Unlocking New Redox Activity in Alluaudite Cathodes Through Compositional Design

The development of energy dense and environmentally friendly secondary batteries is central to meeting the ever-increasing global energy demand. Consequently, sodium (Na)-ion batteries are promising candidates to replace traditional Li-based technology due to their greater relative abundance, lower cost, and ease of processing. Polyanionic alluaudite structures are potential intercalation compounds for Na-ion cathodes. In particular, the sulfate alluaudite variant Na₂Fe₂(SO₄)₃ exhibits the highest ever Fe^2+/3+ redox voltage (3.8V vs Na) observed in any cathode material, which is attributed to the unique edge-sharing <i>M</i>O₆ (<i>M</i> = Fe, Mn, Ni, Co) octahedra in the alluaudite structure.¹ However, sulfates are chemically unstable and suffer from low theoretical capacities. Although phosphate alluaudites can reach high (~140 mAh/g) capacities, they exhibit lower (2.7 V vs Na) Fe^2+/3+ redox potentials, limiting their energy densities.² The development of alluaudite cathodes has been limited by the number of electrochemically active transition metals in this structure type. Barring just a few compositions, only the low voltage Fe^2+/3+ redox has been harnessed, even though structures containing multiple transition metal species have been reported. Herein we successfully apply a set of design principles to activate the Mn^2+/3+ redox couple in alluaudites. First, vanadate compounds were explored in order to increase the electrical conductivity of typically insulating polyanionic structures. Secondly, Al substitution was employed to buffer Jahn-Teller distortions in MnO₆ octahedra, resulting in more facile Mn^2+/3+ redox. We report the synthesis of a new redox active Mn-based alluaudite composition and employ synchrotron X-ray diffraction (SXRD), solid-state nuclear magnetic resonance, density functional theory, X-ray absorption spectroscopy, and scanning electron microscopy to characterize the structure and morphology of this material.³ The electrochemical Na-ion de(intercalation) processes of the alluaudites were probed through galvanostatic cycling, galvanostatic intermittent titration technique, and ex-situ SXRD. The Mn alluaudite compositions were found to be electrochemically active, with Al substitution resulting in a more than twofold increase in discharge capacity. The successful activation of a higher voltage Mn^2+/3+ redox couple via compositional design opens up a new compositional space for alluaudites, paving the way for high energy density Na-ion cathodes.<br/>References:<br/>1) Barpanda, Prabeer, et al. "A 3.8-V earth-abundant sodium battery electrode." Nature communications 5.1 (2014): 1-8.<br/>2) Huang, Weifeng, et al. "Self-Assembled Alluaudite Na2Fe3− xMnx (PO4) 3 Micro/Nanocompounds for Sodium-Ion Battery Electrodes: A New Insight into Their Electronic and Geometric Structure." Chemistry–A European Journal 21.2 (2015): 851-860.<br/>3) Wu, V.; Giovine, R.; Foley, E. E.; Finzel, J.; Balasubramanian, M.; Sebti, E.; Mozur, E.; Clément, R.J. Unlocking new redox activity in alluaudite cathodes through compositional design. <i>In preparation</i>