【Research Background】

In recent years, with the environmental problems brought about by global economic growth and the increasing shortage of fossil energy, the development of energy conversion and storage systems in the whole society has become increasingly demanding. Electrochemical energy storage, including batteries and pseudocapacitors, has become a new and efficient practical energy storage technology, which has promoted the rapid development of different applications from wearable electronics to electric vehicles. The Faraday nature of charge storage requires the release of electrons during the oxidation and reduction of electrochemically active materials. The multivalent state and high theoretical capacity of transition metal oxides are one. There are many uses of transition metal oxides as energy storage materials. Advantages, its richness in nature, low cost, controllable morphology and structure can directly control its electrochemical performance. When used as an electrode material for supercapacitors, it not only stores charge in the electric double layer, but also undergoes a rapid and reversible surface redox reaction. Oxidatively active TMOs have a pseudocapacitance 10 to 100 times higher than that of traditional carbon materials. Very ideal system. Recently, Professor Jilei Liu of Hunan University published a title in the internationally renowned academic journal Advanced Energy Materials : The Role of Cation Vacancies in Electrode Materials for Enhanced Electrochemical Energy Storage: Synthesis, Advanced Characterization and Fundamentals review article, which systematically summarizes the cation vacancy groups The latest progress of electrochemical energy storage materials, including the corresponding synthesis methods and characterization techniques, as well as positioning its role in practical applications from the perspective of chemical materials, key challenges and opportunities for future development, especially transition metal oxides with cation defects And the emerging transition metal carbide (MXene).

【Graphic introduction】

Figure 1. Summary of "external" and "internal" methods to improve electrochemical energy storage performance.

Figure 2. Summary of cation vacancy synthesis, characterization, and electrochemical energy storage in chronological order.

Figure 3. Summary of synthetic strategies introducing cationic defects.

Figure 4. Models used for DFT calculations of OMS-2, Ce-OMS-2 and Ce 2 -OMS-2; the formation process of Ti vacancies in TiO 2 and the structure of a single layer of MnO 2 .

Figure 5. Mesoporous Mg-Co 3 O 4 achieved by leaching of Mg cations . Co / Fe vacancy obtained by Ar plasma stripping CoFe LDH.

Figure 6. Mn vacancy model of δ-MnO 2 nanosheets, samples treated with different pH values.

Figure 7. Raman spectra of TiO 2 and its corresponding anatase samples prepared at 90, 110 and 130 degrees Celsius; infrared spectroscopy of Mo-doped MnO 2 ; -OH at specific vacancy sites in the range of 1300-1800 cm ^-1 Vibration; PL spectrum of NiO at different processing temperatures; positron lifetime spectrum of BiVO 4 rich in V and a small amount of V.

Figure 8. MnO 2 differential charge density and DOS without defects and with Mn defects .

Figure 9. DFT calculation of intercalation potentials ofLi, Mg and Al in anatase TiO 2 and F-doped TiO 2 ; Zn2 + intercalation and deintercalation in spinel ZnMn2O4, and presence and absence of Mn defects Ion diffusion path and GITT test results in the presence.

【Summary and Outlook】

This paper summarizes the research progress of cationic defects of transition metal oxides / carbides for electrochemical energy storage. Many effective strategies can be used to promote the formation of cation defects, adjust the concentration of vacancies, including variable cation / anion doping, maintain balance in solutions with different pH values, and selectively remove cations from the components. Annealing and plasma etching in the atmosphere. The direction of future research is mainly reflected in the following aspects:

i) Quantitative determination of cation vacancies and their spatial distribution, especially in highly disordered nanostructures, is still challenging.

ii) The defect structure is generally metastable. Therefore, cationic defect materials (especially those with large specific surface area and porous nanostructured materials) often encounter physical and chemical instability problems.

iii) To accurately understand the role of cation vacancies, we need to exclude other factors that affect the performance of metal oxides / carbides.

iv) Although the presence of cationic vacancies leads to an increase in the storage charge capacity and rate, it is not clear how cation vacancies improve the energy storage performance of metal oxides / carbides.

Literature link:

https://doi.org/10.1002/aenm.201903780.

Source: MXene Frontier

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