Visible Lithiation Gradients of Bulk MoS2 in Lithium-ion Coin Cells

Abstract

Molybdenum disulfide (MoS₂) is sought out to replace graphite as the negative electrode in lithium-ion batteries (LIBs), due to the higher theoretical capacity of bulk MoS₂ (670 mAh/g) and its natural abundance. However, upon deep discharge (0.01 V) MoS₂ suffers from rapid loss of performance. To further the understanding of the MoS₂ degradation mechanism, ex situ scanning electon microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) are carried out in the first lithiation and delithiation cycle. The study reveals that visible concentric coloured rings form in MoS₂ coin cells, whereby the electrode centre is underused unlithiated 2H MoS₂, the middle ring is partially lithiated 1T/2H Li_xMoS₂, and the outer ring experiences the full electrochemical pathway to form an amorphous product. Since, lithiation inhomogeneities complicate MoS₂ lithiation mechanism studies in coin cells, we propose the use of thin (~ 10 µm) coatings and low current densities (~ 10 mA/g) to enable uniform lithiation.