  Citation: Jeong, H.; Na, D.; Baek, J.; Kim, S.; Mamidi, S.; Lee, C.-R.; Seo, H.-K.; Seo, I. Synthesis of Superionic Conductive Li 1+x+y Al x Si y Ti 2−x P 3−y O 12 Solid Electrolytes. Nanomaterials 2022, 12, 1158. https://doi.org/10.3390/ nano12071158 Academic Editor: Xiang-Hua Zhang Received: 7 March 2022 Accepted: 28 March 2022 Published: 31 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Synthesis of Superionic Conductive Li 1+x+y Al x Si y Ti 2-x P 3-y O 12 Solid Electrolytes Hyeonwoo Jeong 1 , Dan Na 1 , Jiyeon Baek 1 , Sanggil Kim 1 , Suresh Mamidi 1 , Cheul-Ro Lee 1 , Hyung-Kee Seo 2 and Inseok Seo 1, * 1 School of Advanced Materials Engineering, Jeonbuk National University, Baekje-daero 567, Jeonju 54896, Korea; grassmarket@naver.com (H.J.); ld3310@jbnu.ac.kr (D.N.); asssa2089@naver.com (J.B.); agaleon@daum.net (S.K.); sureshmamidi@jbnu.ac.kr (S.M.); crlee7@jbnu.ac.kr (C.-R.L.) 2 Future Energy Convergence Core Center, School of Chemical Engineering, Jeonbuk National University, Baekje-daero 567, Jeonju 54896, Korea; hkseo@jbnu.ac.kr * Correspondence: isseo@jbnu.ac.kr; Fax: +82-63-270-2305 Abstract: Commercial lithium-ion batteries using liquid electrolytes are still a safety hazard due to their poor chemical stability and other severe problems, such as electrolyte leakage and low thermal stability. To mitigate these critical issues, solid electrolytes are introduced. However, solid electrolytes have low ionic conductivity and inferior power density. This study reports the optimization of the synthesis of sodium superionic conductor-type Li 1.5 Al 0.3 Si 0.2 Ti 1.7 P 2.8 O 12 (LASTP) solid electrolyte. The as-prepared powder was calcined at 650 ◦ C, 700 ◦ C, 750 ◦ C, and 800 ◦ C to optimize the synthesis conditions and yield high-quality LASTP powders. Later, LASTP was sintered at 950 ◦ C, 1000 ◦ C, 1050 ◦ C, and 1100 ◦ C to study the dependence of the relative density and ionic conductivity on the sintering temperature. Morphological changes were analyzed using field-emission scanning electron microscopy (FE-SEM), and structural changes were characterized using X-ray diffraction (XRD). Further, the ionic conductivity was measured using electrochemical impedance spectroscopy (EIS). Sintering at 1050 ◦ C resulted in a high relative density and the highest ionic conductivity (9.455 × 10 −4 S cm −1 ). These findings corroborate with the activation energies that are calculated using the Arrhenius plot. Therefore, the as-synthesized superionic LASTP solid electrolytes can be used to design high-performance and safe all-solid-state batteries. Keywords: Ionic conductivity; LASTP; all-solid-state battery; relative density; activation energy 1. Introduction In recent years, renewable energy generation and storage have garnered attention for alleviating emerging environmental concerns. Renewable resources intermittently produce energy [1,2] and require energy storage devices to maintain continuity. Li-ion batteries (LIBs), which have moderate energy and power densities, are currently used in a wide range of applications [3,4]. However, LIBs that use liquid electrolytes suffer from safety issues, such as explosions or fires [5,6]. Some studies report on non-flammable liquid electrolytes, but they require an additional battery pack to secure sealing and avoid leakage [7–9]. Research on solid electrolytes has been conducted to design safe, leakage-free solid-state batteries that use Li metal as the anode and have a theoretical specific capacity of 3860 mAhg −1 , enabling high-energy-density batteries [10–12]. Electrolytes for all-solid-state batteries are divided into oxide-based, sulfide-based electrolytes and polymer electrolytes [13–17]. Sulfide-based electrolytes have relatively high ionic conductivities compared to oxide electrolytes but react with moisture in the air to generate hydrogen sulfide, which is a toxic substance [18]. On the other hand, oxide- based electrolytes have higher chemical stability than sulfide-based electrolytes. Perovskite, garnet, and sodium superionic conductors (NASICON) are commonly used oxide-based electrolytes [19]. Among these oxides, NASICON has high ionic conductivity and superior Nanomaterials 2022, 12, 1158. https://doi.org/10.3390/nano12071158 https://www.mdpi.com/journal/nanomaterials