Alkali Metal Intercalation in MXene/Graphene Heterostructures: A New Platform for Ion Battery Applications

Recent advances of two-dimensional materials-based heterostructures for rechargeable batteries

Because of their unique layer structure, 2D materials have demonstrated to be promising electrode materials for rechargeable batteries. However, individual 2D materials cannot meet all the performance requirements of energy density, power density, and cycle life. Constructing 2D materials-based heterostructures offers an opportunity to synergistically handle the deficiencies of individual 2D materials and modulate the physical and electrochemical properties. The enlarged interlayer distance and increased binding energy with ions of heterostructures can facilitate charge transfer, boost electrochemical reactivities, resulting in an enhanced performance in rechargeable batteries. Here we summarize the latest development of heterostructures consisted of 2D materials and their applications in rechargeable batteries. Firstly, different preparation strategies and optimized structure engineering strategies of 2D materials-based heterostructures are systematically introduced. Secondly, the unique functions of 2D materials-based heterostructures in rechargeable batteries are discussed respectively. Finally, challenges and perspectives are presented to inspire the future study of 2D materials-based heterostructures.

Tuning Na-Ion Diffusion in MXene/Graphene Oxide Heterostructures: An Ab Initio Molecular Dynamics Study

The development of heterostructured anode materials provides an effective approach for enhancing the electrochemical performance of sodium-ion batteries (SIBs). In this work, ab initio molecular dynamics simulations and first-principles calculations are employed to investigate the Na-ion intercalation and diffusion in MXene/graphene oxide heterostructures. The influence of graphene oxidation on interlayer spacing, Na-ion diffusion kinetics, and transport mechanisms is examined at an atomic scale. It has been observed that oxygen functional groups can increase the interspacing between adjacent layers, thereby improving the initial embedding of Na ions. However, overoxidation causes an obstructive effect on the ionic conduction channels. An appropriate oxidation degree enables optimal Na-ion migration kinetics while retaining structural integrity. Our simulation results provide crucial insights into the rational design of high-performance MXene-based anodes for SIBs with excellent capacity and cycling stability.

Effect of Surface Functional Groups on the Electronic Behavior and Optical Spectra of Mn2N Based MXenes

The extensive applications of MXenes, a novel type of layered materials known for their favorable characteristics, have sparked significant interest. This research focuses on investigating the influence of surface functionalization on the behavior of Mn2NTx (Tx=O2, F2) MXenes monolayers using the "Density functional theory (DFT) based full-potential linearized augmented-plane-wave (FP-LAPW)" method. We elucidate the differences in the physical properties of Mn2NTx through the influence of F and O surface functional groups. We found that O-termination results in half-metallic behavior, whereas the F-termination evolves metallic characteristics within these MXene systems. Similarly, surface termination has effectively influenced their optical absorption efficiency. For instance, Mn2NO2 and Mn2NF2 effectively absorb UV light ~50.15×104 cm-1 and 37.71×104 cm-1, respectively. Additionally, they demonstrated prominent refraction and reflection characteristics, which are comprehensively discussed in the present work. Our predictions offer valuable perspectives into the optical and electronic characteristics of Mn2NTx-based MXenes, presenting the promising potential for implementing them in diverse optoelectronic devices.

A first-principles study of the BC3N2 monolayer and a BC3N2/graphene heterostructure as promising anode materials for sodium-ion batteries

High-performance sodium-ion batteries (SIBs) require anode materials with high capacity and fast kinetics. Based on first-principles calculations, we propose BC3N2 and BC3N2/graphene (B/G) heterostructure as potential SIB anode materials. The BC3N2 monolayer exhibits intrinsic metallic behavior. In addition, BC3N2 possesses a low Na+ diffusion barrier (0.15 eV), a high storage capacity (777 mA h g-1), a low open-circuit voltage (0.72 V), and a tiny axial expansion (0.36%). Compared with the BC3N2 monolayer, the B/G heterostructure exhibits a lower diffusion barrier of 0.027 eV, suggesting a much faster diffusion. More importantly, although the B/G heterostructure possesses heavier molar weight, its theoretical capacity (689 mA h g-1) is comparable to that of the BC3N2 monolayer. Based on the above-mentioned properties, we hope both the BC3N2 monolayer and the B/G heterostructure would be promising anodes for SIBs.

Theoretical insights into the intercalation mechanism of Li, Na, and Mg ions in a metallic BN/VS2 heterostructure

Layered VS2 has been widely used as a battery anode material owing to its large specific surface area and controllable ion-transport channel. However, its semiconductor properties and poor cycling stability seriously limit its further applications. Herein, a two-dimensional BN/VS2 heterostructure (BVH) was constructed as an anode material for rechargeable metal-ion batteries (RMIBs). Demonstrated using first principles calculations, BVH exhibits a metallic property due to lattice stress between monolayer BN and VS2. BVH displays low ion diffusion energy barriers (0.13, 0.43, and 0.56 eV) and high theoretical capacities (447, 553.5, and 340.7 mA h g-1) for Li+, Na+, and Mg2+ storage. In BVH, the VS2 layer as the main redox center supports charge transfer, while the inactive BN layer enables high structural stability. This synergistic effect is expected to simultaneously achieve a high rate, high capacity, and long life. This design provides an important insight into developing new anode materials for RMIBs.

Properties of Layered TMDC Superlattices for Electrodes in Li-Ion and Mg-Ion Batteries

In this work, we present a first-principles investigation of the properties of superlattices made from transition metal dichalcogenides for use as electrodes in lithium-ion and magnesium-ion batteries. From a study of 50 pairings, we show that, in general, the volumetric expansion, intercalation voltages, and thermodynamic stability of vdW superlattice structures can be well approximated with the average value of the equivalent property for the component layers. We also found that the band gap can be reduced, improving the conductivity. Thus, we conclude that superlattice construction can be used to improve material properties through the tuning of intercalation voltages toward specific values and by increasing the stability of conversion-susceptible materials. For example, we demonstrate how pairing SnS2 with systems such as MoS2 can change it from a conversion to an intercalation material, thus opening it up for use in intercalation electrodes.

Bilayer and Trilayer C3N/Blue-Phosphorene Heterostructures as Potential Anode Materials for Potassium-Ion Batteries

Two-dimensional (2D) van der Waals heterostructures outperform conventional anode materials for postlithium-ion batteries in terms of mechanical, thermal, and electrochemical properties. This study systemically investigates the performance of bilayer and trilayer C3N/blue phosphorene (C3N/BlueP) heterostructures as anode materials for potassium-ion batteries (KIBs) using first-principles density functional theory calculations. This study reveals that the adsorption and diffusion of K ions on bilayer and trilayer C3N/BlueP heterostructures are markedly superior to those of their monolayer counterparts. A bilayer heterostructure (C3N/BlueP) effectively reduces the bandgap of the BlueP monolayer (1.98 eV) to 0.02 eV, whereas trilayer heterostructures (bilayer-C3N/BlueP and C3N/bilayer-BlueP) exhibit metallic behavior with no bandgap. Additionally, the theoretical capacity of the bilayer and trilayer heterostructures ranges from 636.7 to 755.5 mA h g-1, considerably higher than the theoretical capacity of other prospective 2D heterostructures for KIBs investigated in the literature. This study also shows that the heterostructures exhibit K-ion diffusion barriers as low as 0.042 eV, ensuring the relatively fast diffusion of K ions.

Black phosphorene/SnSe van der Waals heterostructure as a promising anchoring anode material for metal-ion batteries

Black phosphorene (BP) is a glowing two-dimensional semiconducting layer material for cutting-edge microelectronics, with high carrier mobility and thickness-dependent band gap. Here, based on van der Waals (vdW)-corrected first-principles approaches, we investigated stacked BP/tin selenide (BP/SnSe) vdW heterostructure as an anode material for metal ion batteries, which exhibits a significant theoretical capacity, along with relatively durable binding strength compared to the constituent BP and SnSe monolayers. Our calculations demonstrated that the Li/Na adatom favors insertion into the interlayer region of BP/SnSe vdW heterostructure owing to synergistic interfacial effect, resulting in comparable diffusivity to the BP and SnSe monolayers. Subsequently, the theoretical specific capacities for Li/Na are found to be as high as 956.30 mAhg-1and 828.79 mAhg-1, respectively, which could be attributed to the much higher storage capacity of Li/Na adatoms in the BP/SnSe vdW heterostructure. Moreover, the electronic structure calculations reveal that a large amount of charge transfer assists in semiconductor-to-metallic transition upon lithiation/sodiation, ensuring good electrical conductivity. These simulations verify that the BP/SnSe vdW heterostructure has immense potential for application in the design of metal-ion battery technologies.

Prospective utilization of boron nitride and beryllium oxide nanotubes for Na, Li, and K-ion batteries: a DFT-based analysis

CONTEXT

In the present work, we investigated the adsorption mechanism of natural sodium (Na), potassium (K), and lithium (Li) atoms and their respective ion on two nanostructures: boron-nitride nanotubes (BNNTs) and beryllium-oxide nanotubes (BeONTs). The main goal of this research is to calculate the gain voltage for Na, K, and Li ionic batteries. Density function theory (DFT) calculations indicated that the adsorption energy between Na + is higher than that of the other cations, and this is particularly clear in the BeONT. Furthermore, gain voltage calculations showed that BNNTs generate a higher potential than BeONTs, with the most significant difference observed in BNNT/Na + . This research provides theoretical insights into the potential uses of these nanostructures as anodes in Na, K, and Li-ion batteries.

METHOD

Density function theory used to compute the ground state properties for BeONT and BNNT with and without selected atoms and their ions (Li, K, and Na). B3LYP used for exchange correlation between electrons and ions, and 6-31G* basis set used for all atoms and ions. Gauss Sum 2.2 software used for estimate the density of state (DOS) for all structure under investigation.

Effect of surface functional groups on MXene conductivity

We report the in-plane electron transport in the MXenes (i.e., within the MXene layers) as a function of composition using the density-functional tight-binding method, in conjunction with the non-equilibrium Green's functions technique. Our study reveals that all MXene compositions have a linear relationship between current and voltage at lower potentials, indicating their metallic character. However, the magnitude of the current at a given voltage (conductivity) has different trends among different compositions. For example, MXenes without any surface terminations (Ti3C2) exhibit higher conductivity compared to MXenes with surface functionalization. Among the MXenes with -O and -OH termination, those with -O surface termination have lower conductivity than the ones with -OH surface terminations. Interestingly, conductivity changes with the ratio of -O and -OH on the MXene surface. Our calculated I-V curves and their conductivities correlate well with transmission functions and the electronic density of states around the Fermi level. The surface composition-dependent conductivity of the MXenes provides a path to tune the in-plane conductivity for enhanced pseudocapacitive performance.

Hybrid MXene-Graphene/Hexagonal Boron Nitride Structures: Electronic and Molecular Adsorption Properties

Recent advances in experimental techniques allow for the fabrication of hybrid structures. Here, we study the electronic and molecular adsorption properties of the graphene (G)/hexagonal boron nitride (h-BN)-MXenes (Mo2C) hybrid nanosheets. We use first-principles calculations to explore the structure and electronic properties of the hybrid structures of G-2H-Mo2C and h-BN-2H-Mo2C with two different oxygen terminations of the Mo2C surface. The embedding of G or h-BN patches creates structural defects at the patch-Mo2C border and adds new states in the vicinity of the Fermi energy. Since this can be utilized for molecular adsorption and/or sensing, we investigate the ability of the G-M-O1 and BN-M-O1 hybrid structures to adsorb twelve molecules. Generally, the adsorption on the hybrid systems is significantly higher than on the pristine systems, except for N2 and H2, which are weakly adsorbed on all systems. We find that OH, NO, NO2, and SO2 are chemisorbed on the hybrid systems. COOH may be chemisorbed, or it may dissociate depending on its location at the edge between the G/h-BN and the MXene. NH3 is chemisorbed/physisorbed on the BN/G-M-O1 systems. CO, H2S, CO2, and CH4 are physisorbed on the hybrid systems. Our results indicate that the studied hybrid systems can be used for molecular filtration/sensing and catalysis.

Screening and Understanding Li Adsorption on Two-Dimensional Metallic Materials by Learning Physics and Physics-Simplified Learning

Understanding and broad screening Li interaction energetics with surfaces are key to the development of materials for a wide range of applications including Li-based electrochemical capacitors, Li sensors, Li separation membranes, and Li-ion batteries. In this work, we build a high-throughput screening scheme to screen Li adsorption energetics on 2D metallic materials. First, density functional theory and graph convolution networks are utilized to calculate the minimum Li adsorption energies for some 2D metallic materials. The data is then used to find a dependence of the minimum Li adsorption energies on the sum of ionization potential, work function of the 2D metal, and coupling energy between Li⁺ and substrate, and the dependence is used to screen all 2D metallic materials. Physics-simplified learning by splitting the property into different contributions and learning or calculating each component is shown to have higher accuracy and transferability for machine learning of complex materials properties.

Growth Mechanism of Micro/Nano Metal Dendrites and Cumulative Strategies for Countering Its Impacts in Metal Ion Batteries: A Review

Metal-ion batteries are capable of delivering high energy density with a longer lifespan. However, they are subject to several issues limiting their utilization. One critical impediment is the budding and extension of solid protuberances on the anodic surface, which hinders the cell functionalities. These protuberances expand continuously during the cyclic processes, extending through the separator sheath and leading to electrical shorting. The progression of a protrusion relies on a number of in situ and ex situ factors that can be evaluated theoretically through modeling or via laboratory experimentation. However, it is essential to identify the dynamics and mechanism of protrusion outgrowth. This review article explores recent advances in alleviating metal dendrites in battery systems, specifically alkali metals. In detail, we address the challenges associated with battery breakdown, including the underlying mechanism of dendrite generation and swelling. We discuss the feasible solutions to mitigate the dendrites, as well as their pros and cons, highlighting future research directions. It is of great importance to analyze dendrite suppression within a pragmatic framework with synergy in order to discover a unique solution to ensure the viability of present (Li) and future-generation batteries (Na and K) for commercial use.

Chemical Heterointerface Engineering on Hybrid Electrode Materials for Electrochemical Energy Storage

The chemical heterointerfaces in hybrid electrode materials play an important role in overcoming the intrinsic drawbacks of individual materials and thus expedite the in-depth development of electrochemical energy storage. Benefiting from the three enhancement effects of accelerating charge transport, increasing the number of storage sites, and reinforcing structural stability, the chemical heterointerfaces have attracted extensive interest and the electrochemical performances of hybrid electrode materials have been significantly optimized. In this review, recent advances regarding chemical heterointerface engineering in hybrid electrode materials are systematically summarized. Especially, the intrinsic behaviors of chemical heterointerfaces on hybrid electrode materials are refined based on built-in electric field, van der Waals interaction, lattice mismatch and connection, electron cloud bias and chemical bond, and their combination. The strategies for introducing chemical heterointerfaces are classified into in situ local transformation, in situ growth, cosynthesis, and other strategy. The recent progress about the chemical heterointerfaces engineering specially focusing on metal-ion batteries, supercapacitors, and Li-S batteries are introduced in detail. Furthermore, the classification and characterization of chemical heterointerfaces are briefly described. Finally, the emerging challenges and perspectives about future directions of chemical heterointerface engineering are proposed.

MXene-Based Materials for Electrochemical Sodium-Ion Storage

Advanced architecture and rational design of electrode materials for electrochemical sodium-ion storage are well developed by researchers worldwide. MXene-based materials are considered as one of the most potential electrode materials for sodium-ion-based devices, such as sodium-ion batteries (SIBs), sodium-sulfur batteries (SSBs), and sodium-ion capacitors (SICs), because of the excellent physicochemical characteristics of MXenes. Here, in this review, the recent research work and progress, both theoretical and experimental, on MXene-based materials including pure MXenes and MXene-based composites in application of SIBs, SSBs, and SICs are comprehensively summarized. The sodium storage mechanisms and the effective methods to enhance the electrochemical performance are also discussed. Finally, the current critical challenges and future research directions on the development of these MXene-based materials for electrochemical sodium-ion storage are presented.

Rock-Salt MnS0.5Se0.5 Nanocubes Assembled on N-Doped Graphene Forming van der Waals Heterostructured Hybrids as High-Performance Anode for Lithium- and Sodium-Ion Batteries

Manganese-based chalcogenides would be of latent capacity in serving as anodes for assembling lithium- and/or sodium-ion batteries (LIBs/SIBs) due to their large theoretical capacity, low price, and low-toxicity functionality, while the low electroconductivity and easy agglomeration behavior may impede their technical applications. Here, a solid-state solution of MnS_0.5Se_0.5 nanocubes in rock-salt phase has been synthesized for the first time at a relatively low temperature from the precursors of Mn(II) acetylacetonate with dibenzyl dichalcogens in oleylamine with octadecene, and the MnS_0.5Se_0.5 nanocubes have been assembled with N-doped graphene to form a new kind of heterostructured nanohybrids (shortened as MnS_0.5Se_0.5/N-G hybrids), which are very potential for the fabrication of metal-ion batteries including LIBs and/or SIBs. Investigations revealed that there have been dense vacancies generated and active sites increased via nonequilibrium alloying of MnS and MnSe into the solid-solution MnS_0.5Se_0.5 nanocubes with segregation and defects achieved in the low-temperature solution synthetic route. Meanwhile, the introduction of N-doped graphene forming heterojunction interfaces between MnS_0.5Se_0.5 and N-doped graphene would efficiently enhance their electroconductivity and avoid agglomeration of the active MnS_0.5Se_0.5 nanocubes with considerably improved electrochemical properties. As a result, the MnS_0.5Se_0.5/N-G hybrids delivered superior Li/Na storage capacities with outstanding rate performance as well as satisfactorily lasting stability (1039/457 mA h g^-1 at 0.1 A g^-1 for LIBs/SIBs). Additionally, full-cell LIBs of the anodic MnS_0.5Se_0.5/N-G constructed with cathodic LiFePO₄ (LFP) further confirmed the promising future for their practical application.

A V3C2MXene/graphene heterostructure as a sustainable electrode material for metal ion batteries

Individually, MXene and graphene based frameworks have been recognized as promising 2D electrode materials for metal ion batteries. Herein, we have engineered a heterostructure of V₃C₂MXene and graphene using computational design. A comprehensive investigation of designed heterostructure has been reported in this work. Simulated heterostructure has been evaluated for various functionalities such as high performance of thermal stability, metal ion intercalation, diffusion energy using density functional theory method. Interestingly, simulation examinations and obtained calculations demonstrate the high storage capacity of Li and Ca (598.63 mAh g^-1), and Na (555.87 mAh g^-1) with the designed V₃C₂/graphene model. Promising diffusion energy barriers for Li (0.11 eV), Na (0.17 eV) and Ca (0.15 eV) ions are also investigated and have explained systematically in the present work. Moreover, we have achieved high capacity and fast charge/discharge rates of V₃C₂/graphene heterostructure indicating its promising electrode potential efficiency for ion batteries especially for Na ion battery. Thus, our investigation demonstrate the advantages of newly designed V₃C₂MXene and graphene heterostructure for advance metal ion batteries.

Computational insights into modulating the performance of MXene based electrode materials for rechargeable batteries

MXene, a still-growing large family of two-dimensional (2D) materials, has aroused enormous attention in the scientific community. Owing to their high specific surface area, good electronic conductivity, stability, and hydrophilicity, MXene has found a wide application involving electromagnetic interference shielding, sensors, catalysis, and energy storage, etc. In the field of energy storage, MXenes are promising electrode materials for various metal-ion batteries and they are also effective anchoring materials for Li-S batteries. One of the most unique features of MXene is its abundant compositions, which renders us large room to modulate its properties. Besides, other effective approaches applicable to traditional 2D materials can also be used to optimize the performance of MXene. Theoretical calculations have played a significant role in predicting and screening high-performance MXene based electrode materials. So far, theoretical researchers have made much progress in optimizing the performance of MXene as electrode materials for various rechargeable batteries. In the present review, started by a brief introduction of the involved mechanism and basic calculation methods, we comprehensively overview the latest theoretical studies of modulating the performance of MXene based electrode materials for rechargeable batteries.

Interface chemistry of two-dimensional heterostructures - fundamentals to applications

Two-dimensional heterostructures (2D HSs) have emerged as a new class of materials where dissimilar 2D materials are combined to synergise their advantages and alleviate shortcomings. Such a combination of dissimilar components into 2D HSs offers fascinating properties and intriguing functionalities attributed to the newly formed heterointerface of constituent components. Understanding the nature of the surface and the complex heterointerface of HSs at the atomic level is crucial for realising the desired properties, designing innovative 2D HSs, and ultimately unlocking their full potential for practical applications. Therefore, this review provides the recent progress in the field of 2D HSs with a focus on the discussion of the fundamentals and the chemistry of heterointerfaces based on van der Waals (vdW) and covalent interactions. It also explains the challenges associated with the scalable synthesis and introduces possible methodologies to produce large quantities with good control over the heterointerface. Subsequently, it highlights the specialised characterisation techniques to reveal the heterointerface formation, chemistry and nature. Afterwards, we give an overview of the role of 2D HSs in various emerging applications, particularly in high-power batteries, bifunctional catalysts, electronics, and sensors. In the end, we present conclusions with the possible solutions to the associated challenges with the heterointerfaces and potential opportunities that can be adopted for innovative applications.

Nb2CTx MXene as High-Performance Energy Storage Material with Na, K, and Liquid K-Na Alloy Anodes

Two-dimensional MXenes perform well as hosts in batteries, which are promising for next-generation energy storage materials. With low price and high performance, sodium (Na) and potassium (K) own the potential to replace lithium in energy storage devices, but the larger radii and dendrite growth restrict their commercialization. Herein, we successfully synthesized an accordion-like Nb₂CT_x MXene, whose crystal structure integrity and lamellar separation have been confirmed by characterization methods like high-resolution transmission electron microscopy (HR-TEM). Combined with solid Na and K and liquid K-Na alloy as anodes, the Nb₂CT_x MXene shows excellent electrochemical performance, such as high capacity retention after large current shock in tests of rate performance and long time stability for more than 500 cycles, etc. Also, the Nb₂CT_x MXene coupled with liquid K-Na anode performs better than that coupled with solid K for the dendrite-controlling character of the liquid electrode. The Nb₂CT_x MXene would boost the exploitation of more suitable host materials for Na/K-ion batteries and promote an in-depth understanding of MXenes.

Exploring the Efficient Na/K Storage Mechanism and Vacancy Defect-Boosted Li+ Diffusion Based on VSe2/MoSe2 Heterostructure Engineering

As typical 2D materials, VSe₂ and MoSe₂ both play a complementary role in Li/Na/K storage. Therefore, we designed and optimized the VSe₂/MoSe₂ heterostructure to gain highly efficient Li/Na/K-ion batteries. Most importantly, achieving fast Li/Na/K-ion diffusion kinetics in the interlayer of VSe₂/MoSe₂ is a key point. First of all, first-principles calculations were carried out to systematically investigate the packing structure, mechanical properties, band structure, and Li/Na/K storage mechanism. Our calculated results suggest that a large interlayer spacing (3.80 Å), robust structure, and metallic character pave the way for achieving excellent charge-discharge performance for the VSe₂/MoSe₂ heterostructure. Moreover, V and Mo ions both suffer a very mild redox reaction even if Li/Na/K ions fill the interlayer space. These structures were all further verified to show thermal stability (300 K) by means of the AIMD method. By analyzing the Li/Na/K diffusion behavior and the effect of vacancy defect on the structural stability and energy barrier for Li interlayer diffusion, it is found that the VSe₂/MoSe₂ heterostructure exhibits very low-energy barriers for Na/K interlayer diffusion (0.21 eV for Na and 0.11 eV for K). Compared with the VSe₂/MoSe₂ heterostructure, the V_0.92Se_1.84/MoSe₂ heterostructure not only can still maintain a stable structure and metallic character but also has much lower energy barrier for Li interlayer diffusion (0.07 vs 0.48 eV). These discoveries also break new ground to eliminate the obstacles preventing Li⁺ diffusion in the interlayer of other heterostructure materials. Besides, both VSe₂/MoSe₂ and V_0.92Se_1.84/MoSe₂ heterostructures have low average open-circuit voltage (OCV) values during Li/Na/K interlayer diffusion (1.07 V for V_0.92Se_1.84/MoSe₂ vs Li⁺, 0.86 V for VSe₂/MoSe₂ vs Na⁺, and 0.54 V for VSe₂/MoSe₂ vs K⁺), such low OCV values are beneficial for anode materials with excellent electrochemical properties. The above findings offer a new route to design anode materials for Li/Na/K-ion batteries.

Heterogeneous interface design of bimetallic selenide nanoboxes enables stable sodium storage

The heterostructure SnSe2/CoSe2 core encapsulated in a carbon nanobox shell guarantees the structural stability and further ensures stable high performance for sodium ion batteries.

A high-throughput assessment of the adsorption capacity and Li-ion diffusion dynamics in Mo-based ordered double-transition-metal MXenes as anode materials for fast-charging LIBs

Utilizing the latest SCAN-rVV10 density functional, we thoroughly assess the electrochemical properties of 35 Mo-based ordered double transition metal MXenes, including clean Mo2MC2 (M = Sc, Ti, V, Zr, Nb, Hf, Ta) and surface functionalized structures Mo2MC2T2 (T = H, O, F and OH), for the potential use as anode materials in lithium ion batteries (LIBs). The first principles molecular dynamics simulations in combination with the calculations of the site adsorption preferences for Li atoms on all investigated MXenes reveal that both Li-saturated adsorption structures and theoretical capacities of Mo-based MXenes are fundamentally influenced by the surface terminations. We find that the adsorption of Li atoms on either -OH or -F functionalized MXenes is chemically unstable. In particular, the F-groups prefer to form a separate fluoride layer with Li atoms, detaching from the Mo2MC2 substrates. The Li atoms could form a stable single adsorption layer on the -H, -O and intrinsic MXenes surface, exhibiting theoretical capacities in the range from 121 mA h g-1 to 195 mA h g-1. Besides -F and -OH terminations, the remaining Mo-based MXenes also possess superior flat open circuit voltage (OCV) profiles with the most reversible storage capacity below 1.0 V during the charging/discharging cycles. We further predict the low barrier heights of Li-ion diffusion, at a range of 0.03-0.06 eV for most Mo-based MXenes except -O and -H terminations, exceeding that of graphene or Ti3C2. Furthermore, combining the Vineyard transition state theory (TST) with the phonon spectra obtained from density functional perturbation theory (DFPT), the mean planar diffusion coefficient is calculated to be 2 × 10-8 m2 s-1 at 300 K for intrinsic Mo2MC2 monolayers. Although the overall specific capacity is fundamentally restricted with the relatively heavy molecular mass of MXenes, we conclude that Mo-based structures, especially the intrinsic Mo2MC2 (M = Sc, Ti, V) monolayers, might be promising anode materials from the aspect of fast charging/discharging application for LIBs.

Two-dimensional graphene-HfS2 van der Waals heterostructure as electrode material for alkali-ion batteries

Poor electrical conductivity and large volume expansion during repeated charge and discharge is what has characterized many battery electrode materials in current use. This has led to 2D materials, specifically multi-layered 2D systems, being considered as alternatives. Among these 2D multi-layered systems are the graphene-based van der Waals heterostructures with transition metal di-chalcogenides (TMDCs) as one of the layers. Thus in this study, the graphene–hafnium disulphide (Gr–HfS₂) system, has been investigated as a prototype Gr–TMDC system for application as a battery electrode. Density functional theory calculations indicate that Gr–HfS₂ van der Waals heterostructure formation is energetically favoured. In order to probe its battery electrode application capability, Li, Na and K intercalants were introduced between the layers of the heterostructure. Li and K were found to be good intercalants as they had low diffusion barriers as well as a positive open circuit voltage. A comparison of bilayer graphene and bilayer HfS₂ indicates that Gr–HfS₂ is a favourable battery electrode system.

A high rate capacity, moderate volume expansion and energetically stable alkali ion graphene–HfS₂ electrode material.

Fast and Universal Solution-Phase Flocculation Strategy for Scalable Synthesis of Various Few-Layered MXene Powders

MXenes have gained great attention in various fields because of their fascinating properties; however, the preparation of few-layered MXene powders is still limited by serious restacking of MXene nanosheets. Herein, for the first time, we have demonstrated an effective ammonium ion route to fundamentally address restacking and aggregation of the MXene nanosheets, using a solution-phase flocculation method (NH⁴⁺ method and modified NH⁴⁺ method) for large-scale preparation of few-layered Ti₃C₂T_x MXene powders in large quantities. The as-prepared few-layered MXene nanosheet powders show large size in the ab plane without the restacking phenomenon even at scanning electron microscopy measurements of 400× magnification, demonstrating the effectiveness of the proposed method. The method is also suitable for large-scale synthesis of other few-layered MXene powders, including Nb₄C₃T_x, V₂CT_x, Nb₂CT_x, etc., providing a general approach for the preparation of various few-layered MXene nanosheet powders, which represents a significant result for the development of MXenes.

Facile synthesis of CuS/MXene nanocomposites for efficient photocatalytic hydrogen generation

Deposition of covellite CuS nanocrystals on the multilayered MXene and few-layered MXene by a facile reaction of S²⁻ with Cu²⁺ precursors to obtain 0D/2D nanocomposites.

Recent advances in MXenes and their composites in lithium/sodium batteries from the viewpoints of components and interlayer engineering

An up-to-date review about MXenes based on their distinguishing properties, namely, large interlayer spacing and rich surface chemistry.

Modulation engineering of 2D MXene-based compounds for metal-ion batteries

The increasing demand for next generation rechargeable metal-ion batteries (MIBs) has boosted the exploration of high-performance electrode materials. Two-dimensional (2D) transition metal carbides/nitrides (MXenes), the largest family of 2D materials, show extremely competitive potential applications in electrodes due to their excellent electrical conductivity, chemical diversity, and large specific surface area. However, the problems of uncontrollable surface functionalization, interlayer restack and collapse significantly hinder their practical applications. To this end, effective strategies to modify traditional MXenes for targeted electrochemical performance are highly desirable. In this mini review, we briefly summarize the most recent and constructive development in the modulation engineering of 2D MXene-based transition-metal compounds. Firstly, to modify traditional MXenes by intercalating, surface decorating and constructing heterostructures. Secondly, to design novel transition-metal compounds beyond MXenes by precisely controlling the atomic structures, proportions and compositions of constituent elements. Moreover, the critical challenges and perspectives for future research on MXene-based materials are also presented.

Energy storage: The future enabled by nanomaterials

Lithium-ion batteries, which power portable electronics, electric vehicles, and stationary storage, have been recognized with the 2019 Nobel Prize in chemistry. The development of nanomaterials and their related processing into electrodes and devices can improve the performance and/or development of the existing energy storage systems. We provide a perspective on recent progress in the application of nanomaterials in energy storage devices, such as supercapacitors and batteries. The versatility of nanomaterials can lead to power sources for portable, flexible, foldable, and distributable electronics; electric transportation; and grid-scale storage, as well as integration in living environments and biomedical systems. To overcome limitations of nanomaterials related to high reactivity and chemical instability caused by their high surface area, nanoparticles with different functionalities should be combined in smart architectures on nano- and microscales. The integration of nanomaterials into functional architectures and devices requires the development of advanced manufacturing approaches. We discuss successful strategies and outline a roadmap for the exploitation of nanomaterials for enabling future energy storage applications, such as powering distributed sensor networks and flexible and wearable electronics.

Potential Application of Graphene/Antimonene Herterostructure as an Anode for Li-Ion Batteries: A First-Principles Study

To suppress the volume expansion and thus improve the performance of antimonene as a promising anode for lithium-ion batteries, we have systematically studied the stability, structural and electronic properties of the antimonene capped with graphene (G/Sb heterostructure) upon the intercalation and diffusion of Li atoms by first-principles calculations based on van der Waals (vdW) corrected density functional theory. G/Sb exhibits higher Young’s modulus (armchair: 145.20, zigzag: 144.36 N m⁻¹) and improved electrical conductivity (bandgap of 0.03 eV) compared with those of antimonene. Li favors incorporating into the interlayer region of G/Sb rather than the outside surfaces of graphene and antimonene of G/Sb heterostructure, which is caused by the synergistic effect. The in-plane lattice constants of G/Sb heterostructure expand only around 4.5%, and the interlayer distance of G/Sb increases slightly (0.22 Å) at the case of fully lithiation, which indicates that the capping of graphene on antimonene can effectively suppress the volumetric expansion during the charging process. Additionally, the hybrid G/Sb heterostructure has little influence on the migration behaviors of Li on the outside of graphene and Sb surfaces compared with their free-standing monolayers. However, the migration energy barrier for Li diffusion in the interlayer region (about 0.59 eV) is significantly affected by the geometry structure, which can be reduced to 0.34 eV simply by increasing the interlayer distance. The higher theoretical specific capacity (369.03 mAh g⁻¹ vs 208 mAh g⁻¹ for antimonene monolayer) and suitable open circuit voltage (from 0.11 V to 0.89 V) of G/Sb heterostructure are beneficial for anode materials of lithium-ion batteries. The above results reveal that G/Sb heterostructure may be an ideal candidate of anode for high recycling–rate and portable lithium-ion batteries.

First principles study of g-Mg3N2 as an anode material for Na-, K-, Mg-, Ca- and Al-ion storage

Searching for electrode materials for non-lithium metal ion batteries (NLMIBs) is key to the success of NLMIBs. In this work, we investigated the scientific feasibility of using g-Mg₃N₂, which is a novel 2D graphene-like material, as an anode for non-lithium metal-ions (Na, K, Mg, Ca and Al) batteries based on density functional theory calculations. The sequential adsorption energy, Bader charge, intercalation voltage, energy-storage capacity, electronic conductivity and metal-ion diffusion energy barrier are calculated. Results show that the metal-ion intercalation potentials and diffusion energy barriers are suitable for battery application. The maximum specific capacities for Na-, K-, Mg-, Ca- and Al-ion on g-Mg₃N₂ are predicted to be 797, 797, 531, 1594 and 797 mA h g⁻¹, respectively. The excellent structural stability of g-Mg₃N₂ is good for the cycling performance. Moreover, the electronic structure of the g-Mg₃N₂ changes from semiconductor to metal upon metal-ion adsorption, as well as relatively low metal-ion diffusion energy barriers (except for Al-ion diffusion), are beneficial to the charge/discharge rate of the g-Mg₃N₂ anode.

g-Mg₃N₂ is predicted to be a good anode material for non-lithium metal ion batteries.