Mixing Functionality in Polymer Electrolytes: A New Horizon for Achieving High‐Performance All‐Solid‐State Lithium Metal Batteries

Abstract Solid polymer electrolytes (SPEs) are a key materials component for all‐solid‐state lithium metal batteries (ASSLMBs). In these membrane‐like films, accelerating Li + migration while enhancing the mechanical strength of SPEs is challenging. Herein, we introduce a new concept of supramolecularly organized, cross‐linked polymer electrolyte (PCPE) by mixing an ion‐conducting, multi‐arm boron‐containing oligomer (MBO) solid plasticizer into a polyethylene oxide (PEO)‐lithium salt matrix. Studies reveal that the Lewis acid‐base interaction between the Lewis‐acidic boron sites of MBO and lithium‐salt anions induces an amorphous MBO‐salt assembly subphase with percolating nanochannels for rapid Li + transport. Meanwhile, due to the structural compatibility of the multiple linear arms of MBO with PEO, a supramolecular polymer network is obtained which partly crystallizes around the ionic nanodomains, offering an PCPE with improved mechanical strength hosting interconnected ion transport pathways. The resulting PCPE shows simultaneously enhanced ionic conductivity, improved mechanical properties, and film interface stability, which enable a dendrite‐free Li/Li symmetric cell which could be cycled over 2600 h. Excellent electrochemical performance is also demonstrated in a close‐to‐practical high capacity ASSLMBs.