NaLiFe(C₂O₄)₂: a polyanionic Li/Na-ion battery cathode exhibiting cationic and anionic redox

Recently, 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 materials reported so far rely exclusively 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 a promising strategy to enhance the energy density of polyanionic cathode materials for Li/Na-ion batteries. In addition to transition metal redox activity, the oxalate group also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution. Herein, we report NaLiFe(C2O4)2 as a new positive electrode and use different characterization techniques such as Raman spectroscopy and Mössbauer analyses to characterise this dual-ion redox process 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.