The increasing global demand for portable energy means that the search for new electrode materials for rechargeable solid-state batteries is an important scientific challenge. Lithium-ion batteries are used in a variety of portable devices, but the limited supply of lithium and its rising cost have encouraged a shift in interest towards more sustainable sodium-ion battery alternatives. Recently, we have explored the synthesis and properties of various Fe2+-based polyoxyanionic systems related to phases suitable for use in sodium-ion batteries. Since high-spin Fe2+ is a magnetically active species, it is also vital that we explore the magnetic properties of these new materials in order to have a complete understanding of their behaviours. Here, we propose to explore the magnetic behaviour of the sodium-ion battery cathode analogue, KFe(C2O4)F, by muon spin relaxation.