The challenge of polyanion redox in oxalates

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We recently reported dual ion redox in the lithium iron oxalate, Li2Fe(C2O4)2 in which both the iron and the oxalate group appear to exhibit reversible redox activity.1 Our subsequent results suggest that this phenomenon is by no means unique to this compound and is quite widespread among transition metal oxalates, including those of sodium. These give rise to a characteristic dQ/dV plot consisting of a large pair of peaks accompanied by two smaller ones. These are very sharp, implying that an oxalate radical is involved, which may show excellent kinetics. The oxalate anion is a particularly versatile species and may be monodentate, bidentate, tridentate or even tetradendate, giving rise to a huge range of possible compounds and a very rich structural chemistry. It is important to establish the structural requirements that give rise to polyanion redox behaviour and a range materials will be synthesised and characterised. In addition to pure oxalates it is possible to prepare materials with mixed polyanions e.g. oxalate/phosphate. These will be investigated to establish whether polyanionic redox activity extends to this family of compounds.Synthetic methods will include solvothermal with Prof. P. Lightfoot in St Andrews (new to the Faraday Institution) and microwave synthesis at Sheffield. The new materials will be studied by extensive use of the NEXGENNA advanced characterisation platform, including diffraction, XAS, and Mössbauer spectroscopy via our partners in Montpellier. These materials typically exhibit poor electronic conductivity so part of the project will be devoted to electrode optimisation. Interaction with the multiscale modelling project will provide additional insights to this unusual phenomenon. Since materials exhibiting this property are not confined to sodium, but include potassium and lithium we may generate new compounds relevant to either of the cathode projects.1. W. Yao, A. R. Armstrong et al. Nature Commun. (2019) 10, 3483