Quasi-solid-state sodium-ion hybrid capacitors enabled by UiO-66@PVDF-HFP multifunctional separators: Selective charge transfer and high fire safety

The practical application of sodium-ion hybrid capacitors is limited by their low energy densities resulted from the kinetics mismatch between cathodes and anodes, and the fire safety related to the flammable electrolyteseparator system. Hence, we report a rational design of metal-organic frameworks (MOFs, UiO-66) modified PVDF-HFP separator. High tensile strength and dimensional thermal stability of the separator reduce the risk of electrode short circuit caused by the separator deformation. MCC test demonstrates a reduction of 75% in peak heat release rate (pHRR), indicating an enhanced fire-resistant property of the separator. This is due to the transformation of UiO-66 into ZrO2 accompanied by the consumption of oxygen and the formation of the barrier char that suppresses further heat release. Quasi-solid-state electrolyte prepared based on this separator presents an enhanced ionic conductivity of 2.44 mS cm(-1) and Na-ion transference number of 0.55, which are related to the high porosity (>70%) and electrolyte uptake (similar to 320%) of the separator. Moreover, the open metal sites of UiO-66 can capture PF6- and consequently liberate the Na+ for faster migration, thus reducing the kinetics mismatch between cathodes and anodes. Such multifunctional separator enables the quasi-solid-state Na-ion hybrid capacitor to achieve high energy density (182 Wh kg(-1) @31 W kg(-1)) and power density (5280 W kg(-1) @22 Wh kg(-1)), as well as excellent cyclic stability (10,000 cycles @1000 mA g(-1)).

Automated summary

This study successfully addresses the limitations in energy density and fire safety issues of sodium-ion hybrid capacitors by rational design of metal-organic frameworks (MOFs, UiO-66) modified PVDF-HFP separator. The MCC test demonstrates good fire-resistant performance of the separator, and the quasi-solid-state electrolyte prepared also shows enhanced ionic conductivity for the capacitor.