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

Lithium thiophosphate (beta-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 beta-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 degrees C) on the cell behavior is investigated. For an electrode composition of Cu3PS4: beta-Li3PS4: C65=40: 50: 10wt%, the initial discharge capacity at 60 degrees C is 776mAhg(-1) which fades over 60cycles to 508mAhg(-1) when cycled between 0.8 and 2.8V (vs. Li+/Li) at 50mAg(-1) (254.8 mu Acm(-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 S-8 redox.