Formation and conductivity studies of lithium argyrodite solid electrolytes using in-situ neutron diffraction

Lithium-ion conducting argyrodites Li6PS5X (X = Cl, Br, I) are a promising class of fast-ion conductors for all-solid state Li-ion batteries. To gain a deeper insight into the phase formation of Li6PS5Cl, in situ neutron diffraction studies are carried out on a stoichiometric ball-milled precursor mixture during thermal treatment. The evolution of the S2 −/Cl− anion disorder and its correlation with ionic conductivity are reported here. In contrast to earlier reports, an argyrodite phase is found to form between 80 and 150 °C, but the phase shows only moderate conductivity when crystallized at such low temperatures and further thermal treatment is required to access the highly conducting phase. The maximum room-temperature ionic conductivity of 1.1 × 10− 3 S/cm is observed for samples annealed at intermediate temperatures (250 °C). When ball-milled glass-ceramic precursors for Li6PS5Cl are crystallized with a constant slow heating rate, the initially formed argyrodite phase is found to be Li7PS6, which is then gradually converted into Li6PS5Cl at higher temperatures. The industrial requirements for minimizing cost by using lower annealing temperatures thus need to be balanced with the requirements of obtaining the highest conducting composition of the phase for performance in all-solid state batteries.