Materials Exploration in Na-Fe-S-O System: Crystal Structure

The sodium-ion battery system has attracted growing interest as a potential candidate for larger energy storage systems. The ubiquitous element Na allows us to realize the stable production of larger scale systems for broader applications. For such strategies, material design of the electrode should be done by utilizing the abundant elements for not only the guest species but also the host framework. Recently new iron sulphate compounds were discovered by explorations in Li-Fe-S-O(-F) system for example Li₂Fe(SO₄)₂ (3.83 V vs. Li/Li⁺) and various polymorphs LiFeSO₄F (3.6 – 3.9 V vs. Li/Li⁺), which show higher redox-potential than the other Fe oxysalt compounds as expected from the high electronegativity of the sulphate anion [SO₄]^2–.[1-3] Inspired by this strategy, we explored Na-Fe-S-O system for the sodium ion battery and identified some new compounds, a few of which demonstrate good rate-capabilities and high redox-potentials. [4]

One of the Na containing Fe(II) compounds showed a high redox potential of 3.6 V vs. Na/Na⁺, higher than the NASICON-type Fe^III₂(SO₄)₃ of ca. 3.4 V vs. Na/Na⁺. To elucidate the origin of the superior performances, we performed crystal structure determinations and structural analysis of the new phase. It has  a unique Fe local environment with Fe₂O₁₀ dimer. The dimer is interconnected by sulphate anion and forms a 3-dimensional framework, which consists 2 kind of large tunnel-like structure. Na atoms in the tunnels show a significant positional disordering, suggesting a good Na ionic conductivity. For further analysis, we applied a modified bond-valence-sum (mBVS) calculation [5], where deviations of the calculated mBVS from the ideal valence |ΔV| can provide information on preferable positions in the static structural model. Figure 1 shows an example of contour plot of the |ΔV| map. Based on this, we will discuss about the more detailed structural features and expected sodium diffusion mechanisms.

References

1. M. Reynaud et al., Electrochem. Commun. 21:77 – 80, 2012.
2. P. Barpanda et al., Nat. Mater. 10:772–779, 10 2011.
3. N. Recham et al., Nat. Mater, 9:68–74, 11 2010.
4. A. Yamada, P. Barpanda, S. Nishimura, G. Oyama, Patent JP-2013-187914 (2013)
5. S. Adams, Acta Crysta B57: 278 2001.

Acknowledgments

This work was supported by the ‘Element Strategy Initiative for Catalysts & Batteries’ (ESICB) project.