All-solid-state lithium metal batteries promise a specific energy >500 Wh/kg. A solid-state electrolyte (SSE) plays an irreplaceable role in reaching such an energy density goal. Among different SSE types, the sulfide and thiophosphate-based SSE has emerged as a prominent class of soft ionic conductors. Compared to their ceramic and oxide SSE counterparts, sulfide SSEs provide several favorable advantages, including a) high room temperature ionic conductivity up to 10 mS/cm that is comparable to liquid-based electrolytes; b) ductile and mechanically soft SSE that enables better processible and intimate contact with electrodes; and c) scalability with solution-based low-temperature synthesis route.

However, when paring with a high voltage cathode, such as lithium nickel manganese cobalt oxide (NMC), the (electro)chemical instability of the sulfide SSE at the electrode/SSE interfaces becomes a major challenge to tackle with. The interfacial instability can result in up to 50% initial capacity loss in a Li/sulfide SSE/NMC battery, thereby keeping the sulfide SSEs from commercialization. Herein, by using neutron computed tomography, we trace in situ lithium displacement in an all-solid-state battery composed of a ⁷Li anode, ^natLi­­₃PS₄(LPS) electrolyte, and a ^natLiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode. We show that after the first galvanostatic charge/discharge cycle, lithium accumulates at the LPS/NMC811 interface and preferably fills in the pre-formed cracks in the cold-pressed LPS SSE pellet. Such irreversible lithium displacement contributes to the initial capacity loss of the Li/LPS/NMC battery. Our findings suggest that to achieve high-capacity retention of an all-solid-state sulfide-based battery using an NMC cathode, the cathode/sulfide interface should be better engineered and the defects of the LPS pellet should be suppressed.

Acknowledgment

This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is supported by Asst. Secretary, Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program. This research used resources at High Flux Isotope Reactor, a DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory.