Doped NaSICON-Type Solid Electrolytes for Sodium-Ion Batteries from Scalable Spray-Flame Synthesis

Solid-state sodium ion batteries are a viable alternative to lithium ion batteries, especially for use in stationary devices, due to the high availability of raw materials and low costs [1] and Sodium Super Ion Conductors (NaSICON) are promising candidates for solid electrolytes in solid-state sodium-ion batteries. They exhibit good mechanical properties, high ionic conductivity and compatibility with sodium metal based anodes enabling high energy density [2]. One of the best studied NaSICON materials is Na₃Zr₂Si₂PO₁₂ (NZSP) with a high ionic conductivity in the order of 10^-3 Scm^-1 at room temperature [3, 4]. Conventional synthesis methods of this class of materials such as solid-state reaction and liquid-phase synthesis have several drawbacks due to time-consuming process steps such as milling, high temperature sintering, precipitation, washing and drying steps to obtain the final product [5]. We present spray flame synthesis (SFS) as a new approach for the synthesis of nanosized NaSICON materials. Recent studies have shown that sintering of nanoparticular NZSP precursors offers several advantages: A high specific surface area, which increases the sintering activity, and short atomic diffusion paths, allowing high homogeneity and phase purity to be achieved at comparatively low sintering temperatures [6].<br/>In SFS, metal salts dissolved in organic solvents are combusted resulting in fine metal oxide particles. They are characterized by transmission electron microscopy (TEM), X-Ray diffraction (XRD) and Raman-Spectroscopy for structural and morphological investigation. Elemental information is obtained via energy-dispersive X-Ray spectroscopy (EDX) and X-Ray photoelectron spectroscopy (XPS). Ionic conductivities of sintered NZSP pellets are measured by impedance spectroscopy.<br/>In our approach, nanoparticles with a median diameter of around 5 nm are obtained. The pristine particles consist of crystalline ZrO₂, homogeneously covered with an amorphous layer consisting of the elements Na, Si, P and O. After a short annealing step for 1h at 1000°C, this mixture can be converted almost quantitatively into the desired rhombohedral NZSP phase. Moreover, aliovalent dopants were successfully added for the synthesis of Na_3+2xA_xZr_2−xSi₂PO₁₂ with A = Mg or Ca. Pressed pellets sintered at 1100°C for 3h to a relative density of ~90% showed - for a material sintered for such a short period of time - a surprisingly high ionic conductivity of up to 10^-4 Scm^-1 (Mg-doped).<br/>In conclusion, a novel approach for the preparation of NZSP allowing the phase formation at relatively low temperatures is demonstrated. Spray flame synthesis is an elegant and promising possibility for the scalable production of solid electrolytes, which also holds great potential, especially with regard to further improvement of ionic conductivity through targeted doping.<br/><br/>[1] Y.Wang et al., <i>Nano Materials Science</i>, vol. 1, p. 91 (2019)<br/>[2] H.Fu et al., <i>ACS Mater.Lett.</i>, vol. 2, p. 127 (2019)<br/>[3] Goodenough et al., <i>Mater.Res.Bull.</i>, vol. 11, p. 203 (1976)<br/>[4] S. Narayanan et al.,<i>Solid State Ion.</i>, vol. 331, p. 22 (2019)<br/>[5] Z. Yang et al., <i>ChemElectroChem</i>, vol.8, p.1035 (2021)<br/>[6] A. Jalalian-Khakshour et al. <i>J.Mater.Sci.</i>, vol. 55, p.2291 (2019)