A Thermally‐Activated Molecular “Firewall” Composite Solid Electrolyte for Inherently Safe Lithium Metal Batteries

ABSTRACT The pursuit of energy‐dense lithium metal batteries (LMBs) is often compromised by the catastrophic safety risks of flammable polymer electrolytes. Herein, we propose a hierarchical design paradigm for a 20‐µm‐thick composite solid electrolytes (CSE) that orchestrates high ionic influx and intrinsic safety management. This architecture integrates a thermally‐triggered molecular “firewall” composed of trimethyl phosphate (TMP) confined within HKUST‐1 metal‐organic frameworks (HKUST@TMP) and a rigid polyethylene terephthalate mechanical scaffold within the thin‐layer poly(ethylene oxide) (PEO) membrane formation. Notably, the HKUST‐1 framework effectively isolates chemically active TMP from the lithium anode under operating conditions to preserve interfacial integrity on the Li anode, while precisely releasing TMP to scavenge reactive radicals upon thermal abuse (>120°C). Consequently, the as‐formed 20 µm CSE achieves the balanced mechanical strength (25.54 MPa), high ionic conductance (234 mS at 30°C), enhanced Li + transference number of 0.71 as well as the superior flame retardancy with a limiting oxygen index of 24.3%. This controlled‐release strategy enables the LiNi 0.8 Mn 0.1 Co 0.1 O 2 |Li pouch cell to deliver a competitive gravimetric/volumetric energy density of 368.2 Wh kg −1 /693.9 Wh L −1 , together with verified thermal abuse tolerance under the GB/T 31485–2015 protocol, providing a new roadmap for the next generation of safe, high‐energy‐density energy storage.