Degradation of Lithium Iron Phosphate Sulfide Solid-State Batteries by Conductive Interfaces

The superionic solid-state argyrodite electrolyte Li₆PS₅Br can improve lithium and lithium-ion batteries' safety and energy density. Despite many reports validating the conductivity of this electrolyte, it still suffers from passivating electrode degradation mechanisms. At first analysis, lithium iron phosphate (LFP) should be more thermodynamically stable in contact with sulfide electrolytes. However, without substantial improvements to interfacial engineering, we find that LFP is not inherently stable against Li₆PS₅Br. We hypothesize argyrodite oxidation favorably competes with LFPAs delithiation, insulating the electrolyte-electrode interface and causing large overpotential losses. We show that compared to LiNi_xMn_yCo_zO₂, LFP has no actual electrochemical stability advantage despite operating at a lower voltage. We utilize tender energy XAS and XPS to show that chemical reactions occur between LFP and the Li₆PS₅Br solid electrolyte and these reactions are exacerbated by cycling. We also show that electrochemical degradation occurs at the interface between the solid electrolyte ion conductor and any electron conductor, namely the active material and carbon additives. We further demonstrate that LiNbO₃ cathode coatings on LFP can delay electrochemical degradation by electronically insulating the LFP-sulfide electrolyte interface but not prevent its occurrence at the carbon-electrolyte interface.