Ultrastrong nonflammable in-situ polymer electrolyte with enhanced interface stability boosting high-voltage Li metal batteries under harsh conditions

In-situ polymer electrolytes prepared by Li salt-initiated polymerization are promising electrolytes for solid-state Li metal batteries owing to their enhanced interface contact and facile and green preparation process. However, conventional in-situ polymer electrolytes suffer from poor interface stability, low mechanical strength, low oxidation stability, and certain flammability. Herein, a silsesquioxane (POSS)-nanocage-crosslinked in-situ polymer electrolyte (POSS-DOL@PI-F) regulated by fluorinated plasticizer and enhanced by polyimide skeleton is fabricated by Li salt initiated in-situ polymerization. Polyimide skeleton and POSS-nanocage-crosslinked network significantly enhance the tensile strength (22.8 MPa) and thermal stability (200 °C) of POSS-DOL@PI-F. Fluorinated plasticizer improves ionic conductivity (6.83 × 10⁻⁴ S cm⁻¹), flame retardance, and oxidation stability (5.0 V) of POSS-DOL@PI-F. The fluorinated plasticizer of POSS-DOL@PI-F constructs robust LiF-rich solid electrolyte interphases and cathode electrolyte interphases, thereby dramatically enhancing the interface stability of Li metal anodes and LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) cathodes. POSS-DOL@PI-F enables stable, long-term (1200 h), and dendrite-free cycle of Li||Li cells. POSS-DOL@PI-F significantly boosts the performance of Li||NCM811 cells, which display superior cycle stability under harsh conditions of high voltage (4.5 V), high temperature (60 °C), low temperature (−20 °C), and high areal capacity. This work provides a rational design strategy for safe and efficient polymer electrolytes.