Towards flame retardant high-performance solid-state lithium metal batteries: Poly(ionic liquid)-based lithiophilic ion-conductive interfaces and humidity tolerant binders

Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP)-based solid-state lithium metal batteries (SSLMBs) are widely recognized as a leading contender for next-generation energy storage due to their high energy density and safety. However, their performance is hindered by the challenging LAGP/Li interface. In this work, at the LAGP/Li interface, we introduce a novel multifunctional hybrid interlayer composed of a Li_6.4La₃Zr_1.4Ta_0.6O₁₂ ionic filler and poly(ionic liquid) electrolyte (HILP), designed to address incompatibility issues. The HILP exhibits strong lithiophilicity, excellent thermal stability, and continuous Li⁺ conductive pathways across the interface. By stabilizing the interface and inducing a solid electrolyte interphase, the HILP-LAGP configuration achieves a high critical current density of 1.4 mA cm⁻² and demonstrates an extended cycling lifespan without Li dendrite formation. Additionally, SSLMB cells based on LiFePO₄/HILP-LAGP-HILP/Li and LiNi_0.8Mn_0.1Co_0.1O₂/HILP-LAGP-HILP/Li configurations are assembled using polycationic poly(diallyldimethylammonium bis(trifluoromethylsulfonyl)imide) as the cathode binder. This binder not only provides sufficient mechanical strength and strong adhesion to active/conductive/current collector materials but also offers excellent processability. As a result, the full cells deliver a reversible capacity of 146 mAh g⁻¹ at 0.3 C, retaining 93.2 % of the capacity after 200 cycles, along with improved rate performance. The proposed interlayer opens new pathways to enhance the viability of SSLMBs for practical applications.