Spray-Flame Synthesis of NASICON-Type Rhombohedral [sub.3] [x = 0–0.2] Solid Electrolytes

• Published in: Nanomaterials (Basel, Switzerland) Vol. 14; no. 15
• Main Authors: Ali, Md Yusuf, Chen, Tianyu, Orthner, Hans, Wiggers, Hartmut
• Format: Journal Article
• Language: English
• Published: MDPI AG 01.08.2024
• Subjects: Electrolytes; Perovskite; Polymorphism (Crystallography); Germany
• ISSN: 2079-4991; 2079-4991
• DOI: 10.3390/nano14151278

Since solid electrolytes have a broad electrochemical stability window, are exceptionally electrochemically stable against Li metal, and function as a physical separator to prevent dendrite growth, they are at the forefront of alternate possibilities, further increasing the stability and energy density of Li-ion batteries. NASICON-type electrolytes are a promising candidate due to their negligible moisture sensitivity, which results in outstanding stability and a lower probability of Li[sub.2]CO[sub.3] passivity under the ambient atmosphere. However, one of the most promising representatives, Li[sub.1+x]Y[sub.x]Zr[sub.2−x](PO[sub.4])[sub.3] (LYZP), has multiple stable phases with significant variation in their corresponding Li-ion conductivity. In this paper, we have successfully synthesized the highly ionically conductive rhombohedral phase of LYZP via spray-flame synthesis. Two different solvent mixtures (e.g., 2-ethyl hexanoic acid/ethanol, propanol/propanoic acid) were chosen to explore the effect of precursor composition and combustion enthalpy on the phase composition of the nanoparticle. The as-synthesized nanoparticles from spray-flame synthesis consisted of the crystalline tetragonal zirconia (t-ZrO[sub.2]) phase, while lithium, yttrium, and phosphate were present on the nanoparticles’ surface as amorphous phases. However, a short annealing step (1 h) was sufficient to obtain the NASICON phase. Moreover, we have shown the gradual phase conversion from orthorhombic β phase to rhombohedral α phase as the annealing temperature increased from 700 °C to 1300 °C (complete removal of β phase). In this context, Y[sup.3+] doping was also crucial, along with the appropriate solvent mixture and annealing temperature, for obtaining the much-desired rhombohedral α phase. Further, 0.2 at% Y[sup.3]+ doping was added to the solvent mixture of 2-ethyl hexanoic acid/ethanol, and annealing at 1300 °C for 1 h resulted in a high ionic conductivity of 1.14∙10[sup.−5] S cm[sup.−1].