MXene/Graphene Heterostructures as High-Performance Electrodes for Li-Ion Batteries

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.

In Situ Fabricating Oxygen Vacancy-Rich TiO2 Nanoparticles via Utilizing Thermodynamically Metastable Ti Atoms on Ti3C2Tx MXene Nanosheet Surface To Boost Electrocatalytic Activity for High-Performance Li-O2 Batteries

Catalysts with high performance are urgently needed in order to accelerate the reaction kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in lithium-oxygen (Li-O₂) batteries. Herein, utilizing thermodynamically metastable Ti atoms on the Ti₃C₂Tx MXene nanosheet surface as the nucleation site, oxygen vacancy-rich TiO₂ nanoparticles were in situ fabricated on Ti₃C₂Tx nanosheets (V-TiO₂/Ti₃C₂Tx) and used as the oxygen electrode of Li-O₂ batteries. Oxygen vacancy (Vo) can boost the migration rate of electrons and Li⁺ as well as act as the active sites for catalyzing the ORR and OER. Based on the above merits, V-TiO₂/Ti₃C₂Tx-based Li-O₂ battery shows improved performance including the ultralow overpotential of 0.21 V, high specific capacity of 11 487 mA h g^-1 at a current density of 100 mA g^-1, and excellent round-trip efficiency (93%). This work proposes an effective strategy for researching high-performance oxygen electrodes for Li-O₂ batteries via introducing Vo-rich oxides on two-dimensional MXene.

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.

Fast-Charging High-Energy Battery-Supercapacitor Hybrid: Anodic Reduced Graphene Oxide-Vanadium(IV) Oxide Sheet-on-Sheet Heterostructure

The battery-supercapacitor hybrid (BSH) device has potential applications in energy storage and can be a remedy for low-power batteries and low-energy supercapacitors. Although several studies have investigated electrode materials (particularly for a battery-type anode material) and design for BSHs, the energy density and power density are insufficient (far from the levels required for practical applications). Herein, a hierarchical vanadium(IV) oxide on reduced graphene oxide (rGO@VO₂) heterostructure as an anode and activated carbon on carbon cloth (AC@CC) as a cathode are proposed for fabricating an advanced BSH. The mixed valency of V ions inside the as-prepared VO₂ matrix (V³⁺ and V⁴⁺) facilitates redox reactions at a low potential, giving rise to rGO@VO₂ as a typical anode with a working potential of 0.01-3 V (vs Li/Li⁺). The sheet-on-sheet heterostructured rGO@VO₂ yields a high specific capacity of 1214 mAh g^-1 at 0.1 A g^-1 after 120 cycles, with a high rate capability and stability. The rGO@VO₂//AC@CC BSH device exhibits a maximum gravimetric energy density of 126.7 Wh kg^-1 and a maximum gravimetric power density of ∼10 000 W kg^-1 within a working voltage range of 1-4 V. Moreover, it exhibits fast charging times of 5 and 834 s with energy densities of 15.6 and 82 Wh kg ^-1, respectively.

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

The adsorption and diffusion of Na, K, and Ca atoms on MXene/graphene heterostructures of MXene systems Sc₂C(OH)₂, Ti₂CO₂, and V₂CO₂ are systematically investigated by using first-principles methods. We found that alkali metal intercalation is energetically favorable and thermally stable for Ti₂CO₂/graphene and V₂CO₂/graphene heterostructures but not for Sc₂C(OH)₂. Diffusion kinetics calculations showed the advantage of MXene/graphene heterostructures over sole MXene systems as the energy barriers are halved for the considered alkali metals. Low energy barriers are found for Na and K ions, which are promising for fast charge/discharge rates. Calculated voltage profiles reveal that estimated high capacities can be fully achieved for Na ion in V₂CO₂/graphene and Ti₂CO₂/graphene heterostructures. Our results indicate that Ti₂CO₂/graphene and V₂CO₂/graphene electrode materials are very promising for Na ion battery applications. The former could be exploited for low voltage applications while the latter will be more appropriate for higher voltages.

Heterostructures of doped graphene and MoX2 (X = S and Se) as promising anchoring materials for lithium–sulfur batteries: a first-principles study

The heterostructures composed of a MoX₂ (X = S or Se) nanosheet and doped graphene with B or N exhibit superior anchoring effects for soluble Li₂S_n species.

Applications of 2D MXenes in energy conversion and storage systems

This article provides a comprehensive review of MXene materials and their energy-related applications.