Abstract: Li₂ S-P₂ S₅-type inorganic solid electrolytes with cation and/or anion doping are considered to be promis-ing candidates for all-solid-state batteries (ASSBs), due to their high ionic conductivity and electrochem-ical performances. However, compositional tuning of Li₂ S-P₂ S₅ type inorganic solid electrolytes without doping has not been fully studied. In this work, Li-rich Li₇ P₂ S₈ I solid electrolyte was prepared with com-positional tuning by high energy ball mill process. The crystalline nature and the structural character-istics of the prepared solid electrolytes were studied with several physiochemical techniques. The effect of compositional tuning and the associated limitations were analyzed by laser Raman spectroscopy and solid-nuclear magnetic resonance spectroscopy (NMR) analysis techniques. The prepared Li-rich Li₇ P₂ S₈ I solid electrolyte exhibited higher ionic conductivity (6.27 mS cm^-1) than the bare Li₇ P₂ S₈ I solid elec-trolyte (5.16 mS cm^-1). Further, the prepared Li_7.1 P₂ S_8.1 I_0.9 solid electrolyte is highly stable against lithium metal anode and is stable up to 600 charge-discharge cycles. Thus, the fabricated ASSB using Li-rich Li₇ P₂ S₈ I solid electrolyte exhibited excellent cycle stability of 97% specific capacity retention with less interfacial reaction. Electrochemical impedance spectroscopy and the laser Raman spectroscopy analy-sis after galvanostatic charge-discharge cycling confirmed the electrochemical stability of Li-rich Li₇ P₂ S₈ I solid electrolyte.

Key words: Solid electrolyte, Li₇ P₂ S₈ I, Li-rich solid electrolyte, Ionic conductivity, High stability, All-solid-state batteries