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. 2014 Jul 17:5:4358.
doi: 10.1038/ncomms5358.

A 3.8-V earth-abundant sodium battery electrode

Prabeer Barpanda  1 , Gosuke Oyama  2 , Shin-ichi Nishimura  3 , Sai-Cheong Chung  4 , Atsuo Yamada  3
Affiliations

A 3.8-V earth-abundant sodium battery electrode

Prabeer Barpanda et al. Nat Commun. .

Abstract

Rechargeable lithium batteries have ushered the wireless revolution over last two decades and are now matured to enable green automobiles. However, the growing concern on scarcity and large-scale applications of lithium resources have steered effort to realize sustainable sodium-ion batteries, Na and Fe being abundant and low-cost charge carrier and redox centre, respectively. However, their performance is limited owing to low operating voltage and sluggish kinetics. Here we report a hitherto-unknown material with entirely new composition and structure with the first alluaudite-type sulphate framework, Na2Fe2(SO4)3, registering the highest-ever Fe(3+)/Fe(2+) redox potential at 3.8 V (versus Na, and hence 4.1 V versus Li) along with fast rate kinetics. Rare-metal-free Na-ion rechargeable battery system compatible with the present Li-ion battery is now in realistic scope without sacrificing high energy density and high power, and paves way for discovery of new earth-abundant sustainable cathodes for large-scale batteries.

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Figures

Figure 1
Figure 1. XRD pattern of Na2Fe2(SO4)3.
Rietveld refinement pattern of powder XRD data for Na2Fe2(SO4)3. Experimental data and calculated profile and their difference are shown as red crosses and black and purple solid lines, respectively. The theoretical Bragg positions are shown with green ticks. Trace amount (about 4 wt%) of bata-FeSO4 as an impurity was included in the analysis, as indicated by yellow ticks. (Inset) Room temperature Mössbauer spectrum of pristine Na2Fe2(SO4)3 shows the existence of two distinctive Fe(II) sites in 1:1 ratio (red and blue lines).
Figure 2
Figure 2. Crystal structure of Na2Fe2(SO4)3.
(a) The structure of Na2Fe2(SO4)3 projected along the c axis; and (b) local environment of two independent Fe sites. Green octahedra, yellow tetrahedra and blue spheres show FeO6, SO4 and Na, respectively. Fe ions occupy two kinds of crystallographic sites that have distinctive octahedral geometries. Each FeO6 octahedra share an edge with the crystallographically equivalent octahedra and form Fe2O10 dimers. The SO42− anions interconnect these dimers so as to build up a three-dimensional framework structure.
Figure 3
Figure 3. Electrode properties of Na2−xFe2(SO4)3 in Na cell.
(a) Galvanostatic charging and discharging profiles of Na2−xFe2(SO4)3 cathode cycled between 2.0 and 4.5 V at a rate of C/20 (2 Na in 20 h) at 25 °C. First (1st) cycle is shown in dashed black line, and 2nd–5th cycle in solid black lines. (Inset) The differential galvanostatic profiles (dQ/dV) of Na2−xFe2(SO4)3 cathode showing two distinctive peaks during the first charge and broader three peaks upon subsequent discharging/charging processes. (b) Capacity retention upon cycling up to 30 cycles under various rate of C/20 (2 Na in 20 h) to 20C (2 Na in 3 min). (Inset) The discharge curves of Na2−xFe2(SO4)3 as a function of rate (from C/20 to 20C). Before each discharge, the cells were charged at C/10 to 4.2 V.
Figure 4
Figure 4. Na-ion diffusion in Na2Fe2(SO4)3.
(a,b) Equi-value surface of the ΔBVS. The blue and light-blue surfaces are for ΔBVS=0.2 and 0.4, respectively. Inner side of the surface corresponds to accessible spaces for the Na ions. Green and yellow polyhedra are that of FeO6 and SO4, respectively. (c) Migration activation energy of Na+ ion calculated with DFT. Shown are the values (from left to right) for migrations along the c axis for the Na2 sites, between Na2 and Na1 sites, between Na1 and Na3 sites, and along the c axis for the Na3 sites. Calculations are done at low concentration of Na.
Figure 5
Figure 5. Overall comparison of the Fe-based cathode materials that can function as Na sources in Na-ion battery system.
Polyanionic cathode materials are shown as green boxes and simple oxides/fluorides as blue, respectively. Horizontal bars represent average voltage. Yellow band indicates voltage region that can ensure the compatibility with Li-ion batteries. The new compound Na2Fe2(SO4)3 is presented by the red box together with its expected dashed-red region based on the theoretical capacity. (*The capacity and voltage of P2-Na2/3−x[Fe1/2Mn1/2]O2 is assumed by 0<x<2/3 region by inherent amount of Na with large hysteresis including both Fe4+/Fe3+ and Mn4+/Mn3+ redox reactions, as separately denoted with dashed pale blue box. Dashed pale green box for NaFePO4 indicate it cannot be directly synthesized and sluggish kinetics in electrode reaction.)
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