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MXene is investigated as an electrode material for different energy storage systems due to layered structures and metal-like electrical conductivity. Experimental results show MXenes possess excellent cycling performance as anode materials, especially at large current densities. However, the reversible capacity is relatively low, which is a significant barrier to meeting the demands of industrial applications. This work synthesizes N-doped graphene-like carbon (NGC) intercalated Ti3C2Tx (NGC-Ti3C2Tx) van der Waals heterostructure by an in situ method. The as-prepared NGC-Ti3C2Tx van der Waals heterostructure is employed as sodium-ion and lithium-ion battery electrodes. For sodium-ion batteries, a reversible specific capacity of 305 mAh g-1 is achieved at a specific current of 20 mA g-1, 2.3 times higher than that of Ti3C2Tx. For lithium-ion batteries, a reversible capacity of 400 mAh g-1 at a specific current of 20 mA g-1 is 1.5 times higher than that of Ti3C2Tx. Both sodium-ion and lithium-ion batteries made from NGC-Ti3C2Tx shows high cycling stability. The theoretical calculations also verify the remarkable improvement in battery capacity within the NGC-Ti3C2O2 system, attributed to the additional adsorption of working ions at the edge states of NGC. This work offers an innovative way to synthesize a new van der Waals heterostructure and provides a new route to improve the electrochemical performance significantly.

Two-dimensional transition-metal carbides and nitrides "MXenes" have demonstrated great potential as electrode materials for electrochemical energy storage systems. This is especially true for delaminated Ti3C2Tx, which already shows outstanding gravimetric and volumetric capacitance, with areal capacitance limited by thickness (only a few microns). However, the performance of multilayer Ti3C2Txhas been more modest. Here, we report on using metal cation (viz., Na+, K+, and Mg2+) pre-intercalated multilayer Ti3C2Txas electrodes for aqueous supercapacitors. These electrodes are scalable and amenable to roll-to-roll manufacturing, with adjustable areal loadings of 5.2 to 20.1 mg/cm2. K-Ti3C2Txexhibited the highest capacitances at different scan rates. A gravimetric capacitance comparable to that of delaminated MXene of up to 300 F/g was achieved for multilayer K-Ti3C2Txbut with an outstanding ultra-high areal capacitance of up to 5.7 F/cm2, which is 10-fold higher than the 0.5 F/cm2of delaminated MXene and exceeds the 4.0 F/cm2of microengineered MXene electrodes.

Two-dimensional (2D) layered transition metal carbides/nitrides, called MXenes, are attractive alternative electrode materials for electrochemical energy storage. Owing to their metallic electrical conductivity and low ion diffusion barrier, MXenes are promising anode materials for sodium-ion batteries (SIBs). Herein, we report on a new 2D carbonitride MXene, viz., Ti2C0.5N0.5Tx (Tx stands for surface terminations), and the only second carbonitride after Ti3CNTx so far. A new type of in situ HF (HCl/KF) etching condition was employed to synthesize multilayer Ti2C0.5N0.5Tx powders from Ti2AlC0.5N0.5. Spontaneous intercalation of tetramethylammonium followed by sonication in water allowed for large-scale delamination of this new titanium carbonitride into 2D sheets. Multilayer Ti2C0.5N0.5Tx powders showed higher specific capacities and larger electroactive surface area than those of Ti2CTx powders. Multilayer Ti2C0.5N0.5Tx powders show a specific capacity of 182 mAh g−1 at 20 mA g−1, the highest among all reported MXene electrodes as SIBs with excellent cycling stability. (Figure presented.).

Electrical double-layer capacitors (EDLCs) are of increasing importance in energy storage from renewable sources. The properties of the electrode and electrolyte materials influence the energy and power densities of EDLCs. We examined the specific capacitance and ion dynamics of a protic ionic liquid confined in pre-intercalated Ti3C2Tx MXene. Our electrochemical measurements demonstrated that the creation of a protic ionic liquid, 1-butyl-3-H-imidazolium bis(trifluoromethanesulfonyl)imide (BuIMH-NTf2), using a mixture of ionic liquid, 1-butyl imidazole (BuIM), and salt, bis(trifluoromethanesulfonyl)imide (HNTf2), in a ratio of 0.8:0.2 led to the optimal capacitance. Remarkably, quasi-elastic neutron scattering measurements revealed increased particle mobility at this composition, attributed to the more efficient accumulation of BuIMH+ on the electrode surface. This deposit of additional ions results in fewer BuIM molecules away from the surface, enhancing their mobility due to reduced crowding. This composition-dependent electrochemical behavior will guide the formulation of more efficient protic ionic liquid systems, enabling faster ion transport in energy storage devices. [ABSTRACT FROM AUTHOR]

Two‐dimensional (2D) layered transition metal carbides/nitrides, called MXenes, are attractive alternative electrode materials for electrochemical energy storage. Owing to their metallic electrical conductivity and low ion diffusion barrier, MXenes are promising anode materials for sodium‐ion batteries (SIBs). Herein, we report on a new 2D carbonitride MXene, viz., Ti2C0.5N0.5Tx (Tx stands for surface terminations), and the only second carbonitride after Ti3CNTx so far. A new type of in situ HF (HCl/KF) etching condition was employed to synthesize multilayer Ti2C0.5N0.5Tx powders from Ti2AlC0.5N0.5. Spontaneous intercalation of tetramethylammonium followed by sonication in water allowed for large‐scale delamination of this new titanium carbonitride into 2D sheets. Multilayer Ti2C0.5N0.5Tx powders showed higher specific capacities and larger electroactive surface area than those of Ti2CTx powders. Multilayer Ti2C0.5N0.5Tx powders show a specific capacity of 182 mAh g−1 at 20 mA g−1, the highest among all reported MXene electrodes as SIBs with excellent cycling stability. [ABSTRACT FROM AUTHOR]

Layered titanium carbide (Ti 3 C 2 T x ) MXene is a promising electrode material for use in next-generation electrochemical capacitors. However, the atomic-level information needed to correlate the distribution of intercalated cations with surface redox reactions, has not been investigated in detail. Herein we report on sodium preintercalated MXene with high sodium content (up to 2Na per Ti 3 C 2 T x formula) using a solution of Na-biphenyl radical anion complex ( E 0 ≈ -2.6 SHE). Multiple sodiation sites and formation of a two-dimensional sodium domain structure at interfaces/surfaces is identified through combined computational simulations with neutron pair distribution function analysis. The induced layer charges and the redox process characterized by the density-functional tight-binding method on a local scale are found to greatly depend on the location of sodium ions. Electrochemical testing of the pre-sodiated MXene as an electrode material in a sodium-ion capacitor shows excellent reversibility and promising performance, indicating the feasibility of chemical preintercalation as an approach to prepare MXene electrodes for ion capacitors.

The synthesis of heterostructures of different two-dimensional (2D) materials offers an approach to combine advantages of different materials constituting the heterostructure and ultimately enhance their performance for applications such as electrochemical energy storage, achieving high energy, and high-power densities. Understanding the behavior of ions and solvents in confinement between these dissimilar layers is critical to understand their performance and control. Considering aqueous electrolytes, we explore the heterostructure of 2D lepidocrocite-type TiO 2 (2D-TiO 2 ) and hydroxylated or O-terminated Ti 3 C 2 MXene using ReaxFF molecular dynamics simulations and elastic/quasielastic neutron scattering techniques. Simulating a bilayer water intercalation, we find that the extent of interlayer hydration is impacted most by the surface terminations on the MXene and is marginally affected by 2D-TiO 2 . However, the introduction of 2D-TiO 2 decreases the water self-diffusion due to the notch sites (i.e., surface oxygen ridges) entrapping water molecules. Intercalating alkali cations into the heterostructures, we find that Li + is predominantly adsorbed at the 2D-TiO 2 surface instead of the MXenes with the preferential occupation of the notch sites. In contrast, Na + forms a planar solvation with water, while K + is adsorbed both at the O-terminated MXene and 2D-TiO 2 . This behavior is altered when OH-terminated MXene is involved-the repulsion from the protons on the MXene surface forces the K + ions to be adsorbed exclusively to 2D-TiO 2 , while Na + retains some of its solvation in the water layer due to its smaller size. In OH-terminated MXenes, we see a consistent transfer of protons from the MXene surface toward 2D-TiO 2 , implying a greater capacity to store protons in the heterostructures. Of the three cations simulated, Na + hinders the proton migration the least and Li + the most because of its position near the 2D-TiO 2 surface. Therefore, 2D-TiO 2 /MXene heterostructures are likely to exhibit a higher energy density but lower power density, especially with Na + intercalation.

Intercalation in Ti 3 C 2 T x MXene is essential for a diverse set of applications such as water purification, desalination, electrochemical energy storage, and sensing. The interlayer spacing between the Ti 3 C 2 T x nanosheets can be controlled by cation intercalation; however, the impact of intercalation on the Ti 3 C 2 T x MXene chemical and electronic structures is not well understood. Herein, we characterized the electronic structure of pristine, Li-, Na-, K-, and Mg-intercalated Ti 3 C 2 T x MXenes dispersed initially in water and 10 mM sulfuric acid (H 2 SO 4 ) using X-ray absorption spectroscopy (XAS). The cation intercalation is found to dramatically influence the chemical environment of Ti atoms. The Ti oxidation of the MXene increases progressively upon intercalation of cations of larger sizes after drying in air, while interestingly a low Ti oxidation is observed for all intercalated MXenes after dispersion in diluted H 2 SO 4 . In situ XAS at the Ti L-edge was conducted during electrochemical oxidation to probe the changes in the Ti oxidation state in the presence of different cations in H 2 SO 4 aqueous electrolyte. By applying the sensitivity of the Ti L-edge to probe the oxidation state of Ti atoms, we demonstrate that cation-intercalation and H 2 SO 4 environment significantly alter the Ti 3 C 2 T x surface chemistry.