High-temperature-resistant mullite nanofiber membrane as an ion-sieving interlayer for uniform Zn2+ flux in long-life zinc ion hybrid capacitors

Zinc-based energy storage devices are among the most promising candidates for large-scale energy storage owing to their inherent safety and the abundance of raw materials. However, the “tip effect” of zinc metal promotes dendrite growth, significantly limiting the cycle life of zinc-based devices. This study utilizes mullite nanofiber membranes (possessing excellent mechanical strength and high-temperature resistance) as a Zn²⁺-regulating interface layer to achieve uniform Zn²⁺ flux distribution and controlled zinc deposition. Consequently, the cycling stability of both zinc anodes and zinc-based devices is significantly enhanced. Using air-jet spinning followed by high-temperature annealing, this study fabricates a flexible and foldable mullite membrane containing aluminum and silicon, with a uniform pore structure and excellent high-temperature resistance. At a current density of 5 mA cm⁻² and stripping capacity of 1 mAh cm⁻², the Zn//Zn symmetric cell incorporating the mullite membrane achieves a cycle life exceeding 1600 h. The Zn//Mul–GF//AC capacitor demonstrates stable cycling over 20,000 cycles with an average Coulombic efficiency of 99.95 %. This enhanced performance is primarily attributed to the unique structural advantages of the mullite membrane: (1) in conjunction with a glass fiber separator, it forms a gradient-distributed pore architecture that exerts a dielectric shielding effect, facilitating uniform Zn²⁺ flux distribution for homogeneous zinc deposition, and (2) its excellent mechanical integrity suppresses dendrite formation and interfacial side reactions. Furthermore, the soft-pack zinc-ion capacitor incorporating the mullite membrane exhibits excellent electrochemical performance under high-temperature (55°C) and bending conditions, owing to its flexibility and thermal resistance.