MXene-based Electrodes Research Articles

The Emergence of 2D MXenes Based Zn-Ion Batteries: Recent Development and Prospects.

Rechargeable zinc-ion batteries (ZIBs) with exceptional theoretical capacity have garnered significant interest in large-scale electrochemical energy storage devices due to their low cost, abundant material, inherent safety, high specific energy, and ecofriendly nature. Metal carbides/nitrides, known as MXenes, have emerged as a large family of 2D transition metal carbides or carbonitrides with excellent properties, e.g., high electrical conductivity, large surface functional groups (e.g., F, O, and OH), low energy barriers for the diffusion of electrolyte ions with wide interlayer spaces. After a decade of effort, significant development has been achieved in the synthesis, properties, and applications of MXenes. Thus, it has opened up various exciting opportunities to construct advanced MXene-based nanostructures for ZIBs with excellent specific energy and power. Herein, this review summarizes the advances across multiple synthesis routes, related properties, morphological and structural characteristics, and chemistries of MXenes for ZIBs. The recent development of MXene-based electrodes is introduced, and electrolytes for ZIBs are elucidated in detail. MXene-based rocking chair ZIBs, strategies to enhance the performance of MXene-based cathodes, suppress the dendrites in MXene-based anodes, and MXene-based flexible ZIBs are pointed out. A rational design and modification of the MXenes as well as the production of composites with metal oxides exhibits promise in solving issues and enhancing the electrochemical performance of ZIBs. Finally, the present challenges and future prospects for MXene-based ZIBs are discussed.

Self-Supporting, Binder-Free, and Flexible Ti3C2Tx MXene-Based Supercapacitor Electrode with Improved Electrochemical Performance.

MXenes have shown great potential for supercapacitor electrodes due to their unique characteristics, but simultaneously achieving high capacitance, rate capability, and cyclic stability along with good mechanical flexibility is exceptionally challenging. Here, highly enhanced capacitance, rate capability, and cyclic stability, as well as good mechanical flexibility for T3C2Tx MXene-based supercapacitor electrodes are simultaneously obtained by engineering the electrode structure, modifying the surface chemistry, and optimizing the fabrication process via an optimized integration approach. This approach combines and more importantly optimizes three methods that all require a calcination process: carbonizing in situ grown polymer ("Cpolymer") on the MXene, alkali treatment ("A"), and template sacrificing ("P"); and the optimized processes lead to more abundant active sites, faster ion accessibility, better chemical stability, and good mechanical flexibility. The obtained P-MXene/Cpolymer-A electrodes are binder-free and self-supporting and not only have good mechanical flexibility but also demonstrate much larger capacitances and better rate performance than the pristine MXene electrode. Specifically, the P-MXene/CPAQ-A electrode (PAQ: quinone-amine polymer) achieves a high capacitance of 532.9 F g-1 at 5 mV s-1, together with superior rate performance and improved cyclic stability (97.1% capacitance retention after 40 000 cycles at 20 A g-1) compared with the pristine MXene (79.6% retention) and P-MXene-A (77.3% retention) electrodes. In addition, it is discovered that carbonizing in situ grown polymers can variously remove the -F group and the removal effect can be accumulated with that by the alkali treatment.

Mixed-Dimensional MXene-Based Composite Electrodes Enable Mechanically Stable and Efficient Flexible Perovskite Light-Emitting Diodes.

Significant advancements in perovskite light-emitting diodes (PeLEDs) based on ITO glass substrates have been realized in recent years, yet the overall performance of flexible devices still lags far behind, mainly being ascribed to the high surface roughness and poor optoelectronic properties of flexible electrodes. Here, we report efficient and robust flexible PeLEDs based on a mixed-dimensional (0D-1D-2D-3D) composite electrode consisting of 0D Ag nanoparticles (AgNPs)/1D Ag nanowires (AgNWs)/2D MXene/3D poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Our designed MXene-based electrodes combine the advantages of facile formation of a film of low-dimensional materials and excellent optical and electrical properties of metal, inorganic, and organic semiconductors, which endow the electrodes with high electrical/thermal conductivity, flexibility, a smooth surface, and good transmittance. Consequently, the resulting flexible PeLEDs (without a light-coupling structure) demonstrate a record external quantum efficiency of 16.5%, a high luminance of close to 50000 cd/m2, a large emitting area of 8 cm2, and significantly enhanced mechanical stability.

MXene-based film electrode and all-round hydrogel electrolyte for flexible all-solid supercapacitor with extremely low working temperature

The fabrication of a flexible all-solid supercapacitor able to work under various deformations, even under extreme conditions, remains challenging because of the rigidity of electrodes, the freezing of gel electrolytes, and interfacial contact problems. Here, we report a flexible supercapacitor with excellent mechanical deformation and ultra-low temperature tolerance assembled by using the designed MXene/carboxymethyl cellulose film electrode and all-round polyvinyl alcohol/LiCl (PVA/LiCl) hydrogel electrolyte. The supercapacitor combines mechanically flexible (12.7 MPa at 5.9% strain) and highly conductive (267 S/cm) electrodes with the PVA/LiCl hydrogel electrolyte with high ion conductivity, excellent mechanical properties, self-adhesion, and antifreezing ability. As a result, the assembled supercapacitor exhibits a high specific capacitance (113.13 mF cm −2 ) with a retention of ∼95% under mechanical deformations, and more importantly, the electrochemical stability can be maintained even at −40°C when subjected to severe deformations. This work offers an option to design flexible supercapacitors as environment-adaptable energy-storage devices. • The all-round PVA/LiCl hydrogel is prepared by regulating interactions among PVA and H 2 O • The MXene/CMC film electrode shows mechanical softness and high conductivity • The assembled supercapacitor has excellent electrochemical properties, even at −40°C The fabrication of flexible and antifreezing all-solid supercapacitors still remains a challenge. Yin et al. design a flexible supercapacitor with excellent mechanical deformation and ultra-low temperature tolerance by assembling MXene/CMC film electrodes and all-round PVA/LiCl hydrogel electrolyte, which exhibits excellent electrochemical performances under various deformations, even at −40°C.

MXene-based composite electrodes for efficient electrochemical sensing of glucose by non-enzymatic method

A recently discovered 2D transition titanium metal carbides also called as MXenes (Ti3C2Tx)-based nanocomposite was prepared with Cu2O through wet precipitation technique, and these materials were further developed as the electrode for sensing glucose by chronoamperometry technique. The prepared MXene-Cu2O (Ti3C2Tx-Cu2O) nanocomposite was characterized by XRD, FTIR, UV–Vis spectroscopy, FE-SEM, EDAX, and Raman spectroscopy. Morphological studies of the composites revealed that the micro-octahedral shape of Cu2O is distributed on the surface of MXene with size larger than bare Cu2O. Further, the prepared composite material was fabricated as a sensing probe, and the electrochemical activities were examined by cyclic voltammetric analysis (CV) and chronoamperometric (CA) methods. From the CV and CA investigation, the current response was higher for the composite than the bare material (Cu2O & MXene) in the presence of glucose. The amperometric investigation of MXene-Cu2O composite for the detection of glucose shows a broad linear range (0.01–30 mM) with a sensitivity of 11.061/μAmM cm−2 and a detection limit of 2.83 μM. Further, the fabricated sensor exhibits good selectivity with interfering species like NaCl, fructose, sucrose, urea, ascorbic acid, lactose, short response time, stability, good reproducibility, and compatibility with human serum sample. From the investigation, the prepared MXene-Cu2O composite is a good candidate for the direct detection of glucose molecules and is also well suitable for clinical diagnosis.

Sandwich-like high-load MXene/polyaniline film electrodes with ultrahigh volumetric capacitance for flexible supercapacitors

Two-dimensional transition metal carbides/nitrides (MXene) have excellent physicochemical properties, but the restacking of MXene films restricts their development towards flexible supercapacitors with high energy density. Introducing Polyaniline (PANI) into the MXene layer and expanding the interlayer distance of the MXene can reduce the effect of restacking on the MXene after compounding. However, despite the excellent electronic conductivity of the composited MXene/PANI (MP), its internal slow ionic kinetics becomes a fundamental limitation of the electrochemical performance after the MP loading increases. To compensate for this weakness, MP films are often scaled down to a few micrometers in size (<2 mg cm−2), which limits their development. Here, we introduce α-Fe2O3/MnO2 (FM) into MP for the first time by designing a sandwich structure, which significantly improves the bulk capacitance. Due to a large number of active sites and good hydrophilic properties on the MXene surface, FM can interact with the MP. By complexing with MP, the accumulation and loss of FM can be reduced. At the same time, the effect of increasing loading on the electrochemical performance of MP can be compensated. The MXene-PANI/α-Fe2O3-MnO2/MXene-PANI (MP/FM/MP) electrode still exhibits high capacitive performance (661 F g−1, 3138 mF cm−3) when the MP loading reaches 5 mg cm−2, with excellent mechanical properties and increased flexibility. In addition, the corresponding symmetric supercapacitor also shows a remarkable energy density of 53.32 Wh·L−1 (17.45 Wh kg−1). This study provides a way to fabricate MXene-based electrodes with high loadings by designing sandwich-structured electrodes.

Scaled Deposition of Ti3C2Tx MXene on Complex Surfaces: Application Assessment as Rear Electrodes for Silicon Heterojunction Solar Cells.

Two-dimensional transition metal carbides (MXenes) are of great interest as electrode materials for a variety of applications, including solar cells, due to their tunable optoelectronic properties, high metallic conductivity, and attractive solution processability. However, thus far, MXene electrodes have only been exploited for lab-scale device applications. Here, to demonstrate the potential of MXene electrodes at an industry-relevant level, we implemented a scalable spray coating technique to deposit highly conductive (ca. 8000 S/cm, at a ca. 55 nm thickness) Ti3C2Tx films (Tx: surface functional groups, i.e., −OH, −O, −F) via an automated spray system. We employed these Ti3C2Tx films as rear electrodes for silicon heterojunction solar cells as a proof of concept. The spray-deposited MXene flakes have formed a conformal coating on top of the indium tin oxide (ITO)-coated random pyramidal textured silicon wafers, leading to >20% power conversion efficiency (PCE) over both medium-sized (4.2 cm2) and large (243 cm2, i.e., industry-sized 6 in. pseudosquare wafers) cell areas. Notably, the Ti3C2Tx-rear-contacted devices have retained around 99% of their initial PCE for more than 600 days of ambient air storage. Their performance is comparable with state-of-the-art solar cells contacted with sputtered silver electrodes. Our findings demonstrate the high-throughput potential of spray-coated MXene-based electrodes for solar cells in addition to a wider variety of electronic device applications.

Ultrahigh gravimetric and volumetric capacitance in Ti3C2Tx MXene negative electrode enabled by surface modification and in-situ intercalation

Two-dimensional titanium carbide (MXene) has attracted significant attentions in supercapacitors owing to its high metallic conductivity and excellent chemical properties. However, the inherent restacking issue and –F surface termination severely limit the energy storage. Here, an effective strategy to construct high-performance Ti3C2Tx MXene (A-Ti3C2Tx/PANI) based on the synergistic effect of modified surface termination, expanded interlayer spacing and three-dimensional structure is reported. Benefiting from the interconnected structure, enhanced electrochemical activity and efficient ion/electron transport layer, the prepared negative electrode demonstrates an outstanding specific capacitance of 652.3 F g−1 at 1 A g−1 (∼3 times of the pristine MXene), an impressive rate capability with 81% capacitance retention at 50 A g−1, and excellent cycling stability with above 99% capacitance retention after 10, 000 cycles. In addition, a symmetric supercapacitor is fabricated using A-Ti3C2Tx/PANI, and it delivers a high energy density of 20.3 Wh kg−1. Furthermore, the prepared free-stranding film electrode based on A-Ti3C2Tx/PANI shows a high volumetric capacitance of 2368 F cm−3. The improved gravimetric and volumetric capacitances surpass almost all the reported MXene-based electrodes. This study demonstrates a new strategy to enhance the electrochemical performance of Ti3C2Tx electrode and could be a universal way applicable to other MXenes.

Making MXenes more energetic in aqueous battery

MXenes, featuring outstanding electronic conductivity, abundant surface functional groups, tunable interlayer structure, and interfacial chemistry, have aroused increasing attention in safe aqueous batteries (ABs). However, current MXene-based ABs suffer from the issues of inadequate energy density due to their low specific capacity and limited output voltage, which hinders the wide applications of MXenes. Herein, we comprehensively appraise the four charge storage mechanisms in constructing high-capacity/voltage MXene-based electrodes and the state-of-the-art strategies toward energetic ABs. Finally, we render the insights properly for the future development of next-generation MXene-based ABs. MXenes, featuring outstanding electronic conductivity, abundant surface functional groups, tunable interlayer structure, and interfacial chemistry, have aroused increasing attention in safe aqueous batteries (ABs). However, current MXene-based ABs suffer from the issues of inadequate energy density due to their low specific capacity and limited output voltage, which hinders the wide applications of MXenes. Herein, we comprehensively appraise the four charge storage mechanisms in constructing high-capacity/voltage MXene-based electrodes and the state-of-the-art strategies toward energetic ABs. Finally, we render the insights properly for the future development of next-generation MXene-based ABs. Intrinsic voltage plateau of a Nb2CTx MXene cathode in an aqueous electrolyte induced by high-voltage scanningLi et al.JouleOctober 11, 2021In BriefIn the past decade, two-dimensional MXenes have been a rising star in the energy storage field, featured by fast surficial redox and high-rate energy storage. However, typically, MXene electrodes exhibit capacitive behavior with a rapidly descending output voltage during discharge. Moreover, the absence of distinct voltage plateaus deteriorates their capacities and energy densities. Our work outlines an efficient route toward achieving high-energy-density MXene electrodes with distinct discharge voltage plateau through a high-voltage-scanning approach. Full-Text PDF

Headway towards contemporary 2D MXene-based hybrid electrodes for alkali-ion batteries

This review provides a comparative study of contemporary MXene-based hybrid electrode materials in different alkali ion batteries.

Enhancing the ion accessibility of Ti3C2Tx MXene films by femtosecond laser ablation towards high-rate supercapacitors

MXenes have attracted tremendous attention in the area of electrochemical energy-storage devices owing to their high electrical conductivity, pseudocapacitance and two-dimensional lamellar structure. Nevertheless, MXene flakes intrinsically tend to lie flat and restack, resulting in highly tortuous ion transport pathways and inferior ion accessibility. Herein, we develop a femtosecond laser ablation strategy to fabricate flexible and high-performance MXene ribbon supercapacitor electrodes. The fabricated MXene ribbons have porous edges with exposed continuous lamellar channels, which shortens the H+ transport pathways, impregnates with sufficient electrolyte, benefits for H+ intercalation and ion storage. The resultant MXene ribbons exhibit high specific capacitance (1308.3 mF/cm2), good rate capability and long cycling life (95% capacitance retention and 92% coulombic efficiency after 30,000 cycles). This work provides a new strategy for the rational structure design of high-rate MXene-based supercapacitor electrodes and lays the foundation for the next generation high-performance energy storage devices.

Promoting Electrochemical Performance of Ti3C2O2 MXene-Based Electrodes of Alkali-Ion Batteries via S Doping: Theoretical Insight.

Ti3C2O2 MXene has been proposed as a promising electrode material for alkali-ion batteries owing to its tunable physical and chemical properties without sacrificing the excellent metallic conductivity. However, it still suffers from low specific capacity due to its limited interlayer spacing, especially for a larger ion like sodium (Na). Sulfur doping was suggested as a viable strategy to improve the electrode's storage performance. Herein, first-principles calculations and kinetic Monte Carlo (kMC) simulations were carried out to study the role of S doping on Li/Na intercalation. Based on experimental findings, two different doping sites, C (SC) and O (SO), with various S concentrations were reported and therefore used as the models in this study. Computations reveal that S doping on both C and O sites improves the electronic conductivity of the MXenes as their densities of states at the Fermi level are increased. In addition, the doped MXenes reveal an expanded lattice parameter in the normal direction, which agrees with experimental observations. However, only the SO-doped MXenes display an enlarged interlayer spacing, whereas doping at the C site only increases the layer thickness. The enlarged interlayer spacing in the SO-doped MXenes improves stabilities and transport kinetics of ion intercalation as indicated by their significantly lower insertion energies and diffusion barriers when compared with those of the pristine system. The kMC simulations were carried out to account for anisotropic diffusion in the SO-doped system. The obtained macroscopic properties of diffusion coefficients and apparent activation energies of the SO-doped system clearly confirm its superior transport kinetics. The estimated diffusion coefficients of Li(Na) are improved by 4(8) orders of magnitude upon SO doping. A fundamental understanding of the role of S doping on the improved capacitive kinetics serves as a good guide for developing MXene-based electrode materials for Li- and Na-ion batteries.

Capacitive Behavior Study of Free Standing Titanium Carbide (Mxene) Film Under External Pressure

Titanium carbide (Ti3C2Tx, also called Mxene), uniquely combining metallic conductivity with hydrophilicity, has attracted burgeoning attention in batteries and supercapacitors application since reported in 2013. No matter with aqueous electrolytes or organic electrolytes, cation intercalation and pseudocapacitance contributes to the majority of the energy stored in Mxene-based electrochemical capacitors1-3, therefore, it is critical to understand its charge transport behavior within those negatively charged and electrically conductive Ti3C2X layers, especially the ionic diffusion process during charging/discharging.Pressure can cause peculiar and interesting structural changes in Ti3C2X Mxene as compared to other 2D materials such as graphene. Barsoum, et al. 6 have reported the phenomenon of interlayer expansion of Ti3C2X Mxene with external pressure up to 300MPa applied on in the presence of water, and they attribute the expansion to the shear-induced insertion of water molecules. To study its capacitive behavior under pressure will be of significant impact, because as a film of mechanical durability and flexibility, Ti3C2X free-standing films hold great potential in the field of wearable and portable electronics4,5, while in which, structure deformation caused by external pressure or temperature induced internal pressure is inevitable.In this work, the electrochemical performance under pressure of Ti3C2X Mxene free-standing film in various aqueous electrolytes (such as LiOH(aq), NaOH(aq), KOH(aq)) has been systematically investigated. Interestingly, different from other reported carbon-based electrode materials, Ti3C2X Mxene does not simply reach a capacitance saturation state as pressure increases. Further analysis on the structural changes and ionic diffusion under pressure suggests that complex mechanism occurs when charging/discharging under pressure. These findings should contribute to a more comprehensive understanding of the charge transport process within Mxene, and to future work of designing Mxene-based flexible electrodes for more advanced performance.Reference[1] M.R. Lukatskaya, O. Mashtalir, C.E. Ren, Y. Dall’Agnese, P. Rozier, P.L. Taberna, M. Naguib, P. Simon, M.W. Barsoum, Y. Gogotsi, Science, 2013, 341(6153), 1502-1505.[2] X. Wang, X. Chen, Y. Gao, Z. Wang, R. Yu, L. Chen, J. Am. Chem. Soc., 2015, 137, 7, 2715-2721.[3] H. Shao, Z. Lin, K. Xu, P.-L. Taberna, P. Simon, Energy Storage Materials, 2019, 18, 456-461.[4] J. Yan, C.E. Ren, K. Maleski, C.B. Hatter, B. Anasori, P. Urbankowski, A. Sarycheva, Y. Gogotsi, Adv. Funct. Mater., 2017, 27, 1701264.[5] C. Yang, Y. Tang, Y. Tian, Y. Luo, Y. He, X. Yin, W. Que, Adv. Funct. Mater., 2018, 1705487.[6] M. Ghidiu, S. Kota, V. Drozd, M.W. Barsoum, Sci. Adv., 2018, 4, 6850.

Self-assembled Cobalt-doped NiMn-layered double hydroxide (LDH)/V2CTx MXene hybrids for advanced aqueous electrochemical energy storage properties

High-performance aqueous electrochemical energy storage technology has attracted extensive research interest due to its high safety and potential for commercialization. Herein, following the simultaneous doping-electrostatic synergistic assembly strategy, we synthesized the 2D/2D Co-doped NiMn-Layered double hydroxide (LDH)/V2CTx MXene (CNMV) composite materials that serve as advanced electrodes for aqueous energy storage devices, such as supercapacitors and zinc ion batteries (ZIBs). For supercapacitors, benefiting from the hetero ion doping effect, the as-assembled CNMV electrode exhibits a higher specific capacitance of 1005 F g−1 at a current density of 1 A g−1. For the assembled asymmetric supercapacitor (ASC) devices, a high energy density of 30.16 Wh kg−1 at 0.7 kW kg−1 was achieved. Acting as the cathode for ZIBs, CNMV exhibits a high reversible capacity of 322.7 mA h g−1 at 0.2 A g−1 after 100 cycles, and a considerable energy density of 368.7 W h kg−1 at a power density of 246 W kg−1. The battery-type transformation tothe pseudocapacitance-type CNMV electrode for the supercapacitor was demonstrated by quantitative kinetic analysis, while phase transformation and Zn2+ insertion/extraction for zinc ion storage mechanism were demonstrated by ex-situ XRD and XPS. This study provides a facile and effective approach for the design of high-performance MXene-based electrodes for aqueous electrochemical energy storage devices.

MXene for aqueous zinc-based energy storage devices

MXene, a class of 2D transition metal carbide/nitride materials, has attracted widespread attention since its first discovery in 2011. Due to its high electronic conductivity, large specific surface area, good mechanical stability, and adjustable surface functional groups, MXene-based nanomaterials have shown great potential in energy storage devices. Meanwhile, zinc-based aqueous energy storage devices became a hotspot recently in energy storage field on account of their high security and low cost. In this review, the research progress on the preparation routes, preserving method, related structure and properties of MXene is first summarized. Followed by is an introduction of the recent state-of-the-art development of MXene-based electrodes for zinc-based aqueous energy storage devices, including zinc ion batteries (ZIBs), zinc-air batteries (ZABs), and zinc-halide batteries (ZHBs). Finally, the major bottleneck and perspectives for MXene-based nanomaterials in zinc-based aqueous energy storage devices are pointed out.

Oxidized-co-crumpled multiscale porous architectures of MXene for high performance supercapacitors

MXene, an example of two-dimensional (2D) transition metal carbides, is a promising electrode material for batteries and supercapacitors because of its excellent electrochemical properties derived from transition metal carbide/nitride constituents and its high electrical conductivity. However, similar to other 2D nanomaterials, MXene-based electrodes have a notable shortcoming of reduced specific surface area and reduced ion diffusion due to the self-restacking of MXene nanosheets. Here, to overcome these limitations, we present a novel method to create oxidized-co-crumpled multiscale porous architectures of MXene by a sequential process consisting of partial oxidation by hydrogen peroxide and then flocculation in acid. These treatments result in simultaneous generation of mesopores by oxidation and macropores caused by structural crumpling. This oxidized-co-crumpled MXene has a five times larger specific surface area (72 m2/g) than pristine MXene. In addition, the capacitance performance (307 F/g at 20 mV/s under 1 M H2SO4) and the capacitance at a higher scan rate (225 F/g at 100 mV/s) are improved. Additionally, after 6000 cycles, 87.6% of the capacitance is retained. Such an improvement in the properties is attributed to the structural advantages of the multiscale porous oxidized-co-crumpled MXene, wherein ion diffusion through mesopores is facilitated, while the crumpled macropores prevent re-stacking. Overall, this study demonstrates a promising strategy to create MXene-based high performance electrode materials for a variety of electrochemical applications.

MXene-based suspension electrode with improved energy density for electrochemical flow capacitors

The development of high capacitance materials with high packing density and low viscosity in suspension electrodes is critical for progressing towards high-efficiency, low-footprint electrochemical flow capacitors (EFCs). Here, we report on the first electrochemical and rheological characterization of MXene-based suspension electrodes, using multilayer Ti3C2Tx as the active material and carbon black (CB) as the conductive additive in symmetric and asymmetric EFC devices. In the case of symmetric Ti3C2Tx devices, the Ti3C2Tx concentration is fixed to 22 vol.% in the slurry and the CB concentration is varied from 0.5 to 2.0 vol.%. The symmetric device arrangement offers a high capacitance of 240 F ml−1 (2 mV s−1) and volumetric energy density of 2.65 Wh l−1 @ power density of 47.82 W l−1. Additionally, to extend the potential window, an asymmetric device assembly of activated carbon and Ti3C2Tx is investigated. This arrangement allows a stable operating potential window of 1 V with an energy density of 4.12 Wh l−1 and power density of 31.73 W l−1. Overall, multilayer Ti3C2Tx seems to be excellent candidate for flowable electrode applications, offering high capacitance, energy density and low viscosity due to its high electrochemical activity, excellent electrical conductivity, and versatile surface chemistry.

Ti3C2-MXene composite films functionalized with polypyrrole and ionic liquid-based microemulsion particles for supercapacitor applications

Stemming from its unique structure and properties, Ti3C2-MXenes have stood out from the crowd of electrode materials due to effectively ameliorating imperfections of common ones for excellent energy storage. However, a major stumbling block in the further development of MXene-based supercapacitors is the inherent low capacities caused by severe restacking of nanosheets. Herein, we develop a simple, effective and innovative strategy, namely, introducing both polymerized polypyrrole (PPy) particles and ionic liquid (ILs)-based microemulsion particles as “dual spacers”, to fabricate functionalized Ti3C2-MXene composite films for high-performance and wide-temperature application in supercapacitors. The PPy particles acting as one “spacer” circumvent restacking issues of MXene nanosheets, while contribute to desirable capacitance of the hybrid material. ILs-based microemulsions spontaneously adsorbing onto PPy-Ti3C2Tx nanosheets are conceived to be one more liquid “spacer” for fast ion diffusion kinetics and absent electrolyte imbibition steps, which is rarely reported in previous research for MXene-based electrode materials. These features endow the composite films with excellent specific capacitance, rate capability and cycling stability between 4 °C and 50 °C. At room temperature, the symmetric supercapacitor device based on the composite films delivers a maximum gravimetric energy density of 31.2 Wh kg−1 (at 1030.4 W kg−1) and reserves 91% of the initial specific capacitance with a coulombic efficiency of 91% after 2000 cycles. All these impressive results substantiate that the composite films prepared by ingenious structure design showcase huge potentials in advanced MXene-based supercapacitors using various ionic liquid electrolytes.

Interlayer Structure Engineering of MXene-Based Capacitor-Type Electrode for Hybrid Micro-Supercapacitor toward Battery-Level Energy Density.

Micro‐supercapacitors are notorious for their low energy densities compared to micro‐batteries. While MXenes have been identified as promising capacitor‐type electrode materials for alternative zinc‐ion hybrid micro‐supercapacitors (ZHMSCs) with higher energy density, their tightly spaced layered structure renders multivalent zinc‐ions with large radii intercalation inefficient. Herein, through insertion of 1D core‐shell conductive BC@PPy nanofibers between MXene nanosheets, an interlayer structure engineering technique for MXene/BC@PPy capacitor‐type electrodes towards ZHMSCs is presented. Owing to simultaneously achieving two objectives: (i) widening the interlayer space and (ii) providing conductive connections between the loose MXene layers, enabled by the conductive BC@PPy nanospacer, the approach effectively enhances both ion and electron transport within the layered MXene structure, significantly increasing the areal capacitance of the MXene/BC@PPy film electrode to 388 mF cm−2, which is a 10‐fold improvement from the pure MXene film electrode. Pairing with CNTs/MnO2 battery‐type electrodes, the obtained ZHMSCs exhibit an areal energy density up to 145.4 μWh cm−2 with an outstanding 95.8% capacity retention after 25000 cycles, which is the highest among recently reported MXene‐based MSCs and approaches the level of micro‐batteries. The interlayer structure engineering demonstrated in the MXene‐based capacitor‐type electrode provides a rational means to achieve battery‐levelenergy density in the ZHMSCs.

High-performance heterojunction Ti3C2/CoSe2 with both intercalation and conversion storage mechanisms for magnesium batteries

Searching for high-performance cathode materials has become the key to the development of rechargeable magnesium batteries (RMBs). Herein, we successfully constructed a novel heterostructure Ti3C2/CoSe2 by in-situ selenization of Ti3C2/ZIF-67 composite as a cathode for RMBs. In the heterostructure, the unique MXene structure plays a role in maintain the stability of the material skeleton during the charge/discharge process, and the CoSe2 nanoparticles can fully deliver the advantage of high capacity. As a result, the Ti3C2/CoSe2 heterojunction has great obvious improvement in electrochemical performance, leading to a high-rate performance (75.7 mA h g−1 at 1000 m A g−1) and long cycling life with ~79% capacity retention after 500 cycles. Moreover, the Mg storage mechanism and Mg-migration kinetics of Ti3C2/CoSe2 are analyzed and discussed by ex-situ X-ray diffraction and X-ray photoelectron spectroscopy. In particular, the Ti3C2/CoSe2 heterostructures display impressive Mg storage performance (67.3 mA h g−1 after 70 cycles at 55 °C) in quasi-solid-state magnesium batteries. Importantly, the selenization strategy has inspired the exploration of the preparation of other novel MXene-based selenide electrodes for high-performance energy storage devices.