Porous single-crystal NaTi2(PO4)3 via liquid transformation of TiO2 nanosheets for flexible aqueous Na-ion capacitor

Aqueous Na-ion capacitor and battery are of great significance for both wearable electronics and stationary energy storage due to the inherent safety and low cost. However, conventional NaTi2(PO4)3 materials with irregular morphology and large size prepared by solid-state reaction still hinder the application of these systems. Herein, a newly structured porous single-crystal NaTi2(PO4)3 with uniform sizes was fabricated via a well-designed novel liquid transformation of ultrathin TiO2 nanosheets, followed by coating a conductive carbon sheath. To best of our knowledge, this is the first report of the porous single-crystal structure of NaTi2(PO4)3 materials. Examined in a three-electrode cell, this NaTi2(PO4)3 electrode demonstrates an outstanding rate capability of 80–102 mA h g−1 at varied current densities of 0.5–3 A g−1 due to the synergistic effect between porous nanostructure and outstanding stability originated from single-crystal structure. The high-quality NaTi2(PO4)3 was also assembled with N-doped carbon to fabricate an aqueous Na-ion capacitor with robust flexibility. This work paves the way for designing advanced Na super ion conductor (NASICON) based materials for aqueous energy storage systems.