  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 electrolyte-separator
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 (~ 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 (10000 cycles @1000 mA*g-1).
  Keywords: Quasi-solid-state; PVDF-HFP; Metal-organic frameworks; Dimensional
thermal stability; Fire safety; Selective charge transfer
