Abstract
Owing to their high-voltage stabilities, halide superionic conductors such as Li(3)YCl(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(3-3x)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(3)YCl(6) made by SS synthesis, mobile Li(+) carriers in the defect-containing SS-Li(3-3x)Y(1+x)Cl(6) (0 < x < 0.17) and MC-Li(3-3x)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(2.61)Y(1.13)Cl(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(0.8)Mn(0.1)Co(0.1)O(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.