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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. [ABSTRACT FROM AUTHOR]

Lithium thiophosphate (β‐Li3PS4) is a promising solid electrolyte (SE) for solid‐state batteries (SSBs). A major limitation is its very narrow electrochemical stability window which is caused by redox reactions involving sulfur and phosphorous. As these redox processes can be reversible, thiophosphates can be also studied as electrode materials. Herein, the use of Cu3PS4 as an active electrode material for Li SSBs with β‐Li3PS4 as the SE is explored. Both compounds exhibit similarities in crystal structure and composition which may benefit their compatibility. An In/InLi alloy is used as the counter electrode. The influence of electrode composition and temperature (room temperature and 60 °C) on the cell behavior is investigated. For an electrode composition of Cu3PS4: β–Li3PS4: C65 = 40: 50: 10 wt%, the initial discharge capacity at 60 °C is 776 mAh g−1 which fades over 60 cycles to 508 mAh g−1 when cycled between 0.8 and 2.8 V (vs. Li+/Li) at 50 mA g−1 (254.8 μA cm−2). Analysis by X‐ray diffraction and X‐ray photoelectron spectroscopy shows that Cu3PS4 irreversibly decomposes during lithiation. During cycling, the redox activity is found to be due to Cu2S and S8 redox. [ABSTRACT FROM AUTHOR]

Copper sulfide shows some unique physico‐chemical properties that make it appealing as a cathode active material (CAM) for solid‐state batteries (SSBs). The most peculiar feature of the electrode reaction is the reversible formation of µm‐sized Cu crystals during cycling, despite its large theoretical volume change (75%). Here, the dynamic microstructural evolution of CuS cathodes in SSBs is studied using in situ synchrotron X‐ray tomography. The formation of µm‐sized Cu within the CAM particles can be clearly followed. This process is accompanied by crack formation that can be prevented by increasing the stack pressure from 26 to 40 MPa. Both the Cu inclusions and cracks show a preferential orientation perpendicular to the cell stack pressure, which can be a result of a z‐oriented expansion of the CAM particles during lithiation. In addition, cycling leads to a z‐oriented reversible displacement of the cathode pellet, which is linked to the plating/stripping of the Li counter electrode. The pronounced structural changes cause pressure changes of up to 6 MPa within the cell, as determined by operando stack pressure measurements. Reasons for the reversibility of the electrode reaction are discussed and are attributed to the favorable combination of soft materials. [ABSTRACT FROM AUTHOR]