Ultrathin, Flexible Polymer Electrolyte for Cost‐Effective Fabrication of All‐Solid‐State Lithium Metal Batteries

All-solid-state batteries are promising candidates for the next-generation safer batteries. However, a number of obstacles have limited the practical application of all-solid-state Li batteries (ASSLBs), such as moderate ionic conductivity at room temperature. Here, unlike most of the previous approaches, superior performances of ASSLBs are achieved by greatly reducing the thickness of the solid-state electrolyte (SSE), where ionic conductivity is no longer a limiting factor. The ultrathin SSE (7.5 µm) is developed by integrating the low-cost polyethylene separator with polyethylene oxide (PEO)/Li-salt (PPL). The ultrathin PPL shortens Li+ diffusion time and distance within the electrolyte, and provides sufficient Li+ conductance for batteries to operate at room temperature. The robust yet flexible polyethylene offers mechanical support for the soft PEO/Li-salt, effectively preventing short-circuits even under mechanical deformation. Various ASSLBs with PPL electrolyte show superior electrochemical performance. An initial capacity of 135 mAh g−1 at room temperature and the high-rate capacity up to 10 C at 60 °C can be achieved in LiFePO4/PPL/Li batteries. The high-energy-density sulfur cathode and MoS2 anode employing PPL electrolyte also realize remarkable performance. Moreover, the ASSLB can be assembled by a facile process, which can be easily scaled up to mass production.