Bilayered vanadium oxides (BVOs) are a high-capacity intercalation host with affinity for various ions in energy storage systems. However, the electrochemical stability of BVOs upon extended galvanostatic cycling, especially at high rates, is lackluster. In this study, we demonstrate a transformative synthesis of chemically preintercalated BVOs with unique twodimensional (2D) morphology and improved electrochemical stability by oxidation of V 2 CT x MXenes in hydrogen peroxide in the presence of alkali and alkaline-earth metal chlorides. The structure and composition of V 2 CT xderived δ-M x V 2 O 5 •nH 2 O (M = Li, Na, K, Mg, and Ca) phases were examined by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The electrochemical properties of theelectrodes in Li-ion cells were studied. Both materials exhibited high reversible specific capacity, improved cycling stability, and excellent rate capability. Notably, an enhanced tolerance to high current rates is observed with specific discharge capacity dropping from 200 to 130 mAh•g −1 and from 192 to 146 mAh•g −1 when the current rate was changed from C/10 to 5C in the case of V 2 CT x -derived δ-Li x V 2 O 5 •nH 2 O and δ-Mg x V 2 O 5 •nH 2 O electrodes, respectively. The improved capacity retention during electrochemical cycling may be attributed to the 2D morphology and improved crystallinity of the material enabled by the synthesis route.

Synthetic strategies for the improvement in electronic conductivities and electrochemical stabilities of transition metal oxide cathodes, which are limiting factors in the performance of commercial intercalation batteries, are required for next-generation, high-performance battery systems. The chemical preintercalation approach, consisting of a combined sequence of a sol−gel process, extended aging, and a hydrothermal treatment, is a versatile, wet synthesis technique that allows for the incorporation of a polar species between the layers of transition metal oxides. Here, formation of a layered 2D δ-C x V 2 O 5 •nH 2 O heterostructure occurs via chemical preintercalation of dopamine molecules between bilayers of vanadium oxide followed by the hydrothermal treatment of the precipitate, leading to carbonization of the organic molecules. The presence of carbon layers within the structure has been confirmed via a combined analysis of scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, four-probe conductivity measurements, and scanning transmission electron microscopy characterization. 2D δ-C x V 2 O 5 •nH 2 O heterostructure electrodes demonstrated significantly improved electrochemical performance, particularly at higher current densities, in Li-ion cells. The heterostructure electrodes exhibited 75% of the capacity retention when the current was changed from 20 mA g −1 (206 mAh g −1 ) to 300 mA g −1 (155 mAh g −1 ), while the reference δ-V 2 O 5 •nH 2 O electrodes exhibited only 10% capacity retention in the same experiment. Remarkably, 2D δ-C x V 2 O 5 •nH 2 O heterostructure electrodes demonstrated significantly improved capacity retention (94% after 30 cycles) for bilayered vanadium oxide electrodes in Li-ion cells during galvanostatic cycling at 20 mA g −1 . The improved electrochemical performance, in both extended cycling and rate capability studies, of the 2D δ-C x V 2 O 5 •nH 2 O heterostructure electrodes in the Li-ion system is ascribed to the intermittent formation of carbon layers within the bilayered structure, which leads to increased electronic conductivity and improved structural stability of the heterostructure compared to the reference δ-V 2 O 5 •nH 2 O electrodes.

Intercalation redox materials have shown great promise for efficient water desalination due to available faradaic gallery sites. Symmetric capacitive deionization (CDI) cells previously demonstrated using MXenes were often limited in their salt adsorption capacity (SAC) and voltage window of operation. In this study, current collector‐ and binder‐free Ti3C2Tx MXene electrode architectures are designed with porous carbon as the positive electrode to demonstrate hybrid CDI (HCDI) operation. Furthermore, MXene current collectors are fabricated by employing a scalable doctor blade coating technique and subsequently spray coating a layer of a small flake MXene dispersion. Hydrophilic redox‐active galleries of MXenes are capable of intercalating a variety of aqueous cations including Na+, K+, and Mg2+ ions, showing volumetric capacitances up to 250 F cm‐3. As a result, a salt removal capacity of 39 mg g‐1 with decent cycling stability is achieved. This study opens new avenues for developing freestanding, binder‐ and additive‐free MXene electrodes for HCDI applications.

Bilayered vanadium oxides are attractive for energy storage due to their high initial specific capacities, which could be stabilized by integrating the bilayers with conductive nanoflakes often produced in a form of aqueous dispersions. Therefore, exfoliation of the bilayered vanadium oxides in water with high yield is desirable. This work introduces the first aqueous exfoliation of chemically prelithiated bilayered vanadium oxide (i.e., δ-Li x V2O5·nH2O or LVO) followed by vacuum filtration to produce a free-standing film, exhibiting a lamellar stacking of the bilayered vanadium oxide nanoflakes as evidenced by scanning electron microscopy. Due to the hydrated nature of bilayered vanadium oxides, the relationship between interlayer water content and the vacuum drying temperature (105 °C vs 200 °C) was studied using X-ray diffraction, thermogravimetric analysis, and Raman spectroscopy. It was found that vacuum drying the LVO nanoflakes at 200 °C enabled more efficient removal of crystallographic water than drying at 105 °C, and did not induce a phase transformation. Scanning transmission electron microscopy confirmed the layered structure of the samples, which was more well-ordered in the 200 °C case and had no clear boundaries between flakes at the atomic scale. Furthermore, electrochemical testing in nonaqueous Li-ion cells revealed that vacuum drying at 200 °C led to improvements in ion storage capacity and electrochemical stability. Improvements in electrochemical charge storage properties of the electrodes obtained via LVO exfoliation and free-standing film assembly in water dried at 200 °C reveal that conventional battery electrode drying protocols need to be revised as new electrochemically active materials are synthesized, such as hydrated layered oxides with expanded interlayer regions. The remaining capacity fading can be attributed to the structural LVO degradation, dissolution of vanadium oxide in electrolyte, and parasitic effects of the remaining interlayer water molecules. Our results establish an environmentally friendly and safe approach to obtain two-dimensional (2D) bilayered vanadium oxide nanoflakes and create a pathway to constructing novel 2D heterostructures for improved performance in energy storage applications.

Chemically preintercalated dopamine (DOPA) molecules were used as both a reducing agent and a carbon precursor to prepare δ‐V2O5·nH2O/C, H2V3O8/C, VO2(B)/C, and V2O3/C nanocomposites via hydrothermal treatment or hydrothermal treatment followed by annealing under Ar flow. We found that the phase composition and morphology of the produced composites are influenced by the DOPA:V2O5 ratio used to synthesize (DOPA)xV2O5 precursors through DOPA diffusion into the interlayer region of the δ‐V2O5·nH2O framework. The increase of DOPA concentration in the reaction mixture led to a more pronounced reduction of vanadium and a higher fraction of carbon in the composites’ structure, as evidenced by X‐ray photoelectron spectroscopy and Raman spectroscopy measurements. The electrochemical charge storage properties of the synthesized nanocomposites were evaluated in Li‐ion cells with nonaqueous electrolytes. δ‐V2O5·nH2O/C, H2V3O8/C, VO2(B)/C, and V2O3/C electrodes delivered high initial capacities of 214, 252, 279, and 637 mAh g–1, respectively. The insights provided by this investigation open up the possibility of creating new nanocomposite oxide/carbon electrodes for a variety of applications, such as energy storage, sensing, and electrochromic devices.