MXene is a type of layered material of transition metal carbide or nitride. The layers are mainly connected by van der Waals force, which has a series of excellent physical and chemical properties. For example, MXene has good hydrophilicity, adjustable interlayer spacing and surface functional groups. Variety and other characteristics. In terms of structure, MXene is composed of alternating carbon layers and transition metal layers, giving MXene good electrical conductivity and pseudocapacitance characteristics; in terms of composition, compared to single-element two-dimensional materials, MXene contains two elements of M and X (MXene solid solution) And various types of valence bond components between MX give MXene more abundant control space. Reasonable use of the structure and composition characteristics of MXene can prepare electrode materials with excellent performance. Therefore, since its inception, MXene has performed well in the field of energy storage and has high expectations.

Professor Sun Zhengming of School of Materials Science and Engineering of Southeast University has conducted a lot of research work on MXene two-dimensional electrode materials and their applications in the field of energy storage. , This year has published many papers in high-impact journals such as Advanced Functional Materials, Nanoscale, 2D Materials and so on.

1. Revealed the chemical modification mechanism of MXene two-dimensional electrode material

Chemical modification is an effective way to improve the electrochemical performance of two-dimensional materials. At present, a lot of research work has been carried out on the chemical modification of graphene. Taking nitrogen doping as an example, experimental characterization and theoretical simulation results show that the nitrogen element is mainly pyrrolic, pyridinic and quaternary ) Three forms exist in the graphene structure, and by affecting the electronic structure of the material, improve its wettability with the electrolyte, thereby improving the electrochemical performance of the electrode material. As a new type of two-dimensional electrode material, MXene has excellent application prospects in the field of supercapacitors because of its advantages of good conductivity, fast charge response and pseudo-capacitance characteristics. Taking Ti 3 C 2 as an example, the two-dimensional material has a multilayer structure of T-Ti-C-Ti-C-Ti-T, where T is a surface functional group introduced during etching, such as -F, -OH And -O etc. This special structure gives Ti 3 C 2 excellent composition design and structure control space. The chemical modification of the Ti 3 C 2 two-dimensional electrode material has been reported in some research work, but the existence of doping elements, especially the mechanism of the influence on the electrochemical performance of the material is still controversial. To solve this problem, the research team successfully combined the experimental characterization and first-principles calculation research methods to successfully reveal the nitrogen doping mechanism of Ti 3 C 2 and clarify the contribution mechanism of doping elements to the electrochemical performance. Modification provides theoretical guidance.

(1) Determine the existence site of doped N in Ti 3 C 2

In order to reveal the possible existence forms of doped nitrogen in Ti 3 C 2 , the first-principles simulation method was used to calculate the defect formation energy of all doped structures, mainly considering surface adsorption, functional group substitution and lattice substitution possibility. The calculation results show that the -O functional group on the surface has a certain adsorption effect on the N atom, thereby forming a Ti-ON composite bond, and the corresponding formation energy is -2.87 eV; the -OH functional group on the surface may be replaced by the N atom to form -N / -NH functional group, the corresponding formation energy is -4.71 eV; the C atoms of the lattice may also be replaced by N atoms, and the formation energy of the corresponding process is -1.31 eV. Therefore, in the Ti 3 C 2 structure, nitrogen doping may exist in three forms: surface adsorption, functional group substitution and lattice substitution

figure 1

First-principle simulation of Ti 3 C 2 nitrogen doping:

a). Schematic diagram of the atomic structure of possible doping sites;

b). Formation energy calculation results;

c). Ti 3 C 2 supercell schematic diagram of energy feasible doping sites ;

d). Transition state energy.

(2) Clarified the effect mechanism of N doping on the electrochemical performance of Ti 3 C 2

The results of electrochemical performance tests found that the three forms of nitrogen doping can increase the specific capacity of Ti 3 C 2 two-dimensional electrode materials. The analysis shows that the total capacity is composed of two parts: surface control and diffusion control. Among them, the electric double layer capacitance controlled by the surface is determined by the microstructure (interlayer distance) of the material, and the surface pseudocapacitance is provided by the functional group (-O / -N) or the group adsorbed on the surface (N / NH); and the diffusion control part It is affected by the valence state of the outer Ti atoms, that is, the number of outer nuclear orbits.

figure 2

Orbital analysis and electrochemical performance of N-Ti 3 C 2 :

a) Orbital analysis of element N;

b) Orbital analysis of O element;

c) Orbital analysis of Ti element;

d). CV curve of N-Ti 3 C 2 ;

e) Contribution analysis of capacitance-diffusion behavior;

f) Impedance spectrum of N-Ti 3 C 2 .

The research results were published in Advanced Functional Materials.

Original link: https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.20200085

The research group‘s Lu Chengjie and Dr. Yang Li are co-first authors, and associate professors Zhang Wei and Sun Zhengming are co-corresponding authors.

2. Multi-dimensional construction of MXene hydrogel and application in flexible supercapacitors

The development of portable, miniaturized, and wearable electronic devices requires energy storage systems that have both flexibility and excellent electrochemical performance. It is very attractive to design traditional energy storage devices such as supercapacitors into flexible structures to power these wearable devices. Electrode materials are an important part of flexible supercapacitors, and they must have electrochemical activity, mechanical strength, and even stretchability during use. The focus will shift from traditional materials (such as metal oxides and carbon materials) to intrinsically flexible elastomeric polymers. Conductive hydrogel is an elastomeric polymer that combines the advantages of electrochemical activity and three-dimensional polymer network. The challenge is that the mechanical flexibility and electrochemical capacity of the conductive hydrogel must simultaneously meet the requirements for the use of flexible electrodes. Compounding conductive two-dimensional nanosheets into a polymer insulating matrix is considered to be one of the most effective strategies for enhancing conductive hydrogels. While serving as a reinforcing phase, it can also retain the inherent structural advantages and functions of these two-dimensional nanosheets. Graphene can be used as the reinforcing phase of conductive hydrogels due to its excellent electrochemical and mechanical properties, but most graphene hydrogels are formed by the self-assembly of graphene oxide by hydrothermal reduction, which will cause the water of graphene The hydrophilicity of the gel becomes poor, thereby preventing the electrolyte solution from infiltrating. The new two-dimensional material MXene can cooperate with the three-dimensional polymer network to enhance the electrochemical activity and mechanical flexibility of the conductive hydrogel, and is expected to become a candidate material for flexible energy storage. Through reasonable adjustment of the performance of Ti 3 C 2 , two-dimensional Ti 3 C 2 nanosheets were combined with one-dimensional conductive polypyrrole (PPy) nanofibers and three-dimensional polyvinyl alcohol (PVA) hydrogel matrix. A ternary (1, 2 and 3D) conductive hydrogel electrode with excellent flexibility and electrochemical capacity was prepared. Thanks to this unique hierarchical design, the multi-dimensional components are assembled into interconnected porous nanostructures, which not only effectively suppresses the serious stacking of MXene nanosheets, but also promotes the diffusion of the electrolyte solution throughout the network, showing excellent capacitance characteristics And mechanical properties.

(1) Realize multi-dimensional hydrogel structure design

In order to realize this idea, the MXene / PPy-PVA composite hydrogel was prepared by the freeze-thaw cycle method. The stripped Ti 3 C 2 (MXene) few-layer two-dimensional nanosheets have a size between 10 and 30 μm, which can increase the contact area with the electrolyte solution to achieve better ion transmission efficiency. MXene-PVA hydrogels have interconnected networks consisting of thin PVA outer walls ranging in size from nanometers to hundreds of microns. After PPy is incorporated, the composite hydrogel retains the macrostructure of PPy nanofibers interwoven with MXene nanosheets, and forms a good conductive network. The energy dispersion spectrum of MXene / PPy-PVA composite hydrogel shows the distribution relationship between nitrogen and titanium, confirming that PPy nanofibers grow around MXene nanosheets, effectively inhibiting MXene nanosheets in hydrogel Stack again. The MXene-PVA composite hydrogel has the characteristic Raman peaks of both MXene and PVA, indicating that MXene and PVA have good compatibility. The ternary multi-level nanostructure of MXene / PPy-PVA hydrogel can provide a larger usable specific surface area to promote ion diffusion and electron transport, thereby enhancing capacitance characteristics.

image 3

(a) Schematic diagram of preparation of MXene / PPy-PVA hydrogel;

(bd) Microstructure of MXene, MXene-PVA hydrogel and MXene / PPy-PVA hydrogel

(e) Raman spectrum of Mxene composite hydrogel.

(2) MXene hydrogel has excellent mechanical properties

Compared with pure PVA hydrogel, MXene-PVA hydrogel shows higher mechanical flexibility due to nano-enhancement than pure PVA hydrogel. MXene-PVA hydrogel can maintain and restore its original shape under various deformations (such as elongation, compression and knotting). At a fixed PVA concentration of 10 wt%, as the MXene concentration increases from 0.2 to 1 mg mL ^-1 , the MXene hydrogel increases its tensile strength from 1.0 to 5.4 MPa and the elastic modulus from 0.5 to At 1.8 MPa, the deformation energy is increased from 1.4 to 9.0 MJ m ^-3 , while retaining about 300% of similar fracture strain. Through the synergy of one-dimensional nanofibers and two-dimensional nanosheets, the maximum tensile strength of MXene / PPy-PVA hydrogel is 10.3 MPa, which is nearly double the maximum tensile strength of MXene-PVA hydrogel of 5.4 MPa , Which is in sharp contrast to the maximum tensile strength of pure PVA hydrogel, 0.6 MPa.

Figure 4

(ac) Mechanical properties of MXene hydrogel;

(d) Schematic diagram of MXene / PPy-PVA cross-linked network and its reinforcement mechanism under deformation.

(3) MXene hydrogel has high-quality specific capacitance and excellent cycle stability

Compared with MXene-PVA hydrogel, MXene / PPy-PVA hydrogel has higher electric double layer capacitance and larger working voltage window. Introducing PPy can widen the interlayer space between MXene nanosheets and effectively increase the ion exchange interface area. MXene / PPy-PVA hydrogel in specific capacitance A G. 1 ^-1 at a current density of the G F. To 614 ^-1 . In addition, MXene / PPy hydrogels also exhibited surprising capacity retention (100% of their original capacitance after 10,000 cycles) and high coulombic efficiency (99.6%).

Figure 5

(ac) Electrochemical characterization of MXene hydrogel electrodes;

(d) Comparison of cyclic voltammetry curves with MXene-PVA and MXene / PPy-PVA hydrogels at a scan rate of 100 mV s ^{− 1} ;

(e) Comparison of specific capacitance at different current densities;

(f) Comparison of mechanical and electrochemical properties of MXene / PPy hydrogel. The research results were published on Nanoscale.

Original link: https://pubs.rsc.org/en/content/articlelanding/2020/nr/d0nr01414a#!divAbstract

Associate Professor Zhang Wei and graduate student Ma Jing of the research group are co-first authors, and Associate Professor Zhang Wei and Professor Sun Zhengming are co-corresponding authors.

3. Electrostatic self-assembly MXene / carbon ball composite system as an excellent sulfur carrier for lithium-sulfur batteries

Lithium-sulfur batteries have high theoretical energy density (~ 2600 Wh Kg ^-1 ) and high theoretical specific capacity (1675 mAh g ^-1 ); at the same time, elemental sulfur is cheap, non-toxic and harmless, and can meet the needs of new energy electric vehicles and large-scale The demand for renewable energy is considered to be one of the most promising next-generation lithium secondary batteries. However, the adsorption of elemental sulfur in the sulfur positive electrode, the shuttle effect caused by the intermediate product polysulfide, the low conductivity of the final product lithium sulfide, and the volume change of up to 80% limit the commercialization of lithium-sulfur batteries. Based on the above requirements, the research team prepared a composite material (HPCSs @ d-Ti 3 C 2 ) with a sandwich structure of hollow porous carbon spheres (HPCSs) @MXene using electrostatic self-assembly method , which is expected to serve as a sulfur host for lithium-sulfur battery cathodes. Improve the rate performance and cycle stability of lithium-sulfur batteries. The main strategy is: use two porous conductive materials HPCSs and d-Ti 3 C 2 to build a stable three-dimensional conductive network to realize the rapid migration of electrons and enhance the conductivity of the electrode; use the unique hollow structure of HPCSs to increase the load of elemental sulfur Volume and provide space for volume expansion, and at the same time limit the shuttle of polysulfides at the physical level; the use of the polar surface of d-Ti 3 C 2 to chemically adsorb polysulfides, effectively reduce the diffusion inhibition shuttle, and achieve the physical limit Synergy with chemical adsorption to improve the electrochemical performance of lithium-sulfur batteries.

(1) Electrostatic self-assembly to prepare sandwich structures of HPCSs @ Ti 3 C 2

In order to realize the self-assembly of HPCSs and MXene, polydimethyldiallyl ammonium chloride (PDDA) is used to adjust the surface charge of HPCSs, and the negatively charged d-Ti 3 C 2 is electrostatically assembled to form a stable The HPCSs-MXene-HPCSs sandwich stable structure is shown in Figure 6. The d-Ti 3 C 2 nanosheets constitute a stable three-dimensional cross-linked conductive network skeleton, which effectively improves the electrical conductivity of the composite material. HPCSs are uniformly and tightly fixed on both sides of the skeleton by electrostatic attraction. With the help of electrostatic repulsion, there is a clear space between each HPCS. This composite material with a porous structure, a conductive network and a polar surface can alleviate the shuttle effect of the polysulfide of the discharge intermediate product through physical confinement combined with chemical adsorption, improve the conversion kinetics of polysulfide and improve The polarization of the electrode, thereby improving the electrochemical performance.

Image 6

(a) Preparation flow chart of HPCSs @ d- Ti 3 C 2 ;

(Bd) SEM picture and EDS elemental analysis energy spectrum;

(Eg) TEM and HRTEM images.

(2) HPCSs @ d- Ti 3 C 2 / S has excellent rate performance and cycle stability

HPCSs @ d-Ti 3 C 2 has a high specific surface area and a rich pore structure, which provides ample confined space for the containment of elemental sulfur. Under a 75% sulfur load, HPCSs @ d-Ti 3 C 2 / S serves as The sulfur positive electrode exhibits excellent electrochemical performance. The introduction of the MXene conductive network has effectively reduced the voltage lag of the lithium-sulfur battery, reduced the polarization degree of the electrode and accelerated the kinetics of the redox reaction of the sulfur positive electrode, which is beneficial to the improvement of the rate performance of the lithium-sulfur battery. Therefore, HPCSs @ d-Ti 3 C 2 / S showed better rate performance. When the current density returned to 0.1 C and 1 C, the capacity did not decrease significantly (Figure 7c). HPCSs @ d-Ti 3 C 2 / S electrode has excellent capacity retention at 100 cycles of 0.2 C cycle and 500 cycles of 1 C cycle. At a current density of 1 C, the average capacity decay rate per turn is only 0.069%. These results fully demonstrate that the construction of the MXene conductive network not only improves the electrochemical performance of the HPCSs / S electrode, but the structural characteristics of the sandwich also contribute to the stability of the electrode.

Picture 7

The electrochemical performance of HPCSs @ d-Ti 3 C 2 :

(a) Comparison of CV curves at a scan rate of 0.2mV · s ^{− 1} ;

(b) Comparison of voltage lag between charge and discharge curves;

(c) Rate performance;

(d) Constant current charge and discharge performance at 0.2C current density;

(e) Long cycle life curve at 1.0C current density;

(f) Comparison of cyclic decay rates.

The research work was published in the international journal 2D Materials.

Original link: https://iopscience.iop.org/article/10.1088/2053-1583/ab79c1

Source of information: Frontiers of Polymer Science