High Formability Bromide Solid Electrolyte with Improved Ionic Conductivity for Bulk-Type All-Solid-State Lithium–Metal Batteries

Abstract

Solid electrolytes with excellent formability and high room-temperature Li-ion conductivities used in bulk-type all-solid-state lithium-metal batteries (ASSLBs) are expected to suppress the formation of Li dendrites that affects the application of metal Li anodes with high energy density. However, voids and cracks within sintered electrolytes lead to short-circuiting of ASSLBs due to the formation of Li dendrites. This work aimed to develop solid electrolytes with high formability and high room-temperature Li-ion conductivities that can suppress Li dendrites’ formation in ASSLBs. Three indicators of bulk moduli, lithium-ion migration energies, and energies above the hull were investigated: formability, ionic conductivity, and thermodynamic stability. The 109 inorganic materials containing elements of Li and Br listed in the open web-based database of the Materials Project were surveyed due to their weak Coulombic interaction with Li ions and high polarizability. LiGaBr4 with monoclinic symmetry (LGB, S.G.: P121/a1) was focused on due to its lower values for all three analyzed indicators. A ball milling mechanochemical synthesis method was adopted to synthesize LGB. It indicated that the obtained LGB possessed a monoclinic structure with a lithium ionic conduction of 2.0 × 10–5 S cm–1 (cold isostatic pressed powder) at 25 °C. The ionic conductivity was nearly three times higher than that of the conventional sintered LGB and comparable to that of a typical oxide-type electrolyte of LISICON-type Li3.55(Ge0.45Si0.10V0.45)O4 (LGSV) at 25 °C, which is appealing for applying the sintered samples for Li-metal anodes. The SEM and the distribution of relaxation times (DRT) analysis demonstrated that LGB contributed minimal grain-boundary resistance due to tight connections between the particles, while the LGSV oxides contributed an extremely large grain-boundary resistance. An ASSLB using LGB powder as an electrolyte, bare LiCoO2 as a cathode, and lithium metal as the anode exhibited the ability to function smoothly, while short-circuiting occurred at the second cycle with LGSV, demonstrating that LGB is a pure ionic conductor and may be suitable for use in ASSLB that can prevent Li dendrites with electrochemical stability.