Unlocking New Redox Activity in Alluaudite Cathodes through Compositional Design

Abstract

Sodium (Na)-ion conducting polyanionic structures are among the most promising cathode materials to enable more sustainable Na-based battery chemistries to replace current Li-based energy storage systems. Materials adopting the alluaudite structure have been found to reversibly intercalate Na, with phosphate and sulfate derivatives exhibiting moderate to high capacities when used as Na-ion cathodes. However, the development of alluaudite cathodes has been hampered by the fact that largely only Fe2+/3+ redox has been observed in this structure. Herein, we show that the Mn2+/3+ redox couple can be activated through compositional tuning of alluaudite compounds. Specifically, vanadate compounds were explored to increase the electrical conductivity as compared to the phosphates and sulfates. Al substitution was also employed to buffer the Jahn–Teller distortions of Mn(III)O6 octahedra, further facilitating electron transfer and redox processes. We report the synthesis of a series of new Na2Mn3–xAlx(VO4)3 alluaudite cathodes and employ synchrotron X-ray diffraction, solid-state nuclear magnetic resonance, scanning electron microscopy, density functional theory, X-ray absorption spectroscopy, and magnetometry to characterize the structure, morphology, electronic, and magnetic properties of the as-synthesized materials. Electrochemical Na de(intercalation) in Na2Mn3–xAlx(VO4)3 (x = 0, 0.05, 0.2) was probed through galvanostatic cycling, galvanostatic intermittent titration technique, and ex situ synchrotron X-ray diffraction. While these materials are all redox active, Al substitution results in a more than 2-fold increase in discharge capacity. The successful activation of a higher voltage Mn2+/3+ redox couple opens up a new compositional space for alluaudite-type Na-ion cathodes.