Despite the significantly high theoretical energy of conversion-based M‒X (M=Li/Na/K; X=S/Se/Te/Br2/I2) batteries, the electrical insulation intrinsic and severe shuttle behavior result in unsatisfactory energy output and cycle life. Here we develop a new cascade battery that couples two sequential redox reactions in a single system to endow higher output energy. This cascade battery is achieved by simply employing a self-assembled 2D cathode architecture directly synthesized by a eutectoid reaction of 4Se+CI4=4SeI+C, simultaneously constructing metalloid eutectic Se-I compounds encapsulated into carbon nanocages. Such composite cathode exhibits a distinctive 2D core-shell structure, extraordinary electrical conduction and fast synergistic activation, accordingly enabling an ultrahigh energy density (345.7 Wh kg−1) and an ultralong cycle life (90.1% for 1000 cycles) in a representative Li−SeI battery. Ingeniously, the involvement of two-redox in one-cell by synchronous material consolidation strategy offers a promising direction for the design of advanced high-energy batteries with the cascading effects.