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論文題目「Synthesis of Garnet and Double-Perovskite Oxide Electrolyte Nanoparticles for All-Solid-State Battery by Induction Thermal Plasma」
(誘導結合型熱プラズマによる全固体電池用ガーネットとダブルペロブスカイト酸化物電解質ナノ粒子の合成)

Min Byeong-il

　Conventional lithium-ion batteries have reached their limits in capacity, energy density, cycle life, and safety, making them inadequate to meet the growing performance demands of modern society. To address these limitations, all-solid-state lithium-ion batteries (ASSLBs) have emerged as a promising alternative. ASSLBs offer several advantages, including high energy density, broad temperature tolerance, and enhanced safety, as they replace liquid electrolytes with solid ones. The development of ASSLBs relies heavily on the advancement of solid electrolytes, which are essential for their performance. Among the various candidates, 3-lithium garnets (Li3Sm3W2O12 and Li3La3W2O12), double-perovskite (Li3La3W2O12), and 7-lithium garnet (Li7La3Zr2O12) have demonstrated high ionic conductivity and excellent chemical stability through both experimental and computational studies, making them highly promising for commercialization.

　Despite their attractive properties, the practical application of these materials as solid electrolytes is hindered by the limitations of conventional solid-state reaction methods. These methods require prolonged heat treatments at high temperatures and yield particles ranging in size from tens to thousands of micrometers, making it fundamentally difficult to achieve thin-film solid electrolytes under 30 micrometers—a critical requirement for ASSLB commercialization. Therefore, exploring efficient and advanced synthesis methods for producing 3-lithium garnets (Li3Sm3W2O12 and Li3La3W2O12), double-perovskite (Li3La3W2O12), and 7-lithium garnet (Li7La3Zr2O12) nanoparticles is both urgent and necessary.

　Induction thermal plasma (ITP), generated without electrodes, offers a unique and efficient approach for nanoparticle synthesis. In addition to the desirable properties of conventional thermal plasmas—such as high temperature, high chemical reactivity, rapid quenching, and a controllable atmosphere—ITP also features a large plasma volume and low gas flow rates, making it highly suitable for large-scale production. As a result, ITP is considered the most reliable thermal plasma system for synthesizing high-purity nanoparticles. The synthesis of 3-lithium garnets (Li3Sm3W2O12 and Li3La3W2O12), double-perovskite (Li3La3W2O12), and 7-lithium garnet (Li7La3Zr2O12) nanoparticles using ITP offers a compelling alternative to conventional methods and addresses the challenges associated with particle size and processing complexity. In this dissertation, garnet Li3Sm3W2O12, Li7La3Zr2O12, and double-perovskite Li3La3W2O12 nanoparticles were synthesized by induction thermal plasma, and the formation mechanisms during the thermal plasma process were systematically investigated.

　In Chapter 1, The principles of induction thermal plasma and its application to all-solid-state batteries are comprehensively reviewed, while the objectives of this dissertation are clearly articulated.

　In chapter 2, garnet Li3Sm3W2O12 nanoparticles were synthesized. The Sm-to-W ratio in precursors was critical in determining the phase composition. Stoichiometric, Li-rich, and Sm-rich precursors predominantly formed Sm6WO12, while W-rich precursors yielded LiSmW2O8 and Li3Sm3W2O12, with a maximum phase fraction of 0.29 for Li3Sm3W2O12. Quenching gas promoted low-temperature phases in secondary Li-W oxides and suppressed phase transitions between LiSmW2O8 and Li3Sm3W2O12. Formation involved nucleation on Sm2O3 nuclei, followed by co-condensation of vapors into liquid droplets. The Sm-to-W ratio in the precursor controlled the solidification pathway, favoring Li-Sm-W composite oxides at lower ratios and Sm6WO12 at higher ratios.

　In chapter 3, double-perovskite Li3La3W2O12 nanoparticles were synthesized. Stoichiometric precursors showed the highest selectivity, achieving a mole fraction increase from 0.85 to 0.93 as O2 flow increased from 0 to 5 L/min. By-products such as cubic- and hexagonal-La2O3 decreased with higher O2 content. Nucleation occurred on La2O3 nuclei, and higher O2 content increased the nucleation temperature from 2777 K to 2973 K, expanding the coalescence temperature range for non-stoichiometric droplets. This expansion facilitated the conversion of non-stoichiometric droplets into stoichiometric double-perovskite Li3La3W2O12, thereby reducing by-product formation.

　In chapter 4, garnet Li7La3Zr2O12 (cubic and tetragonal phases) was synthesized using ITP and calcination. Amorphous-Li7La3Zr2O12 was the primary product due to rapid quenching of thermal plasma. Higher O2 content enhanced crystallization and formation, with the cubic-Li7La3Zr2O12 fraction reaching 0.1. Combined with the amorphous- and cubic-phase, the total Li7La3Zr2O12 fraction reached 0.45. Morphological differences were observed: low O2 conditions formed spherical particles from liquid droplets, while high O2 conditions resulted in irregular particles from mixed liquid-solid nucleation. Calcination at 1073 K and 1273 K produced tetragonal and cubic-Li7La3Zr2O12, with intensity ratios of 0.995 and 0.982, respectively, within 1 hour without additives.

　In chapter 5, The obtained results and conclusions have been comprehensively summarized and presented.

　In conclusion, this dissertation provides a comprehensive understanding of the synthesis of 3-lithium garnets (Li3Sm3W2O12 and Li3La3W2O12), double-perovskite (Li3La3W2O12), and 7-lithium garnet (Li7La3Zr2O12) electrolyte materials by induction thermal plasma under various experimental conditions. Furthermore, the combined process of ITP and calcination for fabricating ultra-thin films is proposed as a valuable direction for future research. These findings establish ITP as a versatile and scalable platform for producing advanced solid electrolyte materials, thereby facilitating advancements in the development of all-solid-state lithium-ion batteries.


研究論文

国際学会 国内学会