Tape/Freeze-Cast PVDF-HFP/Silica Composite Membranes for Separators in Na-Ion Batteries

Separators for Na-ion batteries benefit from directionally aligned pores for shorter transport paths, high porosity, and hydrophilicity for greater electrolyte uptake. This can be achieved by tape/freeze casting of polymeric solutions with ceramic fillers as reinforcements. In this study, the introduction of silica particles from the sol-gel reaction of tetraethoxysilane (TEOS) into poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) membranes is attained by a co-solvent method in conjunction with dimethyl sulfoxide (DMSO). The effects of TEOS on the microstructure, crystal structure, hydrophilicity and mechanical properties of tape/freeze-cast membranes have been investigated. In addition, the effects of silica reinforcement in the composite membrane separators on ionic conductivity, MacMullin numbers and electrochemical performance of NVP/NTP coin cells have been studied. The tape/freeze-cast PVDF-HFP membranes fabricated with DMSO as the solvent exhibit directionally aligned and dendritic pores, while a hierarchical pore morphology with spherical pores on the aligned pore walls is found in membranes fabricated with the addition of TEOS. With the introduction of silica, as confirmed by EDX and FTIR, composite PVDF-HFP/SiO₂ membranes exhibit similar porosities but greater tensile strengths and a noticeable increase in electrolyte uptake and hydrophilicity compared to its unreinforced polymer counterpart. Due to the incorporation of silica, the composite membrane separators show higher conductivities and lower MacMullin numbers. Furthermore, the coin cells with composite membrane separators outperform those with polymer membrane separators and filter papers as separators in capacity retention from charge-discharge and cycling performance tests.