Electronic structure modulation of MOF-based host–guest recognition polymer electrolytes for high-performance all-solid-state sodium metal batteries

Abstract

Transitioning from gel to all-solid-state sodium metal batteries (ASSMBs) is essential for upgrading the energy density and safety of flexible energy storage devices. Herein, a novel host–guest recognition strategy for all-solid-state composite polymer electrolytes (CPEs) is proposed by introducing an Fe–Ni bimetallic metal–organic framework (MOF) into the PEO matrix. The Fe–Ni MOF with various synergetic sites serves as a ‘host’ platform for CPEs to weaken the coulombic interaction between Na⁺ and bis(trifluoromethysulfonyl)imide (TFSI⁻), thereby promoting the dissociation of sodium salt. They reduce the Na⁺ migration barrier and improve high-voltage stability. The optimized CPE achieves an ionic conductivity of 1.37 × 10⁻⁴ S cm⁻¹ at 30 °C, and the Na⁺ transference number increased threefold (from 0.23 to 0.63). The corresponding Na//Na symmetric cells operate stably for 1150 h at 25 °C, and the Na//Na₃V₂(PO₄)₃ (NVP) cells demonstrate 97.2% capacity retention after 600 cycles at 1C. Thanks to the high oxidation limit of the CPE (4.5 V), the Na₃V₂(PO₄)₂F₃ (NVPF)-based ASSMBs demonstrate stable cycling over 500 cycles within the voltage range of 2.5–4.3 V. Furthermore, an all-solid-state Na metal pouch cell exhibits a high initial discharge specific capacity of 113.4 mAh g⁻¹ and remains stable for 56 cycles at 0.1C. This innovative host–guest recognition polymer electrolyte offers valuable insights for enhancing rapid ion transport and contributing to the better understanding of the underlying Na⁺ kinetics mechanisms.