Submolecular Selenium Enables Low-Pressure and Recyclable All-Solid-State Li-Se Batteries

All-solid-state lithium-selenium (Li-Se) batteries offer high energy density and intrinsic safety, but their practical deployment is hindered by sluggish solid-state redox kinetics, large volume fluctuations, and unstable solid-solid interfaces, particularly under low stack pressure. Here we report a submolecular Se_x cathode by converting crystalline cyclo-Se₈ into amorphous single-atom and short-chain Se (Se_x, x < 4) species anchored on graphene (Se_x@G), which increases the electronic conductivity from 6.3 × 10^-5 to 13 S cm^-1 and Li⁺ diffusivity from 1.7 × 10^-14 to 2.0 × 10^-11 cm² s^-1. Meanwhile, a Li₂O-nanoshell-modified sulfide solid electrolyte is designed to stabilize the interface against both Li metal and Se cathode. As a result, all-solid-state Li-Se batteries deliver the theoretical capacity of 673 mAh g^-1 at 0.1 C and retain 86% capacity over 2800 cycles at 3 C under 8 MPa, with stable operation down to -30°C. All-solid-state Li-Se pouch cells further achieve an energy density of 420 Wh kg^-1 and 94% capacity retention after 300 cycles under 2.5 MPa. Post-mortem analysis reveals that the submolecular Se_x species are not only structurally preserved but also recyclable after electrochemical cycling within the solid-state reaction environment. This work establishes submolecular chalcogen chemistry as a new design paradigm for low-pressure, recyclable, and high-energy all-solid-state batteries.