Reticular framework electrolytes for liquid-free all-solid-state sodium batteries: design principles and evidence standards

All-solid-state sodium batteries are promising for safe, sustainable, and cost-effective energy storage, yet their real progress is often difficult to assess because nominally solid-state cells may still benefit from unrecognized or insufficiently disclosed liquid assistance. This Review adopts a selective and critical perspective on reticular framework-based electrolytes, including MOFs, COFs, and HOFs, with emphasis on whether framework chemistry can genuinely replace the functions normally supplied by liquids, such as wetting, concentration relaxation, and interphase buffering. To make this boundary explicit, we introduce an ionic-liquid involvement (ILI) index to classify liquid-free designs, dry-assembled systems with possible residual liquid effects, and liquid-assisted demonstrations. Recent progress is organized around four mechanism-based design dimensions: bulk Na⁺ percolation, Na⁺-selective transport, solid-state solvation reconfiguration, and targeted interfacial pathway continuity. Within this framework, MOFs are examined as coordination-programmable hosts for anion regulation, channel design, and interfacial transport; COFs are discussed as covalently defined platforms for pore-wall charge engineering, directional ion hopping, and membrane-scale percolation; and HOFs are considered as adaptive supramolecular networks that may improve interfacial contact but require stricter stability validation. Rather than cataloguing reported materials, we highlight representative examples that reveal how topology, coordination chemistry, polymer confinement, and artificial interphases shape Na⁺ migration in liquid-free or near-liquid-free ASSBs. We further identify recurring evidence gaps, including incomplete disclosure of liquid content, overreliance on conductivity values, insufficient interfacial diagnostics, and limited cell-level validation. Finally, we outline design and reporting standards needed to move reticular framework electrolytes from attractive structural concepts toward mechanistically transparent, rigorously validated, and practically relevant liquid-free ASSBs.