Reversible Conversion Reactions and Small First Cycle Irreversible Capacity Loss in Metal Sulfide-Based Electrodes Enabled by Solid Electrolytes

Solid-state batteries can potentially enable new classes of electrode materials which are unstable against liquid electrolytes. Here, SnS nanocrystals, synthesized by a wet chemical method, are used to fabricate a Li-ion electrode, and the electrochemical properties of this electrode are examined in both solid and liquid electrolyte designs. The SnS-based solid-state cell delivers a capacity of 629 mAh g⁻¹ after 100 cycles and exhibits an unprecedentedly small irreversible capacity in the first cycle (8.2%), while the SnS-based liquid cell shows a rapid capacity decay and large first cycle irreversible capacity (44.6%). Cyclic voltammetry (CV) experiments show significant solid electrolyte interphase (SEI) formation in the liquid cell during the first discharge while SEI formation by electrolyte reduction in the solid-state cell appears negligible. Along with CV, X-ray photoelectron spectroscopy and energy dispersive spectroscopy are used to investigate the differences between the solid-state and liquid cells. The reaction chemistry of SnS in solid-state cells is also studied in detail by ex situ X-ray diffraction and X-ray absorption spectroscopy. The overarching findings are that use of a solid electrolyte suppresses materials degradation and electrolyte reduction which leads to a small first cycle irreversible capacity and stable cycling.