Ultrathin Yet Robust Single Lithium‐Ion Conducting Quasi‐Solid‐State Polymer‐Brush Electrolytes Enable Ultralong‐Life and Dendrite‐Free Lithium‐Metal Batteries

Abstract Quasi‐solid‐state polymer electrolytes are one of the most promising candidates for long‐life lithium‐metal batteries. However, introduction of plasticizers for high ion conductivity at room temperature inevitably gives rise to poor mechanical strength and requires a very thick electrolyte membrane, which is detrimental to safety and energy density of the batteries. Herein, inspired by tube brushes coupling hardness with softness, a novel superstructured polymer bottlebrush BC‐ g ‐PLiSTFSI‐ b ‐PEGM (BC = bacterial cellulose; PLiSTFSI = poly(lithium 4‐styrenesulfonyl‐(trifluoromethylsulfonyl) imide); PEGM = poly(diethylene glycol monomethyl ether methacrylate)) with a hard nanofibril backbone and soft functional polymer side‐chains is reported as an effective strategy to well balance the mechanical strength and ion conductivity of quasi‐solid‐state polymer electrolytes. The resulting single lithium‐ion conducting quasi‐solid‐state polymer‐brush electrolytes (SLIC‐QSPBEs) integrate the features of the ultrathin membrane thickness (10 µm), the nanofibril backbone‐strengthened porous nanonetwork (Young's modulus = 1.9 GPa), and the high‐rate single lithium‐ion conducting diblock copolymer brushes. As a result, the ultrathin yet robust SLIC‐QSPBEs enable ultralong‐term (over 3300 h) reversible and stable lithium plating/stripping in Li/Li symmetrical cell at a current density of 1 mA cm −2 for lithium anode. This work affords a promising strategy to develop advanced electrolytes for solid‐state lithium‐metal batteries.