2D nonlayered titanium oxynitrides intercalated with nitrogen-doped graphene for pseudocapacitive energy storage

Abstract

Nanostructured titanium oxynitride (TiN_xO_1-x) is a promising pseudocapacitive material due to its large pseudocapacitance and high conductivity. However, the synthesis of nonlayered two-dimensional (2D) TiN_xO_1-x is challenging and the pseudocapacitive energy storage mechanism of TiN_xO_1-x is still unclear. Herein, accordion-like 2D TiN_xO_1-x nanosheets intercalated with nitrogen-doped graphene (TiN_xO_1-x/NG) are controllably fabricated by a spatially confined thermal nitridation of 1,8-diaminooctane intercalated H_0.8Ni_0.4Ti_1.6O₄·H₂O nanosheets. In situ electrochemical characterizations and theoretical calculations reveal the pseudocapacitance originates from the Faradic reaction between H⁺ and NTiO species accompanied by Ti valence state change and the pseudocapacitance of TiN_xO_1-x/NG is determined by the NTiO content instead of the N/O ratio. Compared to TiN_xO_1-x, the electrons transfer from sandwiched NG to TiN_xO_1-x enhances H⁺ adsorption at N active sites of NTiO within TiN_xO_1-x/NG, giving rise to enhanced pseudocapacitance and cycling stability. Consequently, the TiN_xO_1-x/NG shows a large volumetric capacitance of 739.2 F cm⁻³ (616.0 F g⁻¹) at 1 A cm⁻³ and superior cyclability with 98.5 % capacitance retention after 10,000 cycles, surpassing previously reported metal oxynitrides and nitrides. The results provide insights into the controllable synthesis of 2D nonlayered TiN_xO_1-x and elucidate the pseudocapacitive charge storage mechanism of metal nitrides or oxynitrides.