The sodium-ion battery system is now growing as a potential alternative to the lithium-ion battery for large energy storage systems. The ubiquitous element Na allows us to realize stable production of large scale storages for broader applications. Electrode materials for the Na system is under intensive development by many battery researchers to make properties of the Na-ion battery comparable to those of Li-ion battery. In such pursuit we developed an ideal cathode material with abundant constituents sodium, iron and sulfate ions.¹ Alluaidite-type sodium iron sulfate Na_2+2xFe_2–x(SO₄)₃ exhibits high energy density due to high working potential of 3.8 V vs. Na/Na⁺. Large tunnel structure in the alluaudite framework derives high Na-ion conductivity. Thus Na_2+2xFe_2–x(SO₄)₃ electrode allows high-rate battery operation.In this poster, we present detailed reaction mechanisms of Na_2+2xFe_2–x(SO₄)₃ for Na batteries by means of crystal structural characterizations.

  As mentioned in our first report of this system, Na_2+2xFe_2–x(SO₄)₃ (NFS) electrode undergoes irreversible change during first charing of NFS/Na half cells. Subsequent discharging and charging cycles are reversible. We carried out ex situ structural analysis to understand about the irreversible and reversible changes during electrochemical reaction.

  Crystal structure of NFS powder (x ~ 0.28) was successfully analyzed by the alluaudite structure model with C2/c symmetry.² Ex situ structural analysis of NFS electrodes were carried out by powder X-ray diffraction using synchrotron radiation at Photon Factory in KEK, Japan. Electrochemically reacted samples were also successfully analyzed by using a monoclinic lattice with C2/c symmetry, which is isomorphic with the pristine lattice. Volume change is less than 5% in whole reaction range. The refined structure shows that the desodiation/resodiation reactions are essentially topochemical, while partially irreversible cation rearrangement is observed. One of the Na sites, Na3, is preferentially extracted due to higher site-potential than the other two sites. This model is consistent with computational evaluation of relative defect formation energy at the Na sites.¹ Subsequent extraction sequence shows complex behaviors involving Fe migrations. 

[1] P. Barpanda, G. Oyama, S. Nishimura, S.-C. Chung, and A. Yamada, Nat. Commun., 5:4358, 2014.

[2] G. Oyama, S. Nishimura, Y. Suzuki, M. Okubo, and A. Yamada. ChemElectroChem, 2(7):1019–1023, 2015.

[3] G. Oyama, O. Pecher, K. J. Griffith, S. Nishimura, R. Pigliapochi, C. P. Grey and A. Yamada, in preparation.