Solid polymer electrolytes are considered among the most promising candidates for developing practical solid-state sodium batteries. However, moderate ionic conductivity and narrow electrochemical windows hinder their further application. Herein, inspired by the Na⁺/K⁺ conduction in biological membranes, we report a (–COO^–)-modified covalent organic framework (COF) as a Na-ion quasi-solid-state electrolyte with sub-nanometre-sized Na⁺ transport zones (6.7–11.6 Å) created by adjacent –COO^– groups and COF inwalls. The quasi-solid-state electrolyte enables selective Na⁺ transport along specific areas that are electronegative with sub-nanometre dimensions, resulting in a Na⁺ conductivity of 1.30×10^–4 S cm^–1 and oxidative stability of up to 5.32 V (versus Na⁺/Na) at 25 ± 1 °C. Testing the quasi-solid-state electrolyte in Na||Na₃V₂(PO₄)₃ coin cell configuration demonstrates fast reaction dynamics, low polarization voltages, and a stable cycling performance over 1000 cycles at 60 mA g^–1 and 25 ± 1 °C with a 0.0048% capacity decay per cycle and a final discharge capacity of 83.5 mAh g⁻¹.

Conformal hydrolysis of MOF precursors is a promising strategy to prepare hierarchical metal hydroxide electrode materials on a large scale with low cost and high efficiency. However, a complete transformation is challenging due to the normal “outside-in” conversion process. After studying the hydrolysis of Ni-MOF-74, which has regular 1D channels, we suggest that the transformation to Ni(OH)₂ can occur simultaneously outside and within the precursor depending on the treatment temperature. Molecular dynamics simulations reveal that a higher temperature weakens the steric effects of OH⁻ ions and facilitates the diffusion in the regular channels, and therefore, a complete transformation from Ni-MOF-74 to Ni(OH)₂ is achieved. It is for the first time demonstrated that the 1D channels of MOFs are utilized for the complete conformal hydrolysis of Ni-MOF-74 to Ni(OH)₂ electrode materials. Meanwhile, we also perform pioneering work illustrating that the complete conformal hydrolysis is the key to the improved supercapacitor performances of the MOF-derived Ni(OH)₂ electrodes. The prepared Ni(OH)₂ electrode under the optimized conditions has a specific capacity of 713.2 C g⁻¹ at a current density of 1 A g⁻¹, which is at least 28% larger than those of the Ni(OH)₂ prepared at other temperatures. The detailed analyses based on CV and EIS of the obtained Ni(OH)₂ electrodes indicate that the residual MOFs within electrodes due to incomplete hydrolysis significantly influence the diffusion length and diffusion efficiency of OH⁻, drastically lowering the supercapacitor performances.