安全かつ大容量な全固体リチウム電池の新材料を開発 ～金属リチウム短絡抑制に効果のある新規塩化物固体電解質により安全な車載電池実現に道筋～
谷端 直人 助教（名工大・中山研究室：計画A03）
【概要】
名古屋工業大学大学院工学研究科の谷端直人助教らの研究グループは、高成形性の塩化物固体電解質材料による高エネルギー密度を有するリチウム金属電極の安定した充放電サイクルを実現しました。本研究では固体電解質材料に求められる物性の指標を、構造データベース中の材料に対し網羅的に計算することにより効率的な材料探索を目指しました。その中で、有毒ガスや高温処理が必要であった塩化物材料に対して、不活性ガス常温・常圧下での合成に成功しました。また、圧粉のみというシンプルかつ低環境負荷のプロセスによって、リチウム金属負極の課題である短絡現象を抑制することができました。全固体電池の高エネルギー密度化には固体中をリチウムイオンが伝導する固体電解質が重要な鍵を握ります。この新材料は、次世代型電池として期待される全固体電池の高エネルギー密度化を実現するものであり、さらに高電位材料の電極を開発することで電気自動車の安全性の向上や走行距離の増加が期待されます。
Abstract
Dense solid electrolytes in all-solid-state Li batteries are expected to suppress Li dendrite phenomena that prevent the application of high-energy-density Li metal electrodes. However, voids and cracks in sintered electrolytes still permit short-circuiting due to Li dendrites. This study aimed to investigate solid electrolytes with high formability in which green compacts can prevent Li dendrites. Li+ ion migration energies, bulk moduli, and energies above the hull were comprehensively investigated using first-principles and classical force field calculations as the indicators for ionic conductivity, formability, and thermodynamic stability. The 231 compounds containing Li and Cl listed in the Materials Project database were studied due to their high polarizability and weak Coulombic interaction with Li+ ions. Among them, monoclinic LiAlCl4 (LAC, S.G.: P121/c1) was focused on, owing to its low values of all three indicators. A mechanochemical synthesis was attempted to prepare the metastable phase, where Li ions occupy the interstitial sites, not just the original sites, because the computation for the migration energy suggested conductive pathways between the original Li sites. XRD and 7Li-MAS NMR measurements indicated that the mechanochemically synthesized LAC possessed a monoclinic host structure, while 2.5% Li occupied interstitial tetrahedral sites. Impedance measurements showed that the LAC green compacts exhibited an ionic conductivity of 2.1 × 10–5 S cm–1, 20 times higher than the conventional solid-state synthesized LAC at room temperature. The conductivity was more than one order of magnitude higher than that of garnet-type Li6.6La3Zr1.6Ta0.4O12 (LLZT), which has been attractive for the application of the sintered body for Li metal electrodes. The SEM observations and distribution of relaxation times analysis indicated that dense LAC green compacts with large necking between the particles contributed minimal grain-boundary resistance (7.5%) to the total resistance, while the LLZT green compacts contributed almost completely (99%). Li metal symmetric cells using the LAC pellet showed good cycle performance without short-circuiting and an overvoltage increase for 70 cycles at a current density of 0.1 mA cm–2, while short circuiting occurred at the 1st cycle in the LLZT cells.
Naoto Tanibata, Shuta Takimoto, Koki Nakano, Hayami Takeda, Masanobu Nakayama, Hirofumi Sumi, ACS Mater. Lett. 2, 880-886 (2020). "Metastable Chloride Solid Electrolyte with High Formability for Rechargeable All-Solid-State Lithium Metal Batteries",
DOI: 10.1021/acsmaterialslett.0c00127
Published on June 23, 2020
