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Anodic dissolution of aluminum (commonly called aluminum corrosion) is a potential issue in sodium-ion batteries. Herein, it is demonstrated how different sodium-ion battery electrolyte solutions affect this phenomenon. The type of electrolyte was critical for the presence of anodic dissolution, while the solvent appeared to alter the dissolution process.

Anodic dissolution (often referred to as corrosion) of the aluminum positive electrode current collector above 3 V vs. Li⁺/Li can become performance-limiting in high-voltage Li-ion batteries. Herein, the results of a systematic reevaluation of this phenomenon at potentials up to 5.0 V vs. Li⁺/Li, using different carbonate electrolytes containing LiPF₆, LiFSI or LiTFSI, are presented. The anodic dissolution is most likely caused by etching of the Al₂O₃ passive layer by protons released during the oxidation of the solvent. This sparks off a second oxidation step, involving the oxidation of the aluminum. While a passive AlF₃ layer is formed in 1.0 M LiPF₆, extensive anodic dissolution of aluminum is seen in 1.0 M LiFSI or LiTFSI at potentials where the solvent undergoes oxidation. In 5.0 M LiFSI, a passive layer of AlF₃ is, however, formed most likely due to the presence of fluoride as an impurity in the LiFSI. No significant improvement was seen when using carbon-coated aluminum electrodes.

Energy storage devices such as rechargeable batteries and supercapacitors are of great importance for establishing clean energy sources. Accordingly, the production of these devices needs to rely on sustainable and environmentally friendly materials. This report provides an insight on the use of two-dimensional transition metal carbides (MXene) based electrodes, here shown for Mo1.33CTz-Ti3C2Tz mixed MXene, in Zn-ion hybrid supercapacitors (ZHSC) using aqueous and nonaqueous (acetonitrile-based) electrolytes. The effect of anion carriers on the accessible capacity, rate capability, and life span of the MXene//Zn hybrid supercapacitor is explored in-depth. Halide carriers such as chloride (Cl−) and iodide (I−) feature a superior performance, however, a fast passivation is observed in Cl− based electrolytes and a narrow potential window is achieved in I− based electrolytes. Importantly, a few micron layer of Ti3C2Tz MXene coated on the surface of the Zn anode is found to inhibit the side reactions and passivation observed in ZnCl2 solutions, which enables the use of such low-cost Zn salt in MXene//Ti3C2Tz-coated-Zn cells. The cells can be reversibly cycled over 10,000 cycles, delivering a capacity up to 200 mAh g−1 at low rate (0.5 mV s−1) and a capacity retention of about 36% at high rate (100 mV s−1). Furthermore, the Ti3C2Tz surface coating layer enhanced the coulombic efficiency in Zn(CF3SO3)2 electrolyte without affecting the accessible capacity or the rate capability. This work sheds light on the use of MXenes in sustainable low-cost ZHSC with high energy density and power density as a positive electrode material as well as a surface coating material for the Zn negative electrode.

The high theoretical capacitance of molybdenum trioxide (MoO₃) renders it an attractive supercapacitor electrode material. However, its low electronic conductivity restricts charge transfer and slows its reaction kinetics. Herein, we vacuum filtered porous, free-standing, flexible and highly conductive films comprised of oxygen vacancy-rich MoO_3-x nanobelts and delaminated Ti₃C₂ MXene in a mass ratio of 80:20, respectively. When tested as supercapacitor electrodes, in a 5 M LiCl electrolyte, volumetric capacitances of 631 F cm⁻³ at 1 A g⁻¹, and 474 F cm⁻³ at 10 A g⁻¹ were obtained. To increase the energy density, asymmetric supercapacitors, wherein the anodes were MoO₃-based and the cathodes were nitrogen-doped activated carbon were assembled and tested. The resulting volumetric energy density was 48.6 Wh L⁻¹. After 20,000 continuous charge/discharge cycles at 20 A g⁻¹, 96.3 % of the initial charge remained. These values are outstanding for free-standing supercapacitor electrodes, especially in aqueous electrolytes.

MXenes are two-dimensional (2D) transition metal carbides/nitrides with high potential for energy storage devices owing to their high flexibility, conductivity and specific capacitance. However, MXene films tend to suffer from diffusion limitation of ions within the film, and thus their thickness is commonly reduced to a few micrometers (mass loadings <1 mg cm⁻²). Herein, a straightforward one-step protocol for synthesizing freestanding Mo_1.33CT_z–cellulose composite electrodes with high MXene loading is reported. By varying the amount of the cellulose content, a high gravimetric capacitance (up to 440 F g⁻¹ for 45 wt% cellulose content, ∼5.9 μm thick film) and volumetric capacitance (up to 1178 F cm⁻³ for 5 wt% cellulose content, ∼4.8 μm thick film) is achieved. These capacitance values are superior to those for the pristine MXene film, of a similar MXene loading (1.56 mg cm⁻², ∼4.2 μm thick film), delivering values of about 272 F g⁻¹ and 1032 F cm⁻³. Interestingly, the Mo_1.33CT_z–cellulose composite electrodes display an outstanding capacitance retention (∼95%) after 30 000 cycles, which is better than those reported for other Mo_1.33CT_z-based electrodes. Furthermore, the presence of cellulose inside a thick composite electrode (∼26 μm, MXene loading 5.2 mg cm⁻²) offers a novel approach for opening the structure during electrochemical cycling, with resulting high areal capacitance of about 1.4 F cm⁻². A symmetric device of Mo_1.33CT_z–cellulose electrodes featured a long lifespan of about 35 000 cycles and delivered a device capacitance up to 95 F g⁻¹. The superior performance of the Mo_1.33CT_z–cellulose electrodes in terms of high gravimetric, volumetric, and areal capacitances, long lifespan, and promising rate capability, paves the way for their use in energy storage devices.

Enhancing both the energy storage and power capabilities of electrochemical capacitors remains a challenge. Herein, Ti₃C₂T_z MXene is mixed with MoO₃ nanobelts in various mass ratios and the mixture is used to vacuum filter binder free, open, flexible, and free-standing films. The conductive Ti₃C₂T_z flakes bridge the nanobelts, facilitating electron transfer; the randomly oriented, and interconnected, MoO₃ nanobelts, in turn, prevent the restacking of the Ti₃C₂T_z nanosheets. Benefitting from these advantages, a MoO₃/Ti₃C₂T_z film with a 8:2 mass ratio exhibits high gravimetric/volumetric capacities with good cyclability, namely, 837 C g⁻¹ and 1836 C cm⁻³ at 1 A g⁻¹ for an ≈ 10 µm thick film; and 767 C g⁻¹ and 1664 C cm⁻³ at 1 A g⁻¹ for ≈ 50 µm thick film. To further increase the energy density, hybrid capacitors are fabricated with MoO₃/Ti₃C₂T_z films as the negative electrodes and nitrogen-doped activated carbon as the positive electrodes. This device delivers maximum gravimetric/volumetric energy densities of 31.2 Wh kg⁻¹ and 39.2 Wh L⁻¹, respectively. The cycling stability of 94.2% retention ratio after 10 000 continuous charge/discharge cycles is also noteworthy. The high energy density achieved in this work can pave the way for practical applications of MXene-containing materials in energy storage devices.

MXenes are a class of 2D materials with outstanding properties, including high electronic conductivity, hydrophilicity, and high specific capacitance. In particular, Mo_1.33CT_z MXene has a high specific capacitance, whereas films of Ti₃C₂T_z MXene possess high flexibility and high electronic conductivity. The fabrication of composite materials based on these two MXenes is therefore motivated, taking advantage of combining their good properties. In this article, we introduce a one-step approach to prepare composite MXene films using pristine Mo_1.33CT_z and Ti₃C₂T_z MXenes. The composite films display superior flexibility and electronic conductivity, as well as high capacitance, up to 1380 F cm⁻³ (460 F g⁻¹), in 1 M H₂SO₄. A capacitance retention of 96% is obtained after 17,000 cycles. In addition, the capacitance retentions are about 56% and 25% at scan rates of 200 mV s⁻¹ and 1000 mV s⁻¹, respectively. A significant rise in the capacitance at high rates, 875 F cm⁻³ (282 F g⁻¹) at a current density of 20 A g⁻¹, is achieved by using a 3 M H₂SO₄ solution. The use of composite MXene as negative electrodes for asymmetric supercapacitor devices, as well as lithium-ion batteries, is also discussed. This work suggests new pathways for the use of MXene composites with double transition metals (Mo and Ti) in energy storage devices.

The high demand on fast rechargeable batteries and supercapacitors combined with the limited resources of their active materials (e.g. Li and Co) motivate the exploration of sustainable energy storage systems such as Zn-ion hybrid supercapacitors. MXenes are two-dimensional materials with outstanding properties such as high conductivity and capacitance which enhance their performance in energy storage devices. Herein, we report on the use of freestanding Mo_1.33CT_z–Ti₃C₂T_z mixed MXene films in Zn-ion hybrid supercapacitors. The mixed MXene films are prepared from pristine MXene suspensions using a one-step vacuum filtration approach. The mixed MXene delivers capacities of about 159 and 59 mAh/g at scan rates of 0.5 and 100 mV/s, respectively. These capacity values are higher than the pristine MXene films and previously reported values for MXene electrodes in Zn-ion supercapacitors. Furthermore, the electrodes offer a promising capacity retention of about 90% after 8,000 cycles. In addition, the mixed MXene features energy densities of about 103 and 38 Wh/kg at power densities of 0.143 and 10.6 kW/kg, respectively. Insights into the effect of electrode thickness on rate performance and the mechanism of charge storage are also discussed. This study opens a venue for the use of Mo_1.33CT_z–Ti₃C₂T_z mixed MXene electrodes in sustainable energy storage systems with high energy density and power density.

A vacancy-ordered MXene, Mo_1.33CT_z, obtained from the selective etching of Al and Sc from the parent i-MAX phase (Mo_2/3Sc_1/3)₂AlC has previously shown excellent properties for supercapacitor applications. Attempts to synthesize the same MXene from another precursor, (Mo_2/3Y_1/3)₂AlC, have not been able to match its forerunner. Herein, we show that the use of an AlY_2.3 alloy instead of elemental Al and Y for the synthesis of (Mo_2/3Y_1/3)₂AlC i-MAX, results in a close to 70% increase in sample purity due to the suppression of the main secondary phase, Mo₃Al₂C. Furthermore, through a modified etching procedure, we obtain a Mo_1.33CT_z MXene of high structural quality and improve the yield by a factor of 6 compared to our previous efforts. Free-standing films show high volumetric (1308 F cm⁻³) and gravimetric (436 F g⁻¹) capacitances and a high stability (98% retention) at the level of, or even beyond, those reported for the Mo_1.33CT_z MXene produced from the Sc-based i-MAX. These results are of importance for the realization of high quality MXenes through use of more abundant elements (Y vs. Sc), while also reducing waste (impurity) material and facilitating the synthesis of a high-performance material for applications.

Herein, Zn plating-stripping onto metallic Zn using a couple of acetonitrile (AN)-based electrolytes (0.5 mZn(TFSI)(2)/AN and 0.5 mZn(CF3SO3)(2)/AN) is studied. Both electrolytes show a reversible Zn plating/stripping over 1000 cycles at different applied current densities varying from 1.25 to 10 mA cm(-2). The overpotentials of Zn plating-stripping over 500 cycles at constant current of 1.25 and 10 mA cm(-2)are +/- 0.05 and +/- 0.2 V, respectively. X-ray photoelectron spectroscopy analysis reveals that no decomposition product is formed on the Zn surface. The anodic stability of four different current collectors of aluminum foil (Al), carbon-coated aluminum foil (C/Al), TiN-coated titanium foil (TiN/Ti), and multiwalled carbon nanotube paper (MWCNT-paper) is tested in both electrolytes. As a general trend, the current collectors have a higher anodic stability in Zn(TFSI)(2)/AN compared with Zn(CF3SO3)(2)/AN. The Al foil displays the highest anodic stability of approximate to 2.25 V versus Zn2+/Zn in Zn(TFSI)(2)/AN electrolyte. The TiN/Ti shows a comparable anodic stability with that of Al foil, but its anodic current density is higher than Al. The promising reversibility of the Zn plating/stripping combined with the anodic stability of Al and TiN/Ti current collectors paves the way for establishing highly reversible Zn-ion batteries.