Ultra-stable flexible Zn-ion capacitor with pseudocapacitive 2D layered niobium oxyphosphides

Multivalent aqueous Zn-ion capacitors (ZICs) are promising next-generation electrochemical energy storage systems (ESSs) owing to distinctive features including good safety characteristics, low costs, and better electrochemical parameters than those of conventional supercapacitors. The key challenge of existing ZICs is their low energy density and limited cycling ability due to carbon cathode materials and conventional electrolyte systems. This paper presents a dual strategy in which the electrode and electrolyte features are engineered to improve the overall electrochemical performance of ZICs. First, the capacitance of the cathode material is improved by engineering reduced graphene oxide (rGO)-incorporating, pseudocapacitive, layered niobium oxyphosphide (NbPO) material; second, the electrochemical stability of the Zn metal anode is improved via an additive to the traditional Zn-electrolyte. The aqueous ZIC with rGO–NbPO cathode and NaClO4 additive electrolyte exhibits the highest capacitance (191.88 F g−1), maximal energy density (56.03 Wh kg−1), and excellent energy efficiency (approximately 50% to 55%). The prepared flexible solid-state rGO–NbPO ZIC has an ultra-long lifespan of over 50,000 cycles with approximately 76.81% capacitance retention (at 4 A g−1) and excellent mechanical tractability. The results provide guidance for improving the design of safe aqueous ESSs with high-level efficiency and long-term stability.