Ultrathin, Mechanically Durable, and Scalable Polymer‐in‐Salt Solid Electrolyte for High‐Rate Lithium Metal Batteries

Abstract Polymer‐in‐salt solid‐state electrolytes (PIS SSEs) are emerging for high room‐temperature ionic conductivity and facile handling, but suffer from poor mechanical durability and large thickness. Here, Al 2 O 3 ‐coated PE (PE/AO) separators are proposed as robust and large‐scale substrates to trim the thickness of PIS SSEs without compromising mechanical durability. Various characterizations unravel that introducing Al 2 O 3 coating on PE separators efficiently improves the wettability, thermal stability, and Li‐dendrite resistance of PIS SSEs. The resulting PE/AO@PIS demonstrates ultra‐small thickness (25 µm), exceptional mechanical durability (55.1 MPa), high decomposition temperature (330 °C), and favorable ionic conductivity (0.12 mS cm −1 at 25 °C). Consequently, the symmetrical Li cells remain stable at 0.1 mA cm −2 for 3000 h, without Li dendrite formation. Besides, the LiFePO 4 |Li full cells showcase excellent rate capability (131.0 mAh g −1 at 10C) and cyclability (93.6% capacity retention at 2C after 400 cycles), and high‐mass‐loading performance (7.5 mg cm −2 ). Moreover, the PE/AO@PIS can also pair with nickel‐rich layered oxides (NCM811 and NCM9055), showing a remarkable specific capacity of 165.3 and 175.4 mAh g −1 at 0.2C after 100 cycles, respectively. This work presents an effective large‐scale preparation approach for mechanically durable and ultrathin PIS SSEs, driving their practical applications for next‐generation solid‐state Li‐metal batteries.