Alluaudite Battery Chemistry: Design and Exploration

Abstract

Being one of the most promising technologies in human history, lithium-ion batteries (LIBs) have empowered our lives since their commercialization in 1991 by SONY®. However, all these applications need massive production to match up with the ever-growing energy demand which became expensive because of the scarcity and uneven distribution of Li around the globe. At this juncture, sodium-ion batteries (SIBs) have emerged as a pragmatic alternative to LIBs because of their natural abundance, fast diffusion, and facile interfacial kinetics. Similar to the LIBs, exploration of SIBs is also centered on different oxides and polyanionic materials where the oxides are the champions of yielding high discharge capacity and polyanionic materials can exhibit high working potential. Alluaudites, a subclass of these polyanionic systems, are essentially phosphate-based naturally occurring minerals having open frameworks and broad tunnels for facile alkali ion migration. Their general formula is A(1)A(2)M(1)M(2)2(XO4)3 where A sites are the alkali metal site (Li, Na, K), M is the transition metal site (Fe, Mn, Co, Ni) and XO4 is the polyanionic moiety where X can be S, P, As, V, Mo, W. By tuning XO4 moiety with different electronegative elements, one can play with the redox potential of these insertion materials. The very first report of alluaudite in sodium-ion batteries was in 2010 by Delmas group; they reported NaMnFe2(PO4)3 which was found to act as a 2.7 V cathode with a poor electrochemical performance. To mitigate this voltage issue, in 2014, alluaudite sulfate, Na2Fe2(SO4)3 was reported with the highest ever Fe3+/Fe2+ redox potential at 3.8 V (vs Na) with excellent rate capability. Following this report, there were gamut of research performed on alluaudite sulfates and phosphate frameworks. To find out different type of alluaudites, in 2017, Gao et al reported Na2.67Mn1.67(MoO4)3 which was found to work as a 3.45 V cathode material reporting the first solely Mn-based molybdate alluaudite being electrochemically active. Overall, the thesis mainly focuses on designing novel electrode materials, optimizing synthesis conditions, and understanding the underlying mechanisms with different characterization tools. ‘Alluaudites’ forms a rich treasure house for the development of battery electrode materials