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.