Insights into the MoS₂ lithium-ion battery lithiation mechanism via big data electrochemical analysis

Molybdenum disulfide (MoS₂) is a potential alternative to the commercial graphite anode in lithium-ion batteries (LIBs) due to its higher theoretical capacity (670 mAh g^-1) and natural earth abundance. However, the deep discharge potential range (3.00-0.01 V) enabling MoS₂ high specific capacities is highly debated and suffers from a rapid degradation process (<40 cycles). To investigate the MoS₂ deep lithiation mechanism, long-term electrochemical testing, restrictive potential ranges after an initial deep discharge, and in situ electrochemical impedance spectroscopy (EIS) were applied. Electrochemical analysis reveals that the three cathodic reactions (labelled G, H, and I) are either parallel or series-independent, forming reversible redox couples with the single anodic peak (W). The inclusion of each additional cathodic peak increases cell capacity but accelerates capacity decay. As the redox peaks responsible for the high initial capacities vanish, we verify that capacity is stored entirely through capacitance in the long-term (>40 cycles). Our electrochemical analysis highlights that current MoS₂ lithiation mechanism proposals are unable to accurately describe the phenomena observed when restricting the potential range of MoS₂ after deep lithiation. Therefore, we propose a set of three electrochemical conditions that must be justified by any lithiation mechanism hypothesis.