Role of Surface Structure on Li-Ion Energy Storage Capacity of Two-Dimensional Transition-Metal Carbides

Abstract

A combination of density functional theory (DFT) calculations and experiments is used to shed light on the relation between surface structure and Li-ion storage capacities of the following functionalized two-dimensional (2D) transition-metal carbides or MXenes: Sc2C, Ti2C, Ti3C2, V2C, Cr2C, and Nb2C. The Li-ion storage capacities are found to strongly depend on the nature of the surface functional groups, with O groups exhibiting the highest theoretical Li-ion storage capacities. MXene surfaces can be initially covered with OH groups, removable by high-temperature treatment or by reactions in the first lithiation cycle. This was verified by annealing f-Nb2C and f-Ti3C2 at 673 and 773 K in vacuum for 40 h and in situ X-ray adsorption spectroscopy (XAS) and Li capacity measurements for the first lithiation/delithiation cycle of f-Ti3C2. The high-temperature removal of water and OH was confirmed using X-ray diffraction and inelastic neutron scattering. The voltage profile and X-ray adsorption near edge structure of f-Ti3C2 revealed surface reactions in the first lithiation cycle. Moreover, lithiated oxygen terminated MXenes surfaces are able to adsorb additional Li beyond a monolayer, providing a mechanism to substantially increase capacity, as observed mainly in delaminated MXenes and confirmed by DFT calculations and XAS. The calculated Li diffusion barriers are low, indicative of the measured high-rate performance. We predict the not yet synthesized Cr2C to possess high Li capacity due to the low activation energy of water formation at high temperature, while the not yet synthesized Sc2C is predicted to potentially display low Li capacity due to higher reaction barriers for OH removal.

Effect of Postetch Annealing Gas Composition on the Structural and Electrochemical Properties of Ti2CTx MXene Electrodes for Supercapacitor Applications

“…[18][19][20] In a recent report, Li et al stated that heat treatment is an efficient way to eliminate the surface groups on MXenes and tune their properties 14,21 . As a new class of graphene like 2D materials, properties of MXenes need to be further explored to get a proper understanding of their fundamental properties 22 . It is of great importance to understand how heating in different ambients influences the morphology, structure and electrochemical properties of MXenes, in order to modulate the structure and properties of…”

Transition Metal‐Promoted V2CO2 (MXenes): A New and Highly Active Catalyst for Hydrogen Evolution Reaction

“…Shortly after the synthesis of MXenes,12 their possible application as anode materials for Li ion batteries (LIBs) has been explored greatly and these MXenes exhibit good capability for high charge–discharge rates 14, 17, 18, 19. Besides LIBs, other applications, such as non‐Li‐ion batteries,20, 21 hydrogen storage,22, 23 supercapacitor,24 and thermoelectric materials,25 have also been investigated.…”

Studying of 2D Titanium Carbide Structure by Raman Spectroscopy after Heat Treatment in Argon and Hydrogen Atmospheres

“…MXene has a general formula Mn+1XnT, where "M" stands for early transitional metal, "X" is carbon and/or nitrogen, "T" is a surface termination and n = 1, 2 or 3. MXene has good electrical conductivity and hydrophilic properties [4], which makes it possible to use in energy storage devices [5]. It also has promising performance as an electrode material in electrochemical capacitors [6].…”