Extensive global efforts are underway to employ renewable energy as a future primary electric energy source.¹ The renewable energy sources are intermittent, therefore low-cost and reliable energy storage system is necessary.^1,2 In this scenario, the use of anode materials as sodium has become relevant due to it is high abundance, low cost, high capacity and adequate redox potential to increase the energy density of batteries.³ Among all types of sodium batteries, sodium-metal halide batteries that utilizes transition metals (such as, nickel and iron) as active materials has commercially received considerable attention, because it offers higher specific energy, is chemically safe, has longer cycle-life and is recyclable³. An active challenge in sodium-metal halide technology cathodes is to minimize the quantities of the metals used without decreasing the good electrical conductive of the cathode material in the charging and discharging process.⁴ To overcome the issues of the cathode material, i.e., optimized metal content as well as good electrical contact, the synthesis of higher surface area materials with an interconnected structure may be a promising option. By this away, one of the most utilized methods for synthetizing tridimensional products is through combustion reaction.^5,6 From an industrial point of view, this approach has several advantages, such as lower energy consumable, simple and easily scalable, formation of pure products and high yield.⁶ Herein a synthesis method of obtaining tridimensional interconnected metallic porous structures composed by Ni_xFe_y alloys was developed, aiming higher electrical conductivity with lower amount of metal in the cathode. Briefly, the overall synthesis method involves (i) the pre-formation of a tridimensional (3D) material, followed by a (ii) self-propagating combustion reaction and then a (iii) thermal treatment to generate the porous metallic structures. The combustion reaction step was based on a method previously reported⁵. In the step (i), metallic nitrates (Ni(NO₃)₂ and Fe(NO₃)₃) and triblock copolymer (Pluronic® F-127) were milled, mixed and then heated up to 80 °C to generate a 3D macromolecule. After holding at 80 °C for 18 hours, in the step (ii), the sample was heated to 300 °C and maintained for 1h, allowing the bimetallic structure formation. Finally, in the step (iii), the synthetized samples were reduced under hydrogen atmosphere (4% in N₂) at different temperatures (300, 400, 500 and 600 °C) for 1 hour. XRD patterns showed that the synthesis method employed is effective in the formation of monometallic and bimetallic samples. Morphological analysis indicates that the materials are formed by a porous matrix structure at macro and micro range, with interconnected tridimensional network. The Ni-richer alloys (Ni₃Fe), as well as monometallic nickel, revealed the formation of foam morphology, while Fe-richer alloys (NiFe₃), and also monometallic iron, have agglomerate-collapsed structures. The presented results showed that the developed method is versatile and effective for synthesizing both monometallic and bimetallic (Ni-richer alloys) cathode materials.

REFERENCES

1. Larcher, D. & Tarascon, J.-M. Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 7, 19–29 (2014).
2. Chiang, Y. Electrochemical Energy Storage for the Grid. World (2010).
3. Ellis, B. L. & Nazar, L. F. Sodium and sodium-ion energy storage batteries. Curr. Opin. Solid State Mater. Sci. 16, 168–177 (2012).
4. Li, G., Lu, X., Kim, J. Y., Lemmon, J. P. & Sprenkle, V. L. Cell degradation of a Na–NiCl2 (ZEBRA) battery. J. Mater. Chem. A 1, 14935 (2013).
5. Liang, J., Wang, Y.-Z., Wang, C.-C. & Lu, S.-Y. In situ formation of NiO on Ni foam prepared with a novel leaven dough method as an outstanding electrocatalyst for oxygen evolution reactions. J. Mater. Chem. A 4, 9797–9806 (2016).
6. Liu, G., Li, J. & Chen, K. Combustion synthesis. (Wiley Online Library, 2015).