Hierarchical skeleton-design for high-performance solid-state sodium metal batteries enabled by filler surface modification and in situ polymerization

Gel composite solid electrolytes (GCSEs) possess both high ionic conductivity and good interfacial compatibility, holding great promise for application in solid-state sodium metal batteries (SSSMBs). However, low filler content and widespread agglomeration limit their effective utilization, significantly impairing ion transport efficiency and structural consistency, thus hindering battery consistency and long-term stability. Here, a hierarchical framework-electrolyte design strategy is proposed. First, the NZSP filler is surface-modified with the silane coupling agent KH550 to enhance its compatibility with the PVDF-HFP matrix, significantly improving filler dispersion. Subsequently, a three-dimensional, uniform, porous framework with a high filler content (up to 50 wt%) is prepared using the nonsolvent-induced phase separation (NIPS) technique. Finally, after in-situ thermal polymerization of NaPF₆ electrolyte, the resulting gel composite solid electrolyte GCSE_(KN5P5) exhibits a high ionic conductivity of 1.73 mS cm⁻¹, approximately nine times that of the gel polymer electrolyte GPSE_(PVDF-HFP). In a symmetric Na||Na cell, stable cycling was achieved at a current density of 0.1 mA cm⁻² for 1500 h. Furthermore, the SSSMBs employing an NVPF cathode delivers a specific capacity of 100 mAh g⁻¹ at a high current density of 6.40 A g⁻¹ (33 C) and maintains 82 % of its initial capacity after 1000 cycles at 1.36 A g−¹ . This work provides a robust material and structural design paradigm for high-performance SSSMBs and offers general guidance for the development of next-generation solid-state energy storage systems.