Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.256
Publication date: 18th July 2023
Molybdenum disulphide (MoS2) is seeking to replace graphite as the negative electrode in lithium-ion batteries (LIB), due to the natural abundance of bulk 2H MoS2 and its high theoretical capacity (670 mAh/g). However, bulk MoS2 suffers from rapid loss of performance during the first 50 cycles when fully discharged (0.01 V). Aiming to understand the degradation mechanism at play in bulk MoS2 electrodes, coin cells with electrode material coatings of 10 μm, 40 μm, and 80 μm were opened after lithiation to 0.80 V, 0.40 V, and 0.01 V. Cells lithiated to 0.01 V and then delithiated to 3.00 V were also studied. The cycled electrodes revealed a tri-colored inhomogeneous concentric ring pattern, which remains present during initial delithiation and further cycling. Thicker electrode coatings resulted in a more noticeable ring pattern, which was mimicked by surface corrugation on the Li foil counter electrode. Ex-situ characterization involving scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) was carried out on the 40 μm electrodes to analyze the different regions of the ring pattern. All three segments were covered by a solid electrolyte interphase (SEI) made up of fluoride, carbon, and oxygen. The central segment of the electrode is made up of 2H MoS2, largely unaltered by the lithiation process. The middle segment is composed of predominantly 1T phase MoS2 and a small presence of 2H phase MoS2. The outer segment has a thick SEI cover, with MoS2 underneath whose phase could not be confirmed. Overall, it is postulated that the ring pattern occurs due to poor contact between the bulk MoS2 electrode, separator, and Li counter electrode within the coin cell configuration. Thus, a significant fraction of the bulk MoS2 material is unavailable for lithiation, especially in thicker electrodes.
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