Despite being promising for high-energy multivalent metal-ion batteries, high-potential and multi-electron-involving Se oxidation conversion usually suffers from rigorous performance decay spawned by sluggish kinetics and active intermediate shuttle/dissolution, especially in ionic liquid (IL) electrolytes. Herein, with a novel low-corrosion ZnCl2-based IL electrolyte, we initially report a dual-conversion strategy by in situ activating redox Mo-chloro species to join Se oxidation conversion of the 1T-MoSe2 cathode towards fast-kinetics and durable Zn batteries. The Mo composition in the MoSe2 cathode experiences an unexpected Mo4+ ↔ Mo5+ ↔ Mo6+ transition, which merges with the redox conversion (Se2− ↔ Se0 ↔ Se2+ ↔ Se4+) of Se to form low-dissolution complex charged products. Compared with the conventional Se cathode, the introduction of redox Mo-chloro species catalytically boosts the Se conversion kinetics, remarkably enhancing the reversible capacity and rate capability. Consequently, the Zn–MoSe2 battery exhibits a high discharge plateau of ∼1.47 V, an appreciable capacity of ∼500 mA h g−1 at 0.2 A g−1, and an ultralong lifespan of over 9000 cycles at 5 A g−1 with excellent capacity retention of 99%. The as-assembled Zn–MoSe2 pouch-cell in the IL electrolyte shows decent power supply capacity and extreme safety even when being used under various abuse scenarios. This dual-conversion strategy of transition metal redox chloro-species-coupled chalcogen conversion chemistry sheds new light on unlocking other advanced metal–chalcogenide batteries.