Ionic and Electronic Transport in Alluaudite Na2+2xFe2-x(SO4)3

Alluaudite-type Na2+2xFe2-x(SO4)(3) has recently been discovered as a 3.8 V (highest ever Fe3+/Fe2+ redox potential) cathode material for sodium-ion batteries. Contrary to the sluggish kinetics caused by the large Na+ radius in most cathode materials for sodium-ion batteries, this alluaudite-type compound shows superior high-rate capability during electrochemical cycling. Here, to explore the underlying factors for the fast kinetics, we experimentally investigate the inherent ionic and electronic transport of this alluaudite phase by using impedance spectroscopy and potentiostatic polarization. Compared with the commercial cathode material, LiFePO4, we demonstrate four and two orders of magnitude higher intrinsic ionic and electronic conductivity, respectively, which underpin its superior high-rate capability in terms of electrode performance.