Na2.44Mn1.79(SO4)3: a new member of the alluaudite family of insertion compounds for sodium ion batteries

Abstract

Sodium-ion batteries have been extensively pursued as economic alternatives to lithium-ion batteries. Investigating the polyanion chemistry, alluaudite structured Na₂Fe^II₂(SO₄)₃ has been recently discovered as a 3.8 V positive electrode material (Barpanda et al., Nature Commun., 5:4358, 2014). Registering the highest ever Fe^III/Fe^II redox potential (vs. Na/Na⁺) and formidable energy density, it has opened up a new polyanion family for sodium batteries. Exploring the alluaudite family, here we report isotypical Na_2+2xMn^II_2−x(SO₄)₃ (x = 0.22) as a novel high-voltage cathode material for the first time. Following low-temperature (ca. 350 °C) solid-state synthesis, the structure of this new alluaudite compound has been solved adopting a monoclinic framework (s.g. C2/c) showing antiferromagnetic ordering at 3.4 K. Synergising experimental and ab initio DFT investigation, Na_2+2xMn^II_2−x(SO₄)₃ has been found to be a potential high-voltage (ca. 4.4 V) cathode material for sodium batteries.