Performance-limiting factors of graphite in sulfide-based all-solid-state lithium-ion batteries

A suite of bulk and surface analytical techniques was applied to shed light on the factors limiting fast cycling of composite graphite electrodes in all-solid-state cells based on sulfide electrolytes 0.75Li2S-0.25P2S5 (LPS) and 0.3LiI-0.7(0.75Li2S-0.25P2S5). Cracks in the composite electrodes and poor percolation of the ionic conducting particles were identified by both scanning electron microscopy and X-ray tomography and the slow kinetics during lithiation (limiting practical specific charge at rates >C/10, at geometrical current densities >120 μA cm−2) was monitored by operando X-ray diffraction and supported by Raman microscopy. Operando X-ray photoelectron spectroscopy and X-ray absorption spectroscopy detected the formation of Li2S and LixP at the interface between LPS and graphite, both compounds increasing the interfacial resistance. Despite the kinetic limitations, excellent long-term cycling performance is demonstrated at C/20 rate (at current density of about 60 μA cm−2), revealing slow self-passivation processes at the sulfide/graphite interface which stabilizes after approximately 200 full cycles.