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

Owing to their high-voltage stabilities, halide superionic conductors such as Li₃YCl₆ 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_3-3xY_1+xCl₆ 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 Li₃YCl₆ made by SS synthesis, mobile Li⁺ carriers in the defect-containing SS-Li_3-3xY_1+xCl₆ (0 < x < 0.17) and MC-Li_3-3xY_1+xCl₆ 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_1.13Cl₆) 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_0.1O₂ (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.