With the development of modern society, flexible wearable electronic devices have attracted more and more attention.Because of its unique two-dimensional morphology, controllable surface functional groups, metal-like conductivity and good hydrophilicity, MXene is considered to be an ideal electrode material for the preparation of flexible supercapacitors and secondary batteries. Vacuum filtration is the most commonly used method for the preparation of flexible MXene films, but the tight stacking of two-dimensional MXene layers affects the ion transport and the effective utilization of active sites, resulting in poor electrochemical lithium storage performance of MXene membrane electrodes. Therefore, avoiding / reducing the ordered stacking of MXene layers and regulating the interlayer structure of MXene are of great significance to improve the electrochemical performance of MXene film electrodes.



Enhanced Ionic Accessibility of Flexible MXene Electrodes Produced by Natural Sedimentation.
Ning Sun, Zhaoruxin Guan, Qizhen Zhu, Babak Anasori, Yury Gogotsi*, Bin Xu*.
Nano-Micro Lett. (2020) 12:89.
Https://doi.org/10.1007/s40820-020-00426-0.


The highlight of this article.
1. A simple and efficient strategy for preparing flexible MXene membrane electrode: natural sedimentation method is proposed.
2. The prepared MXene membrane electrode has a large layer spacing, which can effectively improve the ion transport between layers and improve the electrochemical performance of the material.
3. Compared with the MXene membrane electrode prepared by traditional vacuum filtration method, the MXene membrane electrode prepared by natural sedimentation method shows high lithium storage capacity and good cycle and rate performance.


Content introduction.
Professor Xu Bin of Beijing University of Chemical Technology and Professor Yury Gogotsi of Drexel University in the United States proposed a new strategy for the preparation of flexible self-supporting MXene membrane electrode by natural sedimentation to solve the problem that the close stacking of nano-layers in MXene membrane electrode seriously affects the electrochemical performance. Compared with the MXene membrane electrode prepared by traditional vacuum filtration method, the interlayer structure of MXene electrode prepared by natural sedimentation method is looser, the interlayer spacing is larger, and the storage and transport performance of ions between layers are significantly improved. When the flexible self-supporting Ti3C2Tx MXene membrane electrode prepared by natural sedimentation method was used as the negative electrode of lithium ion battery, its reversible capacity was up to 351mAh/g, which was much higher than that of MXene membrane electrode prepared by traditional vacuum filtration method (145mAh/g). At the same time, it also showed excellent cycle stability (no capacity attenuation after 1000 cycles) and rate performance.


Guided reading of picture and text.
Morphology and structure characterization of flexible self-supporting MXene membrane electrode.
The aqueous solution containing 10 mg MXene material was diluted to a solution with a concentration of 0.5,1 and 2 mol/L, respectively, and then placed in a filtration device equipped with Celgard 3501 membrane (diameter 4 cm) until the MXene membrane was dried, and the naturally settling MXene membrane (named Nat-0.5, Nat-1 and Nat-2, respectively) was obtained. The MXene membrane (Vac-0.5) obtained by traditional vacuum filtration and drying of MXene solution with 20 mL concentration of 0.5 mol/L was used as control. As shown in figure 1, the electrode prepared by natural sedimentation method shows good mechanical flexibility. At the same time, the structure of the electrode is looser than that of the vacuum filtered MXene electrode Vac-0.5, the thickness of the electrode increases from 3.31um of Vac-0.5 to 4.05um of Nat-0.5 (increased by ~ 22%), and the (002) peak in the XRD spectrum shifts to a low angle, indicating an increase in interlayer spacing (14.0612.20 vs. 14.7612.56). In the process of natural deposition, the smaller the concentration of MXene solution is, the thicker the electrode thickness of MXene thin film is, and the larger the interlayer spacing of (002) crystal plane is.


Figure 1.
(a) digital photographs of Nat-0.5; (c) cross-sectional SEM diagrams of Nat-0.5 and (d) Vac-0.5; and XRD spectra of Vac-0.5,Nat-2,Nat-1 and Nat-0.5.
Electrochemical Lithium Storage behavior of II MXene membrane electrode.
The vacuum filtration and natural settling MXene films were cut into wafers with a diameter of 10 mm and directly used as working electrodes to assemble lithium-ion batteries for electrochemical tests such as CV and galvanostatic charge-discharge. As shown in figure 2, the Vac-0.5 CV curve of the MXene electrode prepared by vacuum filtration has two obvious redox pairs at 1.54pm 2.10V and 0.71max 1.09 V, corresponding to the intercalation and detachment behavior of lithium ions between MXene layers with different layer spacing. However, the CV curve of Nat-0.5 prepared by natural sedimentation method is obviously different from that of vacuum filtered Vac-0.5, and the two groups of redox pairs tend to merge, because the increased interlayer spacing makes the intercalation of lithium ions change from a sequential process to a simultaneous process. In addition, the large interlayer spacing exposes more active sites and improves the storage and transport performance of lithium ion, so the MXene electrode prepared by natural deposition method shows high lithium storage capacity, in which the reversible capacity of Nat-0.5 is 351 mAh/g, which is much higher than that of Vac-0.5 (145 mAh/g).

Figure 2.
The CV curve of Vac-0.5 and Nat-0.5 at the sweep rate of 0.1 mV/s and the charge-discharge curve at the current density of 30 mA/g.