Probing the Mo-Redox in Alluaudite Battery Materials

Abstract

Alluaudites are essentially naturally occurring mixed metal (Mn and Fe) phosphate-based minerals [1] consisting of an open framework structure to support fast alkali migration. The general formula for alluaudite can be expressed by A(1)A(2)M(1)M(2)2(XO4)3 where A and M sites are alkali ion and transition metals respectively and X can be S, P, As, Mo, W, V and so on [2]. The presence of polyanionic moieties (XO4) assures structural stability and high-voltage operation arising from the inductive effect [3]. Till date, a variety of phosphate and sulfate-based alluaudites have been reported as cathodes for Li-ion and Na-ion batteries [4,5].In 2017, the first molybdate-based alluaudite material, Na2.67Mn1.67(MoO4)3, was reported as a 3.45 V Mn-based cathode [6]. Inspired by this work, we have explored other 3d (Co, Ni) based molybdate alluaudite homologues as potential compounds with desirable electrochemical and electrocatalytic activities. Solution combustion synthesis was employed to prepare phase pure Na36Co1.32(MoO4)3 by restricting the annealing duration to one minute as compared to the reported solid-state route warranting prolonged (100 h) annealing. It was found to act as a high-voltage cathode (4 V vs Na/Na+ and 4.1 V vs Li/Li+) involving Co3+/Co2+ redox center [7] while cycling between 3.0 V to 4.3 V. The above-mentioned alluaudite consists of Mo species, which can be redox-active at lower voltages. When cycled in a low-voltage window (0.01 V to 3.0 V), this material was found to act as an anode for both Li-ion and Na-ion batteries. High capacity (ca 400-500 mAh/g) was obtained with a central potential ~0.6 V involving conversion and (de)insertion reaction mechanism. The Ni-analogue, Na4Ni0.8(MoO4)2 alluaudite, was studied as an anode material in both Li-ion and Na-ion batteries involving conversion and (de)intercalation redox mechanisms like the Co- analogue. The underlying redox mechanism will be described for both cases involving post-mortem diffraction, electron microscopy, and spectroscopic tools as well as with the help of the density functional theory (DFT) approach. Finally, alluaudite Na4Cu(MoO4)3 was prepared by soft chemistry route, which was found to work as an anode material in the voltage window of 0.01 V to 3.0 V. The phase transformation and underlying redox mechanism will be elucidated.