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Two-dimensional transition metal carbide/nitride MXenes have attracted extensive research attention in many research fields, including electrochemical energy storage. Since their discovery in 2011, more than 30 MXenes with different chemical compositions have been synthesized as a new type of 2D material system. Benefiting from their controllable properties, such as surface chemistry, electrical conductivity, and 2D structure, MXenes are considered to be a very promising class of materials. Professor Patrice Simon from the University of Toulouse, Switzerland, and Professor Lin Zifeng from Sichuan University published a short review titled MXenes as High-Rate Electrodes for Energy Storage in the internationally renowned academic journal Trendsin Chemistry, summarizing the advantages of MXene materials as high-rate electrodes. Electrochemical performance, especially the research progress and breakthroughs in non-aqueous electrolyte environments, and prospects for future research directions.

Figure 1. Chemical composition of MAX phase and MXene.

The MXenes etched by HF or HCl/LiF mainly contain -O, -OH and -F functional groups. Quantitative NMR spectroscopy revealed that the HF etched Ti3C2TxMXene had nearly four times more -F functional groups, more -OH and less - compared to HCl/LiF etching. O. At the same time, different concentrations of HF also lead to different compositions of functional groups on the surface of MXene.

Figure 2. Electrochemical characteristics of MXene in aqueous electrolytes.

Figure 3. Electrochemical behavior of MXene-based electrochemical capacitor electrodes in conventional non-aqueous electrolytes.

Figure 4. Electrochemical behavior of MXene electrodes in organic electrolytes containing Li or Na.

In the future, researchers need to conduct in-depth research in the following aspects:

1) Taking the Lewis acid molten salt method as an example, we explore an environmentally friendly, safe, efficient and batch acoustic field method to expand the MAX phase precursor and control the preparation of the surface composition of MXenes.

2) About 70% of all MXene research work is focused on the first discovered MXene, namely Ti3C2Tx. For other types of MXenes, although the preparation process is more complicated, we need to conduct in-depth exploration from both theoretical and experimental aspects.

3) Although MXene electrodes can achieve higher capacity in aqueous electrolytes, considering the wider voltage window, the performance of MXene in non-aqueous electrolytes should be further investigated. The optimal control of the composition and nature of the functional groups on the surface of MXene, as well as the control of the interlayers, is crucial for the performance improvement.

4) For electrochemical capacitors and high-power batteries that need to meet high power and high energy density at the same time, design 3D porous, vertically aligned or other structured MXene electrodes without sacrificing too much volumetric energy density. Ion transport paths in electrodes, which will be the focus of development in the coming years.

Literature link:

https://doi.org/10.1016/j.trechm.2020.04.010