Sodium-Ion Intercalation Mechanism in MXene Nanosheets

MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state ²³Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti₃C₂T_<i>x</i> in a nonaqueous Na⁺ electrolyte exhibits reversible Na⁺ intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti₃C₂T_<i>x</i> undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na⁺ is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na⁺ and the swelling effect of penetrated solvent molecules between the Ti₃C₂T_<i>x</i> sheets. Since Na⁺ intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti₃C₂T_<i>x</i> shows good capacity retention over 100 cycles as well as excellent rate capability

Structure and Dynamic Behavior of the Na–Crown Ether Complex in the Graphite Layers Studied by DFT and ¹H NMR

Diffusion of alkali metals in graphite layers is significant for the chemical and electrochemical properties of graphite intercalation compounds (GICs). Crown ethers co-intercalate into graphite with alkali metal (Na and K) cations and form ternary GICs. The structures and molecular dynamics of 15-crown-5 and 18-crown-6 ether coordinating to Na⁺ or K⁺ in GICs were investigated by DFT calculations and ¹H solid state NMR analyses. DFT calculations suggest a stacked structure of crown ether–metal complex with some offset. ¹H NMR shows two kinds of molecular motions at room temperature: isotropic rotation with molecular diffusion and axial rotation with fluctuation of the axis. The structure and dynamics of crown ether molecules in GIC galleries are strongly affected by the geometry of the crown ether molecules and the strength of the interaction between alkali metal and ligand molecules

Direct Information on Structure and Energetic Features of Cu⁺−Xe Species Formed in MFI-Type Zeolite at Room Temperature

The interacted species of Xe with metal ions that are stable at room temperature are not known and are a subject of interest for fundamental chemistry. We have experimentally found a new and stable Xe species, XeCu⁺, which was formed at room temperature in a copper ion-exchanged MFI-type zeolite. The presence of a prominent interaction between Cu⁺ in MFI and Xe, which has a covalent nature, was for the first time evidenced from experimental in situ synchrotron X-ray absorption fine structure and heat of adsorption measurements: the Cu⁺−Xe bond length of 2.45 Å and the bonding energy of ca. 60 kJ mol⁻¹. The bonding nature between Xe and Cu⁺ in the MFI zeolite was discussed utilizing density functional theory; the observed significant stabilization comes from the d(Cu⁺ in MFI)−p(Xe) orbital interaction. These new findings may pave a new way to developing fundamental chemistry of Xe compounds