The challenge of polyanion redox in oxalates

Abstract

The pursuit of high-energy-density lithium-ion and sodium-ion batteries has intensified the search for advanced cathode materials capable of multielectron redox processes. Polyanionic compounds, particularly those containing oxalate groups, have garnered significant interest due to their potential to exhibit both cationic and anionic redox activity. This study focuses on elucidating the conditions that trigger or suppress the redox behaviour of oxalate within these materials.

We have systematically investigated a series of transition metal oxalates, including those with sodium or potassium, and also mixed with other polyanionic groups. Electrochemical analyses, notably differential capacity (dQ/dV) plots, reveal characteristic peaks indicative of dual redox activity involving both the transition metal (Fe) and the oxalate anion (C₂O₄²⁻). These observations are corroborated by ex-situ Raman spectroscopy, Mössbauer spectroscopy, HAXPES analysis, and density functional theory (DFT) calculations, confirming the reversible participation of oxalate groups in the redox processes.

However, our findings indicate that the anionic redox activity of oxalate is significantly suppressed in the presence of additional polyanionic groups such as phosphate, sulfate, or anions like fluoride. This suppression is attributed to several factors: strong inductive effects from these groups reducing the electron density available for oxalate oxidation, increased structural rigidity hindering necessary lattice adjustments for redox activity, and competition for electron delocalisation that favors more electronegative anions over oxalate.

Understanding these inhibitory effects is crucial for the strategic design of next-generation cathode materials. By balancing structural stability with redox flexibility, it is possible to maximise the dual redox contributions of both cationic and anionic species. Our research provides valuable insights into the structural and compositional prerequisites for enabling oxalate redox behaviour, guiding future development of high-capacity, energy-dense cathode materials for advanced battery applications.

Date of Award	3 Jul 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Robert Armstrong (Supervisor)