Achieving high electrical conductivity, energy storage capacity and cycling stability in ammoniated Mo2TiC2Tx MXenes as an anode for lithium-ion batteries

Abstract

The ammonization treatment is applied to a prototype ordered double-transition metal MXene known as Mo₂TiC₂T_x at different annealing temperatures (400, 500 and 600 °C) and durations (1 h and 2 h). The lattice structures, micro-morphologies, and chemical compositions of the resulting ammoniated MXenes are characterized by XRD, SEM, TEM, XPS and EPMA. All ammoniated MXenes preserve the layered microstructure of the parent carbide 2D material, and N atoms are successfully doped into the lattice sites to form carbonitride MXenes, denoted as Mo₂Ti(C_1−δN_δ)₂T_x. The Hall measurements are carried out to evaluate the electrical conductivity of ammoniated MXenes and raw Mo₂TiC₂T_x, and the results indicate that the carbonitride forms exhibit higher electrical conductivities and electron concentrations than the raw 2D material. The electrochemical properties of ammoniated MXenes are investigated by means of charge–discharge cycling CV tests, the GITT and EIS measurements at various scan rates. The ammoniated MXene sample (N400-1h) could deliver a storage capacity of 750 mA h g⁻¹ (200 mA h g⁻¹) at a current density of 0.03 A g⁻¹ (2 A g⁻¹). It is found that the capacitive charge storage mechanism through the intercalation reaction process dominates the total capacity for the ammoniated MXenes. The diffusion dynamics of Li⁺ in all investigated MXene electrodes show a two-stage lithiation mechanism in which the apparent diffusion coefficient is regulated by the different numbers of vacancies in the absorbate layers at various lithiation stages in the GITT curves. Overall, ammoniated MXenes also show superior performances in Li⁺ diffusion dynamics and charge transfer kinetics at the electrode–electrolyte interface to those of raw Mo₂TiC₂T_x, elucidating a new avenue for the use of ammoniated double-transition metal o-MXenes as high-performance electrode materials for LIBs and supercapacitors in the future.