Copper Thiophosphate (Cu₃PS₄) as an Electrode Material for Lithium Solid-State Batteries with Lithium Thiophosphate (β–Li₃PS₄) Electrolyte

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.