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Structural bidimensional transition-metal carbides and/or nitrides (MXenes) have drawn the attention of the material science research community thanks to their unique physical-chemical properties. However, a facile and cost-effective synthesis of MXenes has not yet been reported. Here, using elemental precursors, we report a method for MXene synthesis via titanium aluminium carbide formation and subsequent in situ etching in one molten salt pot. The molten salts act as the reaction medium and prevent the oxidation of the reactants during the high-temperature synthesis process, thus enabling the synthesis of MXenes in an air environment without using inert gas protection. Cl-terminated Ti 3 C 2 T x and Ti 2 CT x MXenes are prepared using this one-pot synthetic method, where the in situ etching step at 700 °C requires only approximately 10 mins. Furthermore, when used as an active material for nonaqueous Li-ion storage in a half-cell configuration, the obtained Ti 2 CT x MXene exhibits lithiation capacity values of approximately 280 mAh g −1 and 160 mAh g −1 at specific currents of 0.1 A g −1 and 2 A g −1 , respectively. A facile and cost-effective synthesis of MXenes is not yet available. Here, the authors propose a one-pot molten salt-based method of MXenes synthesis from elemental precursors in an air atmosphere. Li-ion storage properties of the MXenes are also reported and discussed.

The energy industry has taken notice of zinc-iodine (Zn-I 2 ) batteries for their high safety, low cost, and attractive energy density. However, the shuttling of I 3 − by-products at cathode electrode and dendrite issues at Zn metal anode result in short cycle lifespan. Here, a tripartite synergistic optimization strategy is proposed, involving a MXene cathode host, a n - butanol electrolyte additive, and the in-situ solid electrolyte interface (SEI) protection. The MXene possesses catalytic ability to enhance the reaction kinetics and reduce I 3 − by-products. Meanwhile, the partially dissolved n - butanol additive can work synergistically with MXene to inhibit the shuttling of I 3 − . Besides, the n - butanol and I − in the electrolyte can synergistically improve the solvation structure of Zn 2+ . Moreover, an organic-inorganic hybrid SEI is in situ generated on the surface of the Zn anode, which induces stable non-dendritic zinc deposition. As a result, the fabricated batteries exhibit a high capacity of 0.30 mAh cm −2 and a superior energy density of 0.34 mWh cm −2 at a high specific current of 5 A g −1 across 30,000 cycles, with a minimal capacity decay of 0.0004% per cycle. This work offers a promising strategy for the subsequent research to comprehensively improve battery performance. Here, authors propose a tripartite synergistic optimization strategy involving cathode host, electrolyte additive, and in-situ anode protection, which enables the zinc-iodine batteries exhibit high capacity, superior energy density, and ultralong cycle life.

Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of materials that have attracted attention as energy storage materials. MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution; most other MAX phases have not been explored. Here a redox-controlled A-site etching of MAX phases in Lewis acidic melts is proposed and validated by the synthesis of various MXenes from unconventional MAX-phase precursors with A elements Si, Zn and Ga. A negative electrode of Ti 3 C 2 MXene material obtained through this molten salt synthesis method delivers a Li + storage capacity of up to 738 C g −1 (205 mAh g −1 ) with high charge–discharge rate and a pseudocapacitive-like electrochemical signature in 1 M LiPF 6 carbonate-based electrolyte. MXenes prepared via this molten salt synthesis route may prove suitable for use as high-rate negative-electrode materials for electrochemical energy storage applications. Two-dimensional transition metal carbides and nitrides, known as MXenes, are currently considered as energy storage materials. A generic Lewis acidic etching route for preparing high-rate negative-electrode MXenes with enhanced electrochemical performance in non-aqueous electrolyte is now proposed.

Pseudocapacitive energy storage in supercapacitor electrodes differs significantly from the electrical double-layer mechanism of porous carbon materials, which requires a change from conventional thinking when choosing appropriate electrolytes. Here we show how simply changing the solvent of an electrolyte system can drastically influence the pseudocapacitive charge storage of the two-dimensional titanium carbide, Ti 3 C 2 (a representative member of the MXene family). Measurements of the charge stored by Ti 3 C 2 in lithium-containing electrolytes with nitrile-, carbonate- and sulfoxide-based solvents show that the use of a carbonate solvent doubles the charge stored by Ti 3 C 2 when compared with the other solvent systems. We find that the chemical nature of the electrolyte solvent has a profound effect on the arrangement of molecules/ions in Ti 3 C 2 , which correlates directly to the total charge being stored. Having nearly completely desolvated lithium ions in Ti 3 C 2 for the carbonate-based electrolyte leads to high volumetric capacitance at high charge–discharge rates, demonstrating the importance of considering all aspects of an electrochemical system during development. Effects from electrolytes on supercapacitor electrodes, especially pseudocapacitive materials, are important but often overlooked. Gogotsi and colleagues demonstrate strong influences from electrolyte solvents on charge-storage processes in a titanium carbide and identify a best-performing electrode/electrolyte couple for supercapacitors.

The use of fast surface redox storage (pseudocapacitive) mechanisms can enable devices that store much more energy than electrical double-layer capacitors (EDLCs) and, unlike batteries, can do so quite rapidly. Yet, few pseudocapacitive transition metal oxides can provide a high power capability due to their low intrinsic electronic and ionic conductivity. Here we demonstrate that two-dimensional transition metal carbides (MXenes) can operate at rates exceeding those of conventional EDLCs, but still provide higher volumetric and areal capacitance than carbon, electrically conducting polymers or transition metal oxides. We applied two distinct designs for MXene electrode architectures with improved ion accessibility to redox-active sites. A macroporous Ti 3 C 2 T x MXene film delivered up to 210 F g −1 at scan rates of 10 V s −1 , surpassing the best carbon supercapacitors known. In contrast, we show that MXene hydrogels are able to deliver volumetric capacitance of ∼1,500 F cm −3 reaching the previously unmatched volumetric performance of RuO 2 . Pseudocapacitors based on redox-active materials have relatively high energy density but suffer from low power capability. Here the authors report that two-dimensional transition metal carbides exhibit high gravimetric, volumetric and areal capacitance values at high charge/discharge rates.

Clarifying the relationship between ion desolvation, ion-electrode interactions, and charge storage capacity during ion intercalation in host electrode materials is crucial for advancing fast and efficient energy storage systems. However, the absence of direct evidence for ion desolvation and lack of detailed understanding of the interactions between surface terminations and intercalated cations (Li ions)/solvents hinder the exploration of their effects on energy storage mechanisms. In this paper, we study the intercalation of Li ions from a non-aqueous electrolyte in two-dimensional metal carbides Ti 3 C 2 MXenes with different surface chemistries: HF-Ti 3 C 2 (F-, OH- and O-terminated) and MS-Ti 3 C 2 (O- and Cl-terminated) MXenes. We are able to visualize the full ion desolvation and solvents-ions co-intercalation in the interlayers of MS-MXene and HF-MXene, respectively at the atomic scale. The combination of several techniques and characterization tools reveal that the complete ion desolvation in Cl- and O-terminated MS-Ti 3 C 2 MXenes is associated with the formation of a dense solid electrolyte interface layer, resulting in improved charge storage capacity. The O-rich surface terminations of MS-MXenes are found to be responsible for the efficient Li ions storage. These findings shed lights on identifying the critical role of non-electrostatic ion-electrode interactions and ion desolvation in designing high-performance energy storage devices. Understanding the charge storage mechanism of electrode materials is critical for designing electrochemical energy storage devices. Here, authors study Li + intercalation into two-dimensional metal carbides in nonaqueous electrolytes, revealing distinct behavior for different surface terminations.

Rechargeable aqueous batteries show promise for large-scale energy storage, yet suffer from low specific energy and poor low-temperature performance. Existing low-temperature aqueous batteries typically use ion-insertion positive electrodes with limited capacity. While aqueous sulfur-based batteries offer high theoretical capacity, their low-temperature operation remains challenging. Current improvement strategies often compromise room-temperature performance due to the inclusion of non-active additives. Here, we present a low-temperature sulfur-based battery using a Cu(BF 4 ) 2 -based electrolyte, which boasts a low glass transition temperature of −115.1 °C, and an ionic conductivity of 5.16 mS cm −1 at −60 °C. This electrolyte enables faster reaction kinetics and higher overall specific energy than traditional CuSO 4 -based systems. The resulting zinc-sulfur battery delivers a discharge capacity of 348 mA h g (S+Zn) −1 and an specific energy of 339 W h kg (S+Zn) −1 at −50 °C, based on the total mass of both the positive and negative electrodes, competitive with existing aqueous batteries. Aqueous batteries suffer from poor performance in extreme cold. Here, authors design an electrolyte enabling a high-energy zinc-sulfur battery that operates at –50 °C, offering a promising solution for low-temperature energy storage.

Highlights A novel and efficient Te etching method for the preparation of Te-functionalized MXene materials is presented This simple etching method enables the processing of V- and Nb-based MAX phases and demonstrates potential for large-scale production. V 2 CTe x MXene has a sodium storage capacity of up to 247 mAh g −1 and maintains 216 mAh g −1 at 23 C. With the rapid development of two-dimensional MXene materials, numerous preparation strategies have been proposed to enhance synthesis efficiency, mitigate environmental impact, and enable scalability for large-scale production. The compound etching approach, which relies on cationic oxidation of the A element of MAX phase precursors while anions typically adsorb onto MXene surfaces as functional groups, remains the main prevalent strategy. By contrast, synthesis methodologies utilizing elemental etching agents have been rarely reported. Here, we report a new elemental tellurium (Te)-based etching strategy for the preparation of MXene materials with tunable surface chemistry. By selectively removing the A-site element in MAX phases using Te, our approach avoids the use of toxic fluoride reagents and achieves tellurium-terminated surface groups that significantly enhance sodium storage performance. Experimental results show that Te-etched MXene delivers substantially higher capacities (exceeding 50% improvement over conventionally etched MXene) with superior rate capability, retaining high capacity at large current densities and demonstrating over 90% capacity retention after 1000 cycles. This innovative synthetic strategy provides new insight into controllable MXene preparation and performance optimization, while the as-obtained materials hold promises for high-performance sodium-ion batteries and other energy storage systems.

HighlightsSmall size V2SnC MAX phase was prepared by the molten salt method.V2SnC MAX phase electrode is able to deliver high gravimetric capacity up to 490 mAh g−1 and volumetric capacity of 570 mAh cm−3A charge storage mechanism with V2C-Li redox and Sn–Li alloying dual reactions was proposedMAX phases are gaining attention as precursors of two-dimensional MXenes that are intensively pursued in applications for electrochemical energy storage. Here, we report the preparation of V2SnC MAX phase by the molten salt method. V2SnC is investigated as a lithium storage anode, showing a high gravimetric capacity of 490 mAh g−1 and volumetric capacity of 570 mAh cm−3 as well as superior rate performance of 95 mAh g−1 (110 mAh cm−3) at 50 C, surpassing the ever-reported performance of MAX phase anodes. Supported by operando X-ray diffraction and density functional theory, a charge storage mechanism with dual redox reaction is proposed with a Sn–Li (de)alloying reaction that occurs at the edge sites of V2SnC particles where Sn atoms are exposed to the electrolyte followed by a redox reaction that occurs at V2C layers with Li. This study offers promise of using MAX phases with M-site and A-site elements that are redox active as high-rate lithium storage materials.

Highlights Decipher the multi-scale causes of interfacial instability in aqueous electrolyte systems via numerical simulations. Develop an electrolyte-triggered interphase construction strategy to achieve synergistic regulation of both the anode and cathode. Achieve high Coulombic efficiency (99.5%) and long-term cycling stability (over 6000 h) at ultra-low current density (0.1 mA cm −2 ) in zinc metal batteries. The advancement of aqueous zinc metal batteries (ZMBs) is constrained by intrinsic interfacial issues in aqueous electrolyte systems. Here, using numerical simulation, we decipher the multi-scale causes of interfacial instability, elucidating the synergistic effect of macroscopic ineffective regions and microscopic passivation. Based on the analysis, we develop an electrolyte-triggered interphase construction strategy to resolve the interfacial failure. This strategy couples the in situ formation of hydrogel interphase on both the anode and cathode with the electrolyte filling process, thereby (1) facilitating contact between electrodes and the separator; (2) promoting anode reversibility through inducing a bilayer SEI that enhances Zn 2+ desolvation kinetics and blocks electron tunneling; (3) ensuring long-term cathode cycling stability via restricting the irreversible dissolution of MnO 2 and side-reactions. The resultant Zn metal anode exhibited a near-unity Coulombic efficiency (99.5%) for Zn plating/stripping at an extremely low current density of 0.1 mA cm −2 and the Zn/MnO 2 full cell sustained 2000 full-duty-cycles with an exceptionally low decay rate of 0.0051% per-cycle. This work unlocks an alternative angle for promoting practical ZMBs toward more sustainable energy storage systems.