Halide Superionic Conductors for All-Solid-State Batteries: Effects of Synthesis and Composition on Lithium-Ion Conductivity

Abstract

Owing to their high-voltage stabilities, halide superionic conductors such as Li ₃YCl ₆ recently emerged as promising solid electrolyte (SE) materials for all-solid-state batteries (ASSBs). It has been shown that by either introducing off-stoichiometry in solid-state (SS) synthesis or using a mechanochemical (MC) synthesis method the ionic conductivities of Li _{3–3 x} Y _{1+ x} Cl ₆ can increase up to an order of magnitude. The underlying mechanism, however, is unclear. In the present study, we adopt a hopping frequency analysis method of impedance spectra to reveal the correlations in stoichiometry, crystal structure, synthesis conditions, Li ⁺ carrier concentrations, hopping migration barriers, and ionic conductivity. We show that unlike the conventional Li ₃YCl ₆ made by SS synthesis, mobile Li ⁺ carriers in the defect-containing SS-Li _{3–3 x} Y _{1+ x} Cl ₆ (0 < x < 0.17) and MC-Li _{3–3 x} Y _{1+ x} Cl ₆ are generated with an activation energy and their concentration is dependent on temperature. Higher ionic conductivities in these samples arise from a combination of a higher Li ⁺ carrier concentration and lower migration energy barriers. A new off-stoichiometric halide (Li _2.61Y _1.13Cl ₆) with the highest ionic conductivity (0.47 mS cm ^–1) in the series is discovered, which delivers exceptional cycling performance (∼90% capacity retention after 1000 cycles) in ASSB cells equipped with an uncoated high-energy LiNi _0.8Mn _0.1Co _0.1O ₂ (NMC811) cathode. This work sheds light on the thermal activation process that releases trapped Li ⁺ ions in defect-containing halides and provides guidance for the future development of superionic conductors for all-solid-state batteries.