Zinc-Ion and Proton as Joint Charge Carriers of S-MoO₂ for High-Capacity Aqueous Zinc-Ion Batteries

Molybdenum-based materials are regarded as promising candidates for aqueous zinc-ion batteries (AZIBs) because of their multi-valences and high specific capacity. However, the structural instability of MoS2 and sluggish reaction kinetics of MoO2 restrict their further development in AZIBs. Herein, the MoO2 with in situ inherited sulfur atoms (S-MoO2) is successfully prepared by heat treatment of MoS2 in static air. Benefiting from the synergistic effects of inherited S atoms and introduced O vacancies, the S-MoO2 exhibits higher specific/rate capacities (236 mAh g(-1) at 0.1 A g(-1) and 105 mAh g(-1) at 5.0 A g(-1)) and better cycling stability (81% capacity retention after 2000 cycles at 2.0 A g(-1)) than the perfect MoO2. More significantly, the in situ electrochemical quartz crystal microbalance (EQCM) and ex situ spectroscopic techniques comprehensively elucidate that zinc-ion and proton as joint charge carriers insert/extract into/from S-MoO2 through the (011) and (020) planes with high reversibility. This work provides a guideline for understanding the multi-ion storage mechanism of cathode materials for high-capacity AZIBs.