Electrochemical Redox Behavior of Li Ion Conducting Sulfide Solid Electrolytes

The lithium-phosphorus-sulfide family of solid electrolytes has attracted much attention for applications in solid-state batteries, as it exhibits some of the highest lithium ion conductivities of solid electrolytes to date. Here, we systematically assess the stability of the beta-Li3PS4 exemplar over a wide electrochemical window, from 0 to 5 V vs Li/Li+, that encompasses the potentials of all negative electrodes and most positive electrodes of interest for high energy density lithium batteries. By use of a unique cell construction in which the solid electrolyte is fabricated as an electroactive electrode by adding carbon as an electronic conductor, redox activity is amplified. The interphase (SEI)-forming solid-state reactions at high potential are found to be irreversible and passivating, whereas those occurring at low potential are reversible. This contrasts with the irreversible anode SEI formed at low potentials in lithium ion and lithium metal batteries and the general absence of an SEI at cathode potentials. Using elemental sulfur and phosphorus as redox-active internal standards, we show that the redox behavior of beta-Li3PS4 upon decomposition is essentially a superposition of that of sulfur and phosphorus species formed at the interphase.