Materials Exploration in Na-Fe-S-O System: Synthesis and Electrochemistry

The current Li ion batteries which employ layered oxides such as LiCoO₂, spinel LiMn₂O₄, and olivine LiFePO₄as cathode materials have dominated global markets of secondary batteries. However, rapid expansion of their application has triggered the growing concerns on scarcity and rising costs of Li resources. As one of the most realistic alternatives, Na ion batteries re-attract massive attention owing to its abundance, lower cost, and sustainable production. In this pursuit, Na-Fe-S-O systems are one of the most desirable systems, which are consisted only by earth-abundant and geographically distributed elements. Especially, sulfate compounds are very cheap, being a byproduct of coal power plant, or petrochemical industries.

During the exploration of in the Na₂O-FeO-SO₃ ternary phase diagrams, we identified some materials with a totally new composition and crystal structure [1]. The target cathode material was obtained via classical solid-state synthesis by ball-milling Na₂SO₄ and anhydrous FeSO₄ followed by annealing the mixed precursors at elevated temperature under an Ar gas flow. The electrode property of as-synthesized cathode material was examined with no special effort for decreasing particle size or carbon coating. The working electrode was formulated by mixing 85 wt% active material, 10 wt% carbon black (Ketjen Black, ECP), and 5 wt% polytetrafluoro- ethylene (PTFE) binder. Figure 1 shows an example of galvanostatic charging and discharging curves measured at a rate of C/10. The initial charging capacity of 98 mAhg^-1 was retained over 10 cycles with small polarization. An average potential of 3.6 V (vs. Na/Na⁺) is the highest-ever Fe³⁺/Fe²⁺redox potential among all known Na-ion cathode materials. The smooth sloppy charging and discharging curves suggests a single-phase reaction mechanism, which is generally favorable from the perspective of homogeneity and kinetics of the reaction.

Details on the compositions and their synthetic conditions will be disclosed in the poster.

References

[1] A. Yamada, P. Barpanda, S. Nishimura, G. Oyama, Patent JP-2013-187914 (2013).

Acknowledgments

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