Nitrogen-Doped Graphene-Like Carbon Intercalated MXene Heterostructure Electrodes for Enhanced Sodium- and Lithium-Ion Storage

Abstract

MXene is investigated as an electrode material for different energy storage systems due to layered structures and metal-like electrical conductivity. Experimental results show MXenes possess excellent cycling performance as anode materials, especially at large current densities. However, the reversible capacity is relatively low, which is a significant barrier to meeting the demands of industrial applications. This work synthesizes N-doped graphene-like carbon (NGC) intercalated Ti₃C₂T_x (NGC-Ti₃C₂T_x) van der Waals heterostructure by an in situ method. The as-prepared NGC-Ti₃C₂T_x van der Waals heterostructure is employed as sodium-ion and lithium-ion battery electrodes. For sodium-ion batteries, a reversible specific capacity of 305 mAh g^-1 is achieved at a specific current of 20 mA g^-1, 2.3 times higher than that of Ti₃C₂T_x. For lithium-ion batteries, a reversible capacity of 400 mAh g^-1 at a specific current of 20 mA g^-1 is 1.5 times higher than that of Ti₃C₂T_x. Both sodium-ion and lithium-ion batteries made from NGC-Ti₃C₂T_x shows high cycling stability. The theoretical calculations also verify the remarkable improvement in battery capacity within the NGC-Ti₃C₂O₂ system, attributed to the additional adsorption of working ions at the edge states of NGC. This work offers an innovative way to synthesize a new van der Waals heterostructure and provides a new route to improve the electrochemical performance significantly.

Keywords: MXene; batteries; energy storage; graphene; heterostructures.

Figure 1

Synthesis process and material characterization of NGC‐Ti₃C₂T _x samples. a) A schematic illustration of the preparation of NGC‐Ti₃C₂T _x . X‐ray diffractograms of pristine Ti₃C₂T _x and D‐Ti₃C₂T _x b) before and c) after annealing at 600 °C for 2 h. d) Raman spectra of various samples. e,f) HAADF‐STEM images of NGC‐Ti₃C₂T _x material and g) corresponding intensity line scan showing the interlayer spacing.

Figure 2

XPS analysis of NGC‐Ti₃C₂T _x . a,d) High‐resolution XPS spectra of Ti 2p, N 1s, C1s, O1s in NGC‐Ti₃C₂T _x .

Figure 3

Electrochemical performance of NGC‐Ti₃C₂T _x as a Na‐ion battery electrode material. a) Cyclic voltammograms of NGC‐Ti₃C₂T _x for the first four cycles at a scan rate of 0.1 mV s⁻¹. b) Galvanostatic charge/discharge testing of NGC‐Ti₃C₂T _x for the first ten cycles at a specific current of 20 mA g⁻¹. c) Rate capabilities of NGC‐Ti₃C₂T _x and Ti₃C₂T _x as Na‐ion battery electrodes. d) Nyquist plots NGC‐Ti₃C₂T _x and Ti₃C₂T _x . Inset is the equivalent circuit. The dots are the original data, and the solid line are the fitting curves. e) Long‐term cycling performance at a current density of 200 mA g⁻¹.

Figure 4

The optimized structures of NGC‐Ti₃C₂O₂ with different Li‐ion concentrations: a) NGC‐Ti₃C₂O₂‐0.083Li, b) NGC‐Ti₃C₂O₂‐0.667Li, c) NGC‐Ti₃C₂O₂‐1.0Li, d) NGC‐Ti₃C₂O₂‐2.0Li, and e) NGC‐Ti₃C₂O₂‐3.0Li, respectively. f) The calculated average voltage for Li‐ and Na‐ion intercalation into NGC‐Ti₃C₂O₂. g) The electron‐density‐difference isosurfaces for Li‐ion intercalation around the edge of NGC (left, side view; right, top view). Isosurface value at 0.002 e Bohr⁻3: electron accumulation, yellow; electron depletion, cyan.