Janus-Faced MgI ₂ Interface Engineering Enables Stable High-Capacity Poly(ethylene oxide)-Based Lithium Batteries

The practical application of poly(ethylene oxide) (PEO)-based polymer electrolytes in all-solid-state lithium–metal batteries (ASSLMBs) is severely restricted by their low energy density and uncontrolled lithium dendrite growth. Herein, we introduced a trace amount of MgI 2 as a dual-functional Janus additive that simultaneously addresses limited capacity and interfacial stability in PEO electrolytes. The Mg 2+ competitively coordinates with both PEO chains and TFSI – anions, effectively weakening the Li + -TFSI – interaction and promoting Li + dissociation, thereby increasing the free Li + concentration and enhancing interfacial lithium-ion transport. Simultaneously, iodine species (I – /I 3 – ) participate in cathode redox reactions to enhance reversible capacity while facilitating the formation of a robust, inorganic-rich solid electrolyte interphase (SEI) at the anode, which effectively suppresses dendrite formation. As a result, the modified electrolyte delivers a recorded critical current density of 1.7 mA/cm 2, and Li||Li symmetric cells achieve ultralong cycling stability for over 10,000 h at 60 °C. A Li||LiFePO 4 full battery exhibits exceptional durability of 10 times that of the blank system, with 93.28% capacity retention at 1 C after 2000 cycles. More impressively, as-fabricated pouch cells demonstrate the capacity retention of 95.80% after 250 cycles at 60 °C. This work presents a facile and economically viable strategy to synergistically regulate additionally reversible capacity and interfacial chemistry for next-generation, high-performance ASSLMBs.