NASICON-Type Sodium Iron Phosphate Cathodes from Scalable Spray-Flame Synthesis

Sodium-ion batteries (SIB) are increasingly in focus of current battery research due to their potential low cost and the abundance of raw materials. While hard carbon is an established and cost-effective anode material, the search for suitable cathode materials for SIB remains a significant area of interest [1]. Lithium Iron Phosphate (LFP), one of the most viable cathode materials for Lithium-ion batteries, offers a theoretical capacity of 170 mAhg^-1 [2]. The sodium-based analog Sodium Iron Phosphate (NFP), theoretically has a similar capacity [1]. However, its most stable crystal structure, maricite, is considered electrochemically inert due to its dramatically low ionic conductivity [3] and the electrochemical properties of NFP are highly dependent on its composition and morphology [3, 4]. In comparison, NASICON-structured polymorphs of NFP exhibit excellent cycling stability and rate capabilities. However, the high phosphate content in the NASICON polyanionic framework typically results in lower theoretical capacity [5, 6].<br/><br/>In this work, we report the production of different compositions of NASICON-type nano NaFe_xPO₄ (x = 1.2, 1, 0.75, 0.67, 0.5) using spray-flame synthesis (SFS). The synthesized NFP materials are characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy for structural and morphological characteristics. Elemental composition is analyzed via energy-dispersive X-ray spectroscopy (EDX). Thermal behavior is studied using thermogravimetric analysis (TGA) with simultaneous differential scanning calorimetry (DSC) and the NFP compositions are tested as cathodes in SIB half-cells.<br/><br/>The pristine materials consist of amorphous nanoparticles with a median diameter of around 12 nm. EDX analysis reveals a homogeneous elemental distribution with quantities close to the intended compositions. After a short annealing step for 1 hour at 600°C, the materials crystallize and, unexpectedly, all materials exhibit the rhombohedral NASICON structure. To our knowledge, no NaFe_xPO₄ with a higher Fe content than x = 0.75 has been reported yet. Initial half-cell tests show that NFP cathodes exhibit capacities close to their theoretical values, with capacity increasing with higher Fe content.<br/><br/>In conclusion, SFS presents a promising method for the scalable production of cathode active nanomaterials for SIB. This method holds significant potential, particularly for the compositional tuning of material classes, to fully utilize their capabilities for various applications.<br/><br/>References:<br/>[1] B. Xiao et al., <i>ChemSusChem</i>, vol. 12, no. 1, 2019<br/>[2] P. Yue, <i>Highl. Sci. Eng. Technol</i>., vol. 58, 2023<br/>[3] K. T. Lee et al., <i>Chem. Mater.</i>, vol. 23, no. 16, 2011<br/>[4] Y. Liu et al., <i>Adv. Funct. Mater</i>, vol. 28, no. 30, 2018<br/>[5] Y. Cao et al., <i>ACS Sustain. Chem. Eng.</i>, vol. 8, no. 3, 2020<br/>[6] L. He et al., <i>Adv. Mater. Interfaces, </i>vol. 9, no. 20, 2022