A 3D Hydroxylated MXene/Carbon Nanotubes Composite as a Scaffold for Dendrite‐Free Sodium‐Metal Electrodes

Sodium metal is a promising anode, but uneven Na deposition with a dendrite growth seriously impedes its application. Herein, a fibrous hydroxylated MXene/carbon nanotubes (h‐Ti₃C₂/CNTs) composite is designed as a scaffold for dendrite‐free Na metal electrodes. This composite displays fast Na⁺/electron transport kinetics and good thermal conductivity and mechanical properties. The h‐Ti₃C₂ contains abundant sodiophilic functional groups, which play a significant role in inducing homogeneous nucleation of Na. Meanwhile, CNTs provide high tensile strength and ease of film‐forming. As a result, h‐Ti₃C₂/CNTs exhibit a high average Coulombic efficiency of 99.2 % and no dendrite after 1000 cycles. The h‐Ti₃C₂/CNTs/Na based symmetric cells show a long lifespan over 4000 h at 1.0 mA cm^?2 with a capacity of 1.0 mAh cm^?2. Furthermore, Na‐O₂ batteries with a h‐Ti₃C₂/CNTs/Na anode exhibit a low potential gap of 0.11 V after an initial 70 cycles.     

The Three‐Dimensional Dendrite‐Free Zinc Anode on a Copper Mesh with a Zinc‐Oriented Polyacrylamide Electrolyte Additive

Rechargeable aqueous zinc‐ion batteries have been considered as a promising candidate for next‐generation batteries. However, the formation of zinc dendrites are the most severe problems limiting their practical applications. To develop stable zinc metal anodes, a synergistic method is presented that combines the Cu‐Zn solid solution interface on a copper mesh skeleton with good zinc affinity and a polyacrylamide electrolyte additive to modify the zinc anode, which can greatly reduce the overpotential of the zinc nucleation and increase the stability of zinc deposition. The as‐prepared zinc anodes show a dendrite‐free plating/stripping behavior over a wide range of current densities. The symmetric cell using this dendrite‐free anode can be cycled for more than 280 h with a very low voltage hysteresis (93.1 mV) at a discharge depth of 80 %. The high capacity retention and low polarization are also realized in Zn/MnO₂ full cells.     

3D crumbled MXene for high-performance supercapacitors

MXene materials have recently attracted considerable attention in energy storage application owing to their metallic conductivity, 2D structure and tunable surface terminations. However, the restacking of 2D MXene nanosheets hinders the ion transport and accessibility to the surface, resulting in adverse effect on their electrochemical performances. Here, with the assistance of hexamethylenetetramine (C₆H₁₂N₄), 2D Ti₃C₂T_x MXene nanosheets were fabricated into a 3D architecture with crumbled and porous structure through an electrostatic self-assembly followed by annealing. The resultant 3D structure can expose massive active sites and facilitates the ion transport, which is beneficial for sufficient utilization of the outstanding superiorities of the MXene. Therefore, as a pseudocapacitive material, the 3D crumpled and porous Ti₃C₂T_x MXene shows a gravimetric capacitance of 333 F/g at 1 A/g, and maintains 261 F/g and 132 F/g at ultrahigh current densities of 100 A/g and 1000 A/g, respectively, revealing promising potential for application in supercapacitors.     

A Hybrid Assembly of MXene with NH2−Si Nanoparticles Boosting Lithium Storage Performance

A facile hybrid assembly between Ti₃C₂T_x MXene nanosheets and (3‐aminopropyl) triethoxylsilane‐modified Si nanoparticles (NH₂?Si NPs) was developed to construct multilayer stacking of Ti₃C₂T_x nanosheets with NH₂?Si NPs assembling together (NH₂?Si/Ti₃C₂T_x). NH₂?Si/Ti₃C₂T_x exhibits a significantly enhanced lithium storage performance compared to pristine Si, which is attributed to the robust crosslinking architecture and considerably improved electrical conductivity as well as shorter Li⁺ diffusion pathways. The optimized NH₂?Si/Ti₃C₂T_x anode with Ti₃C₂T_x: NH₂?Si mass ratio of 4 : 1 displays an enhanced capacity (864 mAh g^?1 at 0.1 C) with robust capacity retention, which is significantly higher than those of NH₂?Si NPs and Ti₃C₂T_x anodes. Furthermore, this work demonstrates the important effect of the MXene‐based electrode architecture on the electrochemical performance and can guide future work on designing high‐performance Si/MXene hybrids for energy storage applications.     

Lithium storage properties of Ti3C2Tx (Tx = F,Cl, Br) MXenes

In this work, Ti₃C₂T_x MXene with -F, -Cl and -Br surface terminations are synthesized and the effect of these halogen terminations on the lithium storage properties is investigated. A maximum Li⁺ storage capacity of 189 mAh/g is achieved with Ti₃C₂Br_x MXene much higher than Ti₃C₂Cl_x and Ti₃C₂F_x with 138 mAh/g and 123 mAh/g, respectively. Density functional theory (DFT) calculation shows that the adsorption formation energy of halogen atoms on Ti atoms follows the trend of Ti-F > Ti-Cl > Ti-Br, leading to the same trend in the content of terminations on corresponding MXenes. In addition, inevitable exposure of MXene to oxygen causes competition between halogen and oxygen. Theoretical results show Ti₃C₂Br_x MXene has the highest Ti to O ratio and the lowest Ti to Br ratio, the high lithium affinity of O explains the maximum Li-ion storage capacity with Ti₃C₂Br_x MXene. This work shed light on the opportunity for achieving improved lithium storage properties of MXene electrodes by regulating the surface chemistry.     

Achieving high yield of Ti3C2Tx MXene few-layer flakes with enhanced pseudocapacior performance by decreasing precursor size

Ti₃C₂T_x, a most studied member of MXene family, shows promise as a candidate electrode for pseudocapacitor due to its electronic conductivity and hydrophilic surface. However, the unsatisfactory yield of Ti₃C₂T_x few-layer flakes significantly restricted it in real applications. Here, we proposed a simple solution to boost the yield of Ti₃C₂T_x few-layer flakes by decreasing precursor size. When using the small 500 mesh Ti₃AlC₂ powders as raw material, high yield of 65% was successfully achieved. Moreover, the as-received small flakes also exhibit an enhanced pseudocapacior performance owing to their excellent electrical conductivity, expanded interlayer space and more O content on the surface. This work not only sheds light on the cost effective mass production of Ti₃C₂T_x few-layer flakes, but also provides an efficient solution for the design of MXene electrodes with high pseudocapacior performance.     

Graphene-Enabled Electric-Field Regulation and Ionic Redistribution Around Lithiophilic Aurum Nanoparticles Toward a Dendrite-Free and 2000-Cycle-Life Lithium Metal Battery

Lithium metal batteries (LMBs) have attracted extensive attention owing to their high energy density. However, the uncontrolled volume changes and serious dendrite growth of the Li metal anode have hindered their commercialization. Herein, a three-dimensional Cu foam decorated with Au nanoparticles and conformal graphene layer was designed to tune the Li plating/stripping behaviors. The 3D−Cu conductive host anchored by lithiophilic Au nanoparticles can effectively alleviate the volume expansion caused by the continuous plating/stripping of Li and reduce the nucleation energy barrier. Notably, the conductive graphene not only facilitates the transfer of electrons, but also acts as an ionic rectifier, thereby avoiding the aggregation of local current density and Li⁺ ions around Au nanoparticles and enabling the uniform Li⁺ flux. As a result, the G−Au@3D−Cu/Li anode ensures the non-dendritic and homogeneous Li⁺ plating/stripping. Electrochemical results show that the symmetric G−Au@3D−Cu/Li cell delivers a low voltage hysteresis of 110 mV after 1000 h at 1 mA cm⁻². Matched with a layered LiNi_0.6Co_0.2Mn_0.2O₂ cathode, the NCM622||G−Au@3D−Cu/Li full cell exhibits a long cycle life of 2000 cycles and an ultra-low capacity decay rate (0.01 % per cycle).     

Selective nitrogen doping on carbon cloth to enhance the performance of zinc anode

Metallic zinc is attractive anode material of rechargeable aqueous Zn-based batteries due to its ambient stability,high volumetric capacity,and abundant reserves.Nonetheless,Zn anodes suffer from issues such as low coulombic efficiency(CE),large polarization and dendrite formation.Herein,uniform Zn electrodeposition is reported on carbon substrates by selective nitrogen doping.Combined experimental and theoretical investigations demonstrate that pyrrolic and pyridinic nitrogen doped in carbon play beneficial effect as zinc-philic sites to direct nucleation and growth of metallic Zn,while negligible effect is observed for graphite nitrogen in Zn plating.The carbon cloth with modified amount of doped pyrrolic and pyridinic nitrogen stabilizes Zn plating/stripping with 99.3% CE after 300 cycles and significantly increases the deliverable capacity at high depth of charge and discharge compared to undoped carbon substrate and Zn foil.This work provides a better understanding of heteroatom doping effect in design and preparation of stable 3 D carbon-supported zinc anode.     

Polycation-Regulated Electrolyte and Interfacial Electric Fields for Stable Zinc Metal Batteries

Zn metal as one of promising anode materials for aqueous batteries but suffers from disreputable dendrite growth, grievous hydrogen evolution and corrosion. Here, a polycation additive, polydiallyl dimethylammonium chloride (PDD), is introduced to achieve long-term and highly reversible Zn plating/stripping. Specifically, the PDD can simultaneously regulate the electric fields of electrolyte and Zn/electrolyte interface to improve Zn²⁺ migration behaviors and guide dominant Zn (002) deposition, which is veritably detected by Zeta potential, Kelvin probe force microscopy and scanning electrochemical microscopy. Moreover, PDD also creates a positive charge-rich protective outer layer and a N-rich hybrid inner layer, which accelerates the Zn²⁺ desolvation during plating process and blocks the direct contact between water molecules and Zn anode. Thereby, the reversibility and long-term stability of Zn anodes are substantially improved, as certified by a higher average coulombic efficiency of 99.7 % for Zn||Cu cells and 22 times longer life for Zn||Zn cells compared with that of PDD-free electrolyte.     

Resistive-type sensors based on few-layer MXene and SnO2 hollow spheres heterojunctions: Facile synthesis,ethanol sensing performances

High-performance and low-cost gas sensors are highly desirable and involved in industrial production and environmental detection. The combination of highly conductive MXene and metal oxide materials is a promising strategy to further improve the sensing performances. In this study, the hollow SnO₂ nanospheres and few-layer MXene are assembled rationally via facile electrostatic synthesis processes, then the SnO₂/Ti₃C₂T_x nanocomposites were obtained. Compared with that based on either pure SnO₂ nanoparticles or hollow nanospheres of SnO₂, the SnO₂/Ti₃C₂T_x composite-based sensor exhibits much better sensing performances such as higher response (36.979), faster response time (5 s), and much improved selectivity as well as stability (15 days) to 100 ppm C₂H₅OH at low working temperature (200 °C). The improved sensing performances are mainly attributed to the large specific surface area and significantly increased oxygen vacancy concentration, which provides a large number of active sites for gas adsorption and surface catalytic reaction. In addition, the heterostructure interfaces between SnO₂ hollow spheres and MXene layers are beneficial to gas sensing behaviors due to the synergistic effect.     

Pt immobilized spontaneously on porous MXene/MAX hybrid monolith for hydrogen evolution reaction

Designing efficient electrocatalysts with low Pt loadings for hydrogen evolution reaction(HER) is urgently required for renewable and sustainable energy conversion.Here,we report a strategy that Pt nanoparticulates are spontaneously immobilized on porous MXene/MAX monolith as HER catalysts by utilizing the redox reaction between Ti_3C_2T_x MXene and [PtCl_4]~2 in H_2 PtCl_6 aqueous solution.By taking advantage of homogeneously distributed Pt nanoparticulates on highly electrically conductive porous Ti_3C_2T_x/Ti_3AlC_2 monolith,the as-prepared electrocatalysts show high catalytic performance for hydrogen evolution.Specifically,the binder-free electrocatalysts have Pt loadings as low as 8.9 μg/cm~2,with low overpotential of 43 mV at a curre nt density of 10 mA/cm~2 and low Tafel slope that three times lower than porous Ti_3C_2T_x/Ti_3AlC_2 without Pt loading.This strategy offers a new approach to constructing ultra-low Pt-loading HER catalysts on the basis of in situ redox reaction between noble metal ions and MXenes.     

An Ultrathin Inorganic Molecular Crystal Interfacial Layer for Stable Zn Anode

Zn metal anode suffers from dendrite growth and side reactions during cycling, significantly deteriorating the lifespan of aqueous Zn metal batteries. Herein, we introduced an ultrathin and ultra-flat Sb₂O₃ molecular crystal layer to stabilize Zn anode. The in situ optical and atomic force microscopes observations show that such a 10 nm Sb₂O₃ thin layer could ensure uniform under-layer Zn deposition with suppressed tip growth effect, while the traditional WO₃ layer undergoes an uncontrolled up-layer Zn deposition. The superior regulation capability is attributed to the good electronic-blocking ability and low Zn affinity of the molecular crystal layer, free of dangling bonds. Electrochemical tests exhibit Sb₂O₃ layer can significantly improve the cycle life of Zn anode from 72 h to 2800 h, in contrast to the 900 h of much thicker WO₃ even in 100 nm. This research opens up the application of inorganic molecular crystals as the interfacial layer of Zn anode.     

Highly Reversible Zinc Metal Anode in a Dilute Aqueous Electrolyte Enabled by a pH Buffer Additive

Aqueous zinc-ion batteries have drawn increasing attention due to the intrinsic safety, cost-effectiveness and high energy density. However, parasitic reactions and non-uniform dendrite growth on the Zn anode side impede their application. Herein, a multifunctional additive, ammonium dihydrogen phosphate (NHP), is introduced to regulate uniform zinc deposition and to suppress side reactions. The results show that the NH₄⁺ tends to be preferably absorbed on the Zn surface to form a “shielding effect” and blocks the direct contact of water with Zn. Moreover, NH₄⁺ and (H₂PO₄)⁻ jointly maintain pH values of the electrode-electrolyte interface. Consequently, the NHP additive enables highly reversible Zn plating/stripping behaviors in Zn//Zn and Zn//Cu cells. Furthermore, the electrochemical performances of Zn//MnO₂ full cells and Zn//active carbon (AC) capacitors are improved. This work provides an efficient and general strategy for modifying Zn plating/stripping behaviors and suppressing side reactions in mild aqueous electrolyte.     

Functional MXene Materials: Progress of Their Applications

Nowadays, two‐dimensional materials have many applications in materials science. As a novel two‐dimensional layered material, MXene possesses distinct structural, electronic, and chemical properties; thus, it has potential applications in many fields, including battery electrodes, energy storage materials, sensors, and catalysts. Up to now, more than 70 MAX phases have been reported. However, in contrast to the variety of MAX phases, the existing MXene family merely includes Ti₂C, Ti₃C₂, (Ti_1/2, Nb_1/2)₂C, (V_1/2, Cr_1/2)₃C₂, Nb₂C, Ti₃CN, Ta₄C₃, V₂C, and Nb₄C₃. Among these materials, the Ti₃C₂T_x MXene exhibits prominently high volumetric capacitance, and the rate at which it transports electron is suitable for electrode materials in batteries and supercapacitors. Hence, Ti₃C₂T_x is commonly utilized as an electrode material in ion batteries such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Al³⁺ batteries. What is more, Ti₂C has the biggest specific surface area among all of these potential MXene phases, and therefore, Ti₂C has remarkably high gravimetric hydrogen storage capacities. In addition, Ti₂CO₂ materials display extremely high activity for CO oxidation, which makes it possible to design catalysts for CO oxidation at low temperatures. Furthermore, Ti₃C₂T_x with O, OH, and/or F terminations can be used for water purification owing to excellent water permeance, favorable filtration ability, and long‐time operation ability. This review supplies a relatively comprehensive summary of various applications of MXenes over the past few years.     

Electrochemical and in-situ X-ray diffraction studies of Ti3C2Tx MXene in ionic liquid electrolyte

2D titanium carbide (Ti₃C₂T_x MXene) showed good capacitance in both organic and neat ionic liquid electrolytes, but its charge storage mechanism is still not fully understood. Here, electrochemical characteristics of Ti₃C₂T_x electrode were studied in neat EMI-TFSI electrolyte. A capacitive behavior was observed within a large electrochemical potential range (from − 1.5 to 1.5 V vs. Ag). Intercalation and de-intercalation of EMI⁺ cations and/or TFSI⁻ anions were investigated by in-situ X-ray diffraction. Interlayer spacing of Ti₃C₂T_x flakes decreases during positive polarization, which can be ascribed to either electrostatic attraction effect between intercalated TFSI⁻ anions and positively charged Ti₃C₂T_x nanosheets or steric effect caused by de-intercalation of EMI⁺ cations. The expansion of interlayer spacing when polarized to negative potentials is explained by steric effect of cation intercalation.     

A 3D Hydroxylated MXene/Carbon Nanotubes Composite as a Scaffold for Dendrite-Free Sodium-Metal Electrodes

Sodium metal is a promising anode, but uneven Na deposition with a dendrite growth seriously impedes its application. Herein, a fibrous hydroxylated MXene/carbon nanotubes (h-Ti₃C₂/CNTs) composite is designed as a scaffold for dendrite-free Na metal electrodes. This composite displays fast Na⁺/electron transport kinetics and good thermal conductivity and mechanical properties. The h-Ti₃C₂ contains abundant sodiophilic functional groups, which play a significant role in inducing homogeneous nucleation of Na. Meanwhile, CNTs provide high tensile strength and ease of film-forming. As a result, h-Ti₃C₂/CNTs exhibit a high average Coulombic efficiency of 99.2 % and no dendrite after 1000 cycles. The h-Ti₃C₂/CNTs/Na based symmetric cells show a long lifespan over 4000 h at 1.0 mA cm⁻² with a capacity of 1.0 mAh cm⁻². Furthermore, Na-O₂ batteries with a h-Ti₃C₂/CNTs/Na anode exhibit a low potential gap of 0.11 V after an initial 70 cycles.     

In situ decoration of CoP/Ti3C2Tx composite as efficient electrocatalyst for Li-oxygen battery

Application of Li-oxygen (Li-O₂) battery is in urgent need of bifunctional ORR/OER electrocatalyst. A surface-functionalization CoP/Ti₃C₂T_x composite was fabricated theoretically, with the optimized electronic structure and more active electron, which is beneficial to the electrochemical reaction. The accordion shaped Ti₃C₂T_x is featured with large specific surface area and outstanding electronic conductivity, which is beneficial for the adequate exposure of active sites and the deposition of Li₂O₂. Transition metal phosphides provide more electrocatalytic active sites and present good electrocatalytic effect. The CoP/Ti₃C₂T_x composite served as the electrocatalyst of Li-O₂ battery reaches a high specific discharge capacity of 17,413 mAh/g at 100 mA/g and the lower overpotential of 1.25 V, superior to those of the CoP and Ti₃C₂T_x individually. The composite of transition metal phosphides and MXene are applied in Li-O₂ battery, not only demonstrating higher cycling stability of the prepared CoP/Ti₃C₂T_x composite, but pointing out the direction for their electrochemical performance improvement.     

Toward Understanding the Enhanced Pseudocapacitive Storage in 3D SnS/MXene Architectures Enabled by Engineered Surface Reactions

The optimization of three-dimensional (3D) MXene-based electrodes with desired electrochemical performances is highly demanded. Here, a precursor-guided strategy is reported for fabricating the 3D SnS/MXene architecture with tiny SnS nanocrystals (≈5 nm in size) covalently decorated on the wrinkled Ti₃C₂T_x nanosheets through Ti−S bonds (denoted as SnS/Ti₃C₂T_x-O). The formation of Ti−S bonds between SnS and Ti₃C₂T_x was confirmed by extended X-ray absorption fine structure (EXAFS). Rather than bulky SnS plates decorated on Ti₃C₂T_x (SnS/Ti₃C₂T_x-H) by one-step hydrothermal sulfidation followed by post annealing, this SnS/Ti₃C₂T_x-O presents size-dependent structural and dynamic properties. The as-formed 3D hierarchical structure can provide short ion-diffusion pathways and electron transport distances because of the more accessible surface sites. In addition, benefiting from the tiny SnS nanocrystals that can effectively improve Na⁺ diffusion and suppress structural variation upon charge/discharge processes, the as-obtained SnS/Ti₃C₂T_x-O can generate pseudocapacitance-dominated storage behavior enabled by engineered surface reactions. As predicted, this electrode exhibits an enhanced Na storage capacity of 565 mAh g⁻¹ at 0.1 A g⁻¹ after 75 cycles, outperforming SnS/Ti₃C₂T_x-H (336 mAh g⁻¹), SnS (212 mAh g⁻¹), and Ti₃C₂T_x (104 mAh g⁻¹) electrodes.     

Recent progress in liquid electrolytes for lithium metal batteries

Li metal batteries are revived as the next-generation batteries beyond Li-ion batteries. The Li metal anode can be paired with intercalation-type cathodes LiMO₂ and conversion-type cathodes such as sulfur and oxygen. Then, energy densities of Li/LiMO₂ and Li/S,O₂ batteries can reach 400 Whkg^?1 and more than 500 Whkg^?1, respectively, which surpass that of the state-of-the-art LIB (280 Whkg^?1). However, replacing the intercalation-type graphite anode with the Li metal anode suffers from low coulombic efficiency during repeated Li plating/stripping processes, which leads to short cycle lifetime and potential safety problems. The key solution is to construct a stable and uniform solid electrolyte interphase with high Li⁺ transport and high elastic strength on the Li metal anode. This review summarizes recent progress in improving the solid electrolyte interphase by tailoring liquid electrolytes, a classical but the most convenient and cost-effective strategy.     

Tetraphenylporphyrin-based Chelating Ligand Additive as a Molecular Sieving Interfacial Barrier toward Durable Aqueous Zinc Metal Batteries

The sustained water consumption and uncontrollable dendrite growth strongly hamper the practical applications of rechargeable zinc (Zn) metal batteries (ZMBs). Herein, for the first time, we demonstrate that trace amount of chelate ligand additive can serve as a “molecular sieve-like” interfacial barrier and achieve highly efficient Zn plating/stripping. As verified by theoretical modeling and experimental investigations, the benzenesulfonic acid groups on the additive molecular not only facilitates its water solubility and selective adsorption on the Zn anode, but also effectively accelerates the de-solvation kinetics of Zn²⁺. Meanwhile, the central porphyrin ring on the chelate ligand effectively expels free water molecules from Zn²⁺ via chemical binding against hydrogen evolution, and reversibly releases the captured Zn²⁺ to endow a dendrite-free Zn deposition. By virtue of this non-consumable additive, high average Zn plating/stripping efficiency of 99.7 % over 2100 cycles together with extended lifespan and suppressed water decomposition in the Zn||MnO₂ full battery were achieved, thus opening a new avenue for developing highly durable ZMBs.