Lithium-ion conducting argyrodites are among the most studied solid electrolytes due to their high ionic conductivities. A major concern in a solid-state battery is the stability of the solid electrolyte. Here, we present a systematic study on the influence of cationic and anionic substitution on the electrochemical stability of Li6PS5X using stepwise cyclic voltammetry, optical band gap measurements, and hard X-ray photoelectron spectroscopy along with first-principles calculations. We observe that on going from Li6PS5Cl to Li6+xP1-xMxS5I (M = Si4+, Ge4+), the oxidative stability does not change. Considering the chemical bonding shows that the valence band edges are mostly populated by nonbonding orbitals of the PS43- units or unbound sulfide anions and that simple substitutions in these sulfide-based solid electrolytes cannot improve oxidative stabilities. This work provides insights into the role of chemical bonding on the stability of superionic conductors and shows that alternative strategies are needed for long-term stable solid-state batteries.
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