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

Abstract

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 NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> 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 NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> with uniform sizes was fabricated via a well-designed novel liquid transformation of ultrathin TiO<inf>2</inf> 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 NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> materials. Examined in a three-electrode cell, this NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> 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 NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> 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.