Paul R. C. Kent

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

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.

Prediction and characterization of MXene nanosheet anodes for non-lithium-ion batteries.

Rechargeable non-lithium-ion (Na(+), K(+), Mg(2+), Ca(2+), and Al(3+)) batteries have attracted great attention as emerging low-cost and high energy-density technologies for large-scale renewable energy storage applications. However, the development of these batteries is hindered by the limited choice of high-performance electrode materials. In this work, MXene nanosheets, a class of two-dimensional transition-metal carbides, are predicted to serve as high-performing anodes for non-lithium-ion batteries by combined first-principles simulations and experimental measurements. Both O-terminated and bare MXenes are shown to be promising anode materials with high capacities and good rate capabilities, while bare MXenes show better performance. Our experiments clearly demonstrate the feasibility of Na- and K-ion intercalation into terminated MXenes. Moreover, stable multilayer adsorption is predicted for Mg and Al, which significantly increases their theoretical capacities. We also show that O-terminated MXenes can decompose into bare MXenes and metal oxides when in contact with Mg, Ca, or Al. Our results provide insight into metal ion storage mechanisms on two-dimensional materials and suggest a route to preparing bare MXene nanosheets.

Hybrid Density Functional Study of Structural and Electronic Properties of Functionalized Tin+1Xn (X=C, N) Monolayers

Density functional theory simulations with conventional (PBE) and hybrid (HSE06) functionals were performed to investigate the structural and electronic properties of MXene monolayers, \ce{Ti_{n+1}C_n} and \ce{Ti_{n+1}N_n} (

Accurate static and dynamic properties of liquid electrolytes for Li-ion batteries from ab initio molecular dynamics.

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Systematic Study of d-Wave Superconductivity in the 2D Repulsive Hubbard Model

= 1--9) with surfaces terminated by O, F, H, and OH groups. We find that PBE and HSE06 give similar results. Without functional groups, MXenes have magnetically ordered ground states. All the studied materials are metallic except for \ce{Ti_{2}CO_{2}}, which we predict to be semiconducting. The calculated density of states at the Fermi level of the thicker MXenes (

Carrier localization and the origin of luminescence in cubic InGaN alloys

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Atomic Defects in Monolayer Titanium Carbide (Ti3C2Tx) MXene

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Monte Carlo energy and variance-minimization techniques for optimizing many-body wave functions

5) is much higher than for thin MXenes, indicating that properties such as electronic conductivity and surface chemistry will be different. In general, the carbides and nitrides behave differently with the same functional groups.

Understanding the origin of high-rate intercalation pseudocapacitance in Nb2O5 crystals

Lithium-ion batteries have the potential to revolutionize the transportation industry, as they did for wireless communication. A judicious choice of the liquid electrolytes used in these systems is required to achieve a good balance among high-energy storage, long cycle life and stability, and fast charging. Ethylene-carbonate (EC) and propylene-carbonate (PC) are popular electrolytes. However, to date, almost all molecular-dynamics simulations of these fluids rely on classical force fields, while a complete description of the functionality of Li-ion batteries will eventually require quantum mechanics. We perform accurate ab initio molecular-dynamics simulations of ethylene- and propylene-carbonate with LiPF(6) at experimental concentrations to build solvation models which explain available neutron scattering and nuclear magnetic resonance (NMR) results and to compute Li-ion solvation energies and diffusion constants. Our results suggest some similarities between the two liquids as well as some important differences. Simulations also provide useful insights into formation of solid-electrolyte interphases in the presence of electrodes in conventional Li-ion batteries.