High-Temperature Neutron and X-ray Diffraction Study of Fast Sodium Transport in Alluaudite-type Sodium Iron Sulfate

Sodium-ion battery is a potential alternative to replace lithium-ion battery, the present main actor in electrical energy storage technologies. A recently discovered cathode material Na2.5Fe1.75(SO4)3 (NFS) derives not only high energy density with very high voltage generation over 3.8 V, but also high-rate capability of reversible Na insertion as a result of large tunnels in the alluaudite structure. Here we applied high-temperature X-ray/neutron diffraction to unveil characteristic structural features related to major Na transport pathways. Thermal activation and nuclear density distribution of Na demonstrate one-dimensional Na diffusion channels parallel to [001] direction in full consistence with computational predictions. This feature would be common for the related (sulfo-)alluaudite system, forming emerging functional materials group for electrochemical applications.