For high capacities and extremely fast charging rates, two-dimensional (2D) crystals exhibit a significant promising application on lithium-ion batteries. With density functional calculations, this paper systematically investigated the Li storage properties of eight 2D M2CO2 (M = V, Cr, Ta, Sc, Ti, Zr, Nb, and Hf), which are the recently synthesized transition-metal carbides (called MXenes) with O groups. According to whether the structural transformation occurs or not during the adsorption of the first Li layer, the adsorption of Li can be grouped into two types: V-type (V2CO2, Cr2CO2, and Ta2CO2) and Sc-type (Sc2CO2, Ti2CO2, Zr2CO2, Nb2CO2, and Hf2CO2). The structural transformation behaviors of V-type are reversible during lithiation/delithiation and are confirmed by ab initio molecular dynamic simulations. Except for Nb-MXene, the V-type prefers the sandwich H2H1T-M2CO2Li4 structure and the Sc-type prefers the TH1H2-M2CO2Li4 structure during the adsorption of the second Li layer. The H2H1T-M2CO2Li4 structure of O layer sandwiched by two Li layers preferred by V-type can prevent forming Li dendrite and therefore stabilize the lithiated system. The tendency of O bonding to Li rather than M in V-type is bigger than that in Sc-type, which causes that the sandwich structure of H2H1T-M2CO2Li4 is more suitable for V-type than Sc-type.Keywords: Li ion batteries; MXene; structural transformation; two-dimensional; V2CO2

Here a novel material for methane adsorption was synthesized and studied, which is a graphene-like two-dimensional (2D) carbide (Ti2C, a member of MXenes), formed by exfoliating Ti2AlC powders in a solution of lithium fluoride (LiF) and hydrochloric acid (HCl) at 40 °C for 48 h. Based on first-principles calculation, theoretically perfect Ti2C with O termination has a specific surface area (SSA) of 671 m2 g−1 and methane storage capacity is 22.9 wt%. Experimentally, 2.85 % exfoliated Ti2C with mesopores shown methane capacity of 11.58 cm3 (STP: 0 °C, 1 bar) g−1 (0.82 wt%) under 5 MPa and the SSA was 19.1 m2 g−1. For Ti2C sample intercalated with NH3·H2O, the adsorbed amount was increased to 16.81 cm3 (STP) g−1 at same temperature. At the temperature of 323 K, the adsorbed amount of as-prepared Ti2C was increased to 52.76 cm3 (STP) g−1. For fully exfoliated Ti2C, the methane capacity was supposed to be 28.8 wt% or 1148 V (STP)v−1. Ti2C theoretically has much larger volume methane capacity than current methane storage materials, though its SSA is not very high.

•We first predicted the hydrogen storage properties of 2D Sc2C phase.•The maximum hydrogen storage capacity was calculated to be 9.0 wt.%.•Adsorption energies of H2 molecule are suitable for applications.•3.6 wt.% H2 molecules are adsorbed and released reversibly at ambient conditions.Recently, a new family of two-dimensional (2D) MXene materials was prepared by exfoliating the MAX phases (ACS Nano 2012, 6, 1322). Among all possible MXene phases, theoretically 2D Sc2C possesses the highest surface area per weight and thus is expected to have the highest gravimetric hydrogen storage capacities. In this work, using first-principles total energy pseudopotential calculations, we systematically investigated the hydrogen storage properties of 2D Sc2C phase. Depending on different adsorption sites, the hydrogens are bound by three modes: chemisorption, physisorption and Kubas-type interactions with the binding energies of 4.703, 0.087 and 0.164 eV respectively. The maximum hydrogen storage capacity was calculated to be 9.0 wt.%, which meets the gravimetric storage capacity target (5.5 wt.% by 2015) set by the U.S. DOE. Ab-initio molecular dynamic simulations confirmed that 3.6 wt.% hydrogen molecules storaged by Kubas-type interactions can be adsorbed and released reversibly at ambient conditions.

Searching for reversible hydrogen storage materials operated under ambient conditions is a big challenge for material scientists and chemists. In this work, using density functional calculations, we systematically investigated the hydrogen storage properties of the two-dimensional (2D) Ti2C phase, which is a representative of the recently synthesized MXene materials ( ACS Nano 2012, 6, 1322). As a constituent element of 2D Ti2C phase, the Ti atoms are fastened tightly by the strong Ti–C covalent bonds, and thus the long-standing clustering problem of transition metal does not exist. Combining with the calculated binding energy of 0.272 eV, ab initio molecular dynamic simulations confirmed the hydrogen molecules (3.4 wt % hydrogen storage capacity) bound by Kubas-type interaction can be adsorbed and released reversibly under ambient conditions. Meanwhile, the hydrogen storage properties of the other two MXene phases (Sc2C and V2C) were also evaluated, and the results were similar to those of Ti2C. Therefore, the MXene family including more than 20 members was expected to be a good candidate for reversible hydrogen storage materials under ambient conditions.