Recently there has been renewed interest towards the argyrodite family Li6PS5X (X = Cl, Br, I) as Solid electrolyte (SE) with Li-conductivity &gt; 1 mS/cm for next generation all solid-state battery. Aliovalent substitution results in further enhancement in ionic conductivity for Li6.6Si0.6Sb0.4S5I and Li6+xP1–xGexS5I of ~ 25 mS/cm and 18.4 mS/cm, respectively. Notably, cation doping increases conductivity by elevating the prefactor without significantly lowering the activation energy, often resulting in sluggish Li+-transport at low temperatures. We have developed an entropy-enhanced ceramic class as a novel space of SEs, Li6-xPS5-xClBrx (0 ≤ x ≤ 0.8), that yields stable structures with both low activation barriers and significantly increased prefactor for ion migration. Highest conductivity of 24 mS/cm at 25 oC with a low activation energy of 0.155 eV has been achieved for the composition of Li5.3PS4.3ClBr0.7 which can be potential to stabilize the all-solid-state-battery operation from freezing temperature to ambient condition. Diversity in chemical species in the Li6-xPS5-xClBrx SE results in flat energy landscape for anion ordering and varying Li-coordination environment. Such Li-coordination entropy gives rise to the fast Li+ ion transport due to no net change in Li-X coordination numbers during the hopping process. These insights serve as a new strategy for the design of novel solid electrolytes.

Lithium-ion transport in mixed-halide argyrodites

ACS Fall 2021

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