigation of MeFOFs' structure-property relationships, not only will their design principle and optimisation be established to facilitate KIB commercialisation, but a novel laboratory-based instrumentation for advanced atomic structure characterisation via pair distribution function technique will also be developed to expedite the development of novel functional materials that have complex structural properties, with a broader implication beyond energy storage.