【Research Background】

       MXene materials have been widely studied in the fields of electrochemical energy storage and electrocatalysis due to their high electrical conductivity, large redox active area, rich surface chemistry, and adjustable layer structure. It is worth noting that the electrochemical performance of MXene is closely related to its synthesis conditions, interface chemistry and structural configuration. Recently, the United States Date Maosi Li Wei瑒school professors and professors Taoxin Yong Zhejiang University (Corresponding author) cooperation in high-impact information in the field of new materials Wiley Publishing Group journals InfoMat published an article on the subject for the " Interfacial Structure-based Design of MXene " Nanomaterials for electrochemical energy storage and conversion " review paper, postdoctoral fellow Luo Jianmin is the first author of this article . This paper systematically summarizes the synthesis technology of MXene in recent years, the design of the interface structure of MXene-based nanomaterials and its development in electrochemical energy storage and conversion applications. Among them, the interface structure of MXene nanomaterials mainly includes: (1) MXene layered structure with larger interlayer spacing ; (2) Multi-level structure of MXene nanosheet assembly ; (3) MXene-based hybrid nanostructure (including Mixed nanostructures of one-dimensional and two-dimensional nanomaterials) . In addition, from the perspective of energy storage and electrocatalysis mechanism, the relationship between the interface structure of MXene and its electrochemical performance was deeply analyzed. Finally, it outlines the challenges faced by MXene‘s interface structure design in the future and some of the author‘s insights.

【Graphic introduction】

Figure 1.  Summary of representative MXene synthesis technology, structural design, and electrochemical applications

Figure 2.  Classification, synthesis methods, basic features and electrochemical applications of MXene interface structure design

Figure 3.  Synthesis method and electrochemical application of MXene materials with pillar spacing with larger layer spacing

Table 1. Different types of MXene with larger layer spacing

      In the process of constructing a MXene layered structure with a large interlayer spacing, different intercalating agents can be used, such as cations (including alkali metal ions, metal ions (Sn ²⁺ , Sn ⁴⁺ and Co ^2+, etc.), large-size Cations (surfactant ions CTA ⁺ , STA ⁺ and ammonium ions, etc.), polymers (including PVA, PDDA, PPy, SA, etc.), heteroatoms (including N, S, etc.) and some nanomaterials (including CNT, Graphene , MoS 2 etc.), through different methods (including liquid phase pre-pillar and pillar method, layer assembly method and precursor embedding annealing method, etc.) embedded in the MXene layer, so that the MXene layer spacing increases to varying degrees (1 ~ 3 nm). After the MXene layer spacing is increased, the following advantages can be obtained, for example: (1) The energy storage space between the MXene layers is increased, and the energy storage performance is improved; (2) The ion diffusion path is shortened and the ion migration is faster; Controllable modification of terminal groups on the MXene interface, etc. In the liquid phase pre-pillaring and pillaring method and the precursor embedding annealing method, the research team first tried to pre-pillarize MXene using alkali metal cations / surfactant cations, and then used the ion exchange method to construct Sn ²⁺ / Sn ^{4 +} pillared MXene nanocomposite materials, used as lithium ion battery / capacitor anode materials and metal Na anode frame materials ( ACS Nano  2016, 10, 2491-2499;  ACS Nano 2017, 11, 2459-2469;  Adv. Funct . Mater.2019, 29, 1805946). In addition, the research group used surfactant cations to pre-treat MXene to increase its interlayer spacing, then thermally diffused elemental sulfur into the MXene layers, and then used high-temperature annealing to construct S-atomic pillared MXene nanocomposites, which were applied as sodium ions Battery / capacitor negative material ( Adv. Funct. Mater.  2019, 29, 1808107). Compared with other pillaring agents, cationic surfactants are used as pillaring agents. By screening cationic surfactants of different sizes (for example, CTAB, STAB, DDAC, etc.), the precise adjustment of MXene material layer spacing can be achieved (1 ~ 2.7 nm) ( ACS Nano  2017, 11, 2459-2469), and the surfactant cations embedded in the MXene layer can provide ion exchange sites for the embedding of other metal ions, and realize the construction of different types of pillared MXene materials.

Figure 4.  (AC) 3D hollow MXene sphere preparation process, SEM image and sodium ion storage performance diagram; (DE) wrinkled N-doped Ti 3 C 2 MXene nanomaterial preparation process, SEM image and S carrier Electrochemical performance diagram; (GI) MXene aerogel preparation flow chart, SEM picture and electrochemical performance diagram in supercapacitor; (JL) plush 3D MXene structure preparation process, SEM picture and OER performance diagram  

In addition, through MXene nanosheet assembly and freeze-drying, capillary force assembly and spray drying, and sacrificial template and other methods, MXene materials (including aerogel, hollow spheres and other structures) with multi-level structure are constructed. Such a multi-level structure MXene has a large specific surface area and a large pore volume, which can achieve high ion / electron conductivity, and by designing a multi-level structure MXene can effectively suppress the agglomeration of MXene nanosheets and optimize its electrochemical performance.

Figure 5.  (AC) Zero-dimensional black phosphorus quantum dot / MXene nanocomposite preparation flow chart, TEM picture and lithium ion storage performance diagram; (DF) SnO 2 nanowire / MXene composite material, TEM picture and lithium ion storage performance diagram ; (GI) One-dimensional bacterial cellulose / MXene nanocomposite material flow chart, SEM picture and supercapacitor performance diagram; (JL) Two-dimensional MOF / MXene nanocomposite material, TEM picture and OER performance diagram

      In addition, MXene can be combined with zero-dimensional nanomaterials (including Pt, TiO 2 , P, SnO 2 , Mn 3 O 4 , Ag Etc.), one-dimensional nanomaterials (including CNTs, bacterial cellulose, etc.) and two-dimensional nanomaterials (including LDHs, MOFs, TMDs, etc.) are compounded to obtain MXene-based hybrid nanomaterials. MXene has the following effects in mixed nanostructures: (1) MXene can effectively inhibit the reunion of zero-dimensional / one-dimensional / two-dimensional nanomaterials; effectively fix single atoms; (2) MXene can give nanomaterials high electrical conductivity and effectively relieve Stress generated during service.

        In the outlook part, MXene‘s interface structure design still has many challenges, such as (1) obtaining uniform and controllable terminal groups (eg, -OH, -F, -Cl) on the specified MXene surface is essential for its application Less; (2) A comprehensive understanding of the properties (stability, transport and bonding environment) of intercalators (ionic, molecular) embedded between MXene layers and their impact on the physical, chemical and electrochemical properties of layered MXene It is necessary to conduct systematic research; (3) Understanding the effect of active centers (end groups or interfacial transition metal layers) on ion dynamics and energy storage / electrocatalysis mechanisms from experimental and theoretical perspectives needs further exploration.

【About the Author】

Introduction to the first author:

Jianmin Luo , a postdoctoral fellow at Dartmouth College in the United States, whose research direction is the structure and interface construction of new energy storage materials, published 7 SCI papers with the first author, cited more than 1,500 times, H factor 20, and part of the research results were published in ACS Nano , Adv. Funct. Mater. And other international journals.

Corresponding author profile:

Tao Xinyong is a professor at Zhejiang University of Technology and an associate dean of the School of Materials. His research direction is new carbon materials, advanced secondary battery materials, and won the National Natural Science Foundation Outstanding Youth Fund. More than 150 SCI papers have been published, cited nearly 10,000 times, H factor 52, some research results have been published in international journals such as Nature Communi., Sci. Adv .

Weiyang Li, Assistant Professor of William P. Harris Career Development at Dartmouth College, USA , won the NASA Early Career Faculty Award, Air Force Young Investigator Program Award, etc. The research direction is the design and synthesis of energy materials. More than 70 SCI papers have been published, cited 20,000 times, H factor 54 and some research results have been published in international journals such as Nature Communi., PNAS, Nano Lett .

Literature link: https://onlinelibrary.wiley.com/doi/full/10.1002/inf2.12118

Source: MXene Frontier 

【expand】

The team of Professor Li Weiyuan from Dartmouth College in the United States and Professor Tao Xinyong of Zhejiang University of Technology have been committed to using MXene‘s new two-dimensional materials as electrochemical energy storage in recent years, and have made certain research progress:

1. 1.    JianminLuo, Weiyang Li, * and Xinyong Tao *  et al.  Atomic Sulfur CovalentlyEngineered Interlayers of Ti 3 C 2  MXene for Ultra-FastSodium-Ion Storage by Enhanced Pseudocapacitance.  Adv. Funct. Mater.  2019, 29, 1808107.

2. 2.    Jianmin Luo, Xinyong Tao, * and WeiyangLi *  et al.  Pillared MXene with Ultralarge Interlayer Spacing as a Stable Matrix for High Performance SodiumMetal Anodes, Adv. Funct. Mater.  2019, 29, 1805946.    

3. 3.    Jianmin Luo, Weiyang Li * and Xinyong Tao *  et al.  Tunable Pseudocapacitance Storage of MXene by Cation Pillaring for High Performance Sodium-IonCapacitors,  J. Mater. Chem. A , 2018, 6, 7794-7806.

4. 4.    Jianmin Luo, Wenkui Zhang, Xinyong Tao * et al.  Pillared Structure Design ofMXene with Ultralarge Interlayer Spacing for High-Performance Lithium-IonCapacitors.  ACS Nano  2017, 11, 2459-2469.

5. 5.    JianminLuo, Xinyong Tao, * Wenkui Zhang *  et al. Sn ⁴⁺  ion decorated highly conductive Ti 3 C 2 MXene: Promising lithium-ion anodes with enhanced volumetric capacity and cyclic performance.  ACS Nano 2016 , 10, 2491-2499.