UC Santa Barbara

The continued reliance on a select few elements, namely Li, Ni, and Co, for rechargeable battery cathodes is untenable with the increasing demand for batteries in electric vehicles, grid-scale energy storage, and portable electronics. In this dissertation we have explored three alternative cathode classes exhibiting improved sustainability: 1) Li-ion disordered rocksalt oxyfluorides, 2) cryolite-like Na3FeF6, and 3) weberite Na2Fe2F7. The first class enables a greater variety of transition metals to be utilized, but their long-range disorder necessitates short-range characterization which we have accomplished using 7Li and 19F solid-state nuclear magnetic resonance (ss-NMR). The second and third class, forming the bulk of this dissertation, completely depart from Li as the intercalant ion and instead utilize Na.While Na-ion batteries have emerged as a clear alternative for Li-ion batteries in select applications, their energy densities are still largely limited by the cathode materials. Thus, a paradigm shift in the development of competitive Na cathodes hinges on the investigation of new structural frameworks and anion chemistries. Here, we investigated two sodium iron fluoride materials whose differing structures lead to significantly different cycling mechanisms – conversion in cryolite-like Na3FeF6 due to its dense structure, and intercalation in weberite Na2Fe2F7 due to its more open framework structure. The former material, Na3FeF6, we find via an ex situ analytical method based on 23Na NMR and operando magnetometry, forms NaF and Fe on discharge, which is only moderately reversible due to sluggish conversion reaction kinetics. In the latter material, weberite Na2Fe2F7, we find more reversible electrochemical intercalation behavior, but a phase transformation to perovskite NaFeF3 still occurs upon cycling. Through compositional tuning and synthetic control, this phase transformation can be suppressed leading to a promising new Na-ion cathode material. Additional computational characterization highlights the complex phase stability landscape inherent to weberite materials, leading to polymorphism and metastability as observed experimentally. Thus, we have developed a set of computationally informed design rules to guide further exploration of the weberite structure class.