The Challenge of Polyanion Redox Process in Oxalates
Authors: Manche Alexis Gilles , Pramanik Atin , Armstrong A. Robert 
Published: August 2024

Lately, polyanionic compounds have received great interest as alternative cathode materials to conventional oxides due to their different advantages in cost, safety, structural stability, as well as being environmentally friendly. However, the vast majority of polyanionic cathodes reported so far rely primarily upon the redox reaction of the transition metal for lithium/sodium transfer.

The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is considered to be a promising strategy to successfully enhance the energy density of polyanionic cathodes for Li/Na-ion batteries. In addition to transition metal redox activity, the oxalate group itself also shows redox behaviour enabling reversible charge/discharge and high capacity without gas evolution.

Recently, our group reported dual ion redox in the lithium iron oxalate, in the potassium iron oxalate and in the sodium lithium iron oxalate, Li2Fe(C2O4)2, K2Fe(C2O4)2 & NaLiFe(C2O4)2
1
,2,3
in which both the iron and the oxalate group appear to exhibit reversible redox activity. We also found this phenomenon present in the lithium-rich KLi3Fe(C2O4)3.4

Different characterisation techniques such as Raman spectroscopy or Synchrotron HAXPES and XAS analyses make it possible to show this phenomenon experimentally. First-principles calculations also help to understand the interactions between the transition metal and the oxalate group as the main factor that modulates the cationic and polyanionic redox couples in these materials.

Our current results suggest that this phenomenon could be widespread among transition metal oxalates.

Moreover, the oxalate anion is a particularly versatile species and may be monodentate, bidentate, tridentate, or even tetradentate, 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 of materials are being synthesised and characterised by our group. In addition to pure oxalates, it is possible to prepare materials with mixed polyanions e.g., oxalate/phosphate.

In our work, we demonstrate that oxalate has a role as a family of cathode materials and suggests a direction for the identification and design of electrode materials with polyanionic frameworks.

(1) Yao, W.; Armstrong, A. R.; Zhou, X.; Sougrati, M. T.; Kidkhunthod, P.; Tunmee, S.; Sun, C.; Sattayaporn, S.; Lightfoot, P.; Ji, B.; Jiang, C.; Wu, N.; Tang, Y.; Cheng, H. M. An Oxalate Cathode for Lithium Ion Batteries with Combined Cationic and Polyanionic Redox. Nat. Commun.
2019, 10 (1), 1–9.

(2) Pramanik, A.; Manche, A. G.; Sougrati, M. T.; Chadwick, A. G.; Lightfoot, P.; Armstrong, A. R. K2Fe(C2O4)2: a New Oxalate Cathode for Li/Na ion Batteries Exhibiting a Combination of Multielectron Cation and Anion Redox. Chem.of Mat.
2023, 35, 2600–2611.

(3) Manche, A. G.; Pramanik, A.; Lindgren, F.; Ericsson, T.; Häggström, L.; Armstrong, A. R. NaLiFe(C2O4)2: A New Iron-Based Polyanionic Cathode for Next-Generation Li/Na-ion Battery Exhibiting Reversible Cationic and Anionic Redox Behaviour. Energy Storage Mat. 2024,
In review

(4) Pramanik, A.; Manche, A. G.; Clulow, R.; Lightfoot, P.; Armstrong, A. R. Exploiting Anion and Cation Redox Chemistry in Lithium-Rich Perovskite Oxalate: A Novel next-Generation Li/Na-Ion Battery Electrode. Dalt. Trans.
2022, 51 (33), 12467–12475.