Ultrathin cellulose separators with sustained release enabling safe, energy-dense lithium metal batteries

The electrochemical deposition of metallic lithium, though a promising anode for next-generation high-energy batteries, is challenged by the formation of an unstable solid-electrolyte interphase (SEI), leading to dendrite growth and posing safety risks and limiting life cycles. Traditional strategies such as electrolyte additives offer improvements but struggle under harsh conditions due to insufficient durability of concentration and high flammability. Inspired by sustained drug-delivery systems, we introduce ultrathin (5 μm) cellulosic composite paper separators that enable the controlled release of antimony oxychloride (SbOCl) nano-additives via sustained first-order release kinetics from the intermolecular entanglement within the cellulose fibers. These additives form Li 3 Sb clusters at the anode interface, enhancing SEI stabilization and uniformity of lithium deposition, thereby mitigating dendrite formation. Integrating SbOCl and cellulose effectively suppresses dendrites and improves battery safety through flame-retardant (limiting oxygen index: 39.27%) and thermal stability. The Li anodes with the paper separator achieve over 6000 h of stable cycling in symmetric cells and over 250 cycles in full cells with NCM622 cathodes, realizing an energy density of 368.65 Wh kg⁻¹ (904.90 Wh L⁻¹). Ah-level pouch cells demonstrate robust safety in nail penetration tests, underscoring the potential of this method to advance safe, sustainable, high-energy lithium metal batteries (LMBs) for future wearable applications.