Halloysite Nanotube-Assisted Morphology Engineering in Quasi-Solid-State Composite Polymer Electrolytes for High-Performance and Flexible Lithium Metal Batteries

Lithium metal batteries (LMBs) require electrolytes with high ionic conductivity, stability, and mechanical robustness to mitigate dendrite growth, unstable SEI formation, and safety issues associated with liquid electrolytes. Here, we report quasi-solid-state composite polymer electrolytes (QSCPEs) fabricated by incorporating single-ion-conducting halloysite nanotubes (HNTs) into a thermoplastic polyurethane (TPU) matrix. Increasing the HNT content induces controllable surface morphologies with interconnected pores, which facilitate Li⁺ transport and markedly enhance ionic conductivity while maintaining mechanically robust, dimensionally stable, and flexible films with excellent thermal stability. The optimized QSCPE-70 (containing 70 wt % HNT relative to TPU) delivers a high ionic conductivity of 1.04 × 10^-3 S cm^-1 at room temperature, a Li⁺ transference number of 0.767, and stable cycling for over 2500 h in Li|Li symmetric cells. A full cell employing this electrolyte exhibits significant capacity retention and rate capability, attributed to the promoted Li⁺ transport. The corresponding pouch cell preserves both electrochemical performance and structural integrity, even under mechanical deformation. These results demonstrate that compositional tuning of QSCPEs enables simultaneous control over morphology, selective Li-ion transport, and flexibility, providing a practical pathway toward safe, high-performance, and flexible LMBs.