F-ion (de)Insertion

Our group is studying how the fundamental crystal chemistry of intercalation hosts influences the transport of ions through the solid state. We are particularly interested in polyanionic compounds, like sulfates and silicates, which contain rigid subunits consisting of strong covalent bonds between a main group element and its ligands, because of their robust structures and rich compositional phase diagrams. We have done extensive work to demonstrate that these materials exhibit significantly different structural distortions compared to traditional oxide based hosts. Whereas oxides may experience short-range structural distortions to accommodate the volumetric changes required as Li is (de)inserted into the framework, polyanionic hosts must perform cooperative rotations of the rigid subunits in order to satisfy the local bonding within the polyanionic group. This mechanism can be highly beneficial as it is a highly reversible process that maintains the integrity of the structure over thousands of cycles, yet this rigidity may also limit the directions along which ions can hop from site to site, potentially slowing their mobility.

More recently, we have begun to explore intercalation hosts that can reversibly cycle F-ions in a similar way to Li. By doing so, we envision a new paradigm for energy storage based on the concept for energy storage, that relies on the sequential deinsertion of anions followed by the insertion of cations during discharge and vice versa on reversal.