Understanding the Role of Triple Phase Boundaries on Coating-Free Solid-State Cathodes

Sulfide solid electrolytes have high ionic conductivities necessary to achieve high-rate solid-state cathodes at room temperature and low pressure. Cathode active materials generally require coatings to avoid deleterious oxidative decomposition reactions with the electrolyte. Coatings add cost and complexity to the manufacture. Here we decouple the effect of double and triple phase boundaries on the decomposition in the thick (i.e., ∼110 μm) uncoated solid state cathode. We show that more severe oxidative decomposition of solid electrolytes occurs when the cathode active materials, carbon, and the solid electrolyte coexist, highlighting the importance of the triple phase boundary concerning the decomposition. By regulating the electronic pathways at the triple phase boundary, a thick uncoated electrode at 1 mA cm^-2 and 2 MPa stack pressure, delivers an initial areal capacity of ∼4.6 mAh cm^-2 at 30 °C and ∼85% capacity retention after 500 cycles.