Metastable Chloride Solid Electrolyte with High Formability for Rechargeable All-Solid-State Lithium Metal Batteries

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.: P12_1/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 Li_6.6La₃Zr_1.6Ta_0.4O₁₂ (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.