Exfoliated MoS₂ as Electrode for All-Solid-State Rechargeable Lithium-Ion Batteries

The electrode behavior of exfoliated MoS₂ is studied in an all-solid-state lithium-ion battery. MoS₂ nanosheets with a crystallite thickness of about 6 nm are synthesized by chemical exfoliation of bulk MoS₂ and characterized by XRD, SEM, and TEM. MoS₂ composite electrodes are obtained by uniaxial cold-pressing at 3 tons and contain 60 wt % MoS₂, 30 wt % β-Li₃PS₄ solid electrolyte, and 10 wt % carbon black. Solid-state lithium-ion batteries are assembled using β-Li₃PS₄ as the solid electrolyte and a Li-In alloy as the counter electrode. The electrode performance is well above state-of-the-art with an initial specific capacity of about 439 mAh g^-1 when discharged at 67 mA g^-1 (C/10) in the potential range of 0.01-3.0 V. The initial irreversible capacity is only 9%. The specific capacity retention is excellent with 312 mAh g^-1 obtained after 500 cycles. In view of the theoretical capacity of MoS₂ (q_th = 670 mAh g^-1), the solid-state reaction in the cell is incomplete; yet the end phases of the conversion reaction after discharge (Mo and Li₂S) are confirmed by XRD. We also studied the behavior of bulk MoS₂ (same mass loading) and found that the performance is inferior as compared to MoS₂ nanosheets. The initial discharge capacity of bulk MoS₂ is only 259 mAh g^-1, and the initial irreversible capacity is as large as 26%. Overall, the study shows that MoS₂ can be effectively cycled in all-solid-state batteries with β-Li₃PS₄ as solid electrolyte and that the electrode performance can be significantly improved by nanostructuring.