The vast majority of rechargeable batteries currently used for mobile electronics and vehicle applications are lithium-ion, exploiting their impressive energy densities and relatively high voltage. However, concerns around factors including safety, high cost and geopolitical distribution of the raw materials has motivated studies of other battery chemistries. Na⁺-ion cells are at the most advanced stage of development, driven by the high earth abundance of sodium, whilst multivalent mobile cations such as Mg²⁺ and Al³⁺ potentially offer increased energy densities, but are hampered by large structural changes associated with the insertion/removal of these highly charged species into/from the electrodes.

Until recently, batteries based on mobile anions (rather than cations) have been largely ignored, despite reports of high mobility of F^- ions within solids dating back to Michael Faraday’s studies of the ‘fluoride of lead’, β-PbF₂, in 1838. A major challenge for cells based on fluorine chemistry is the lack of a suitable liquid electrolyte, as most candidates contain HF2- ions with a small voltage stability window and/or react with atmospheric moisture to form HF. However, a significant advance occurred in 2011, with reports of viable cells using solid electrolytes such as BaSnF₄ [1], which has recently attracted the interest of car companies including Honda and Toyota. However, these batteries have a major drawback, needing to be operated at elevated temperatures (typically 80-150^oC) to achieve sufficiently high F^--ion conductivity within the solid electrolyte [2].

This presentation will focus on the structure-property relationships within a number of F^--ion conductors, including a discussion of our current understanding of the archetypal compound β-PbF₂. This will be followed by a more detailed presentation of a number of ternary derivatives of PbF₂, such as KPbF₃ [3], PbSnF₄ [4], CsSn₂F₅ [5] and β-KSbF₄ [6]. Information on the crystal structure, including the nature of the dynamic F^- disorder, has been provided by neutron powder diffraction studies, exploiting the sensitivity of the technique to the locations of the anions in the presence of heavier cations. Finally, the factors that promote extensive F^- disorder within such solid phases will be considered, including the influence of crystal structure on the preferred anion diffusion mechanisms and the role of electron lone-pairs associated with cations such as Pb²⁺ and Sn²⁺ in promoting high F^- mobility.