Silver Thiophosphate (Ag3PS4) as a Multielectron Reaction Active Material for Lithium Solid-State Batteries

Abstract

Beyond its Li-ion conductivity, the solid electrolyte lithium thiophosphate (β-Li₃PS₄) exhibits redox activity when its electrochemical stability window is exceeded. As this redox activity can be (partially) reversible, thiophosphates may be used as cathode active materials (CAM). Silver thiophosphate (Ag₃PS₄) is a well-known Ag-ion conductor, which has the same crystal structure and similar chemical composition as β-Li₃PS₄. Here, Ag₃PS₄ is selected and studied as the CAM for Li solid-state batteries (Li-SSBs) with the configuration (In/InLi| β-Li₃PS₄| Ag₃PS₄: β-Li₃PS₄: C65 = 40: 50: 10 wt%). The cells provide a discharge capacity of 325 mAh g⁻¹ at 10 mA g⁻¹, but suffer from continuous capacity fading during cycling with an average Coulomb efficiency of 97% at 50 mA g⁻¹. The reaction mechanism is studied using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and impedance spectroscopy. Overall, the reaction of Li with Ag₃PS₄ is found to be initially partially reversible, but over cycling Ag₂S and S₈ become the active materials along with the formation of other byproducts such as Ag₂P₂S₆ and Li₂P₂S₆.