Professor Lin He, JPS view: High-performance zinc ion battery positive electrode material based on innovative ion exchange mechanism [Article information] High-performance anti-spinel Mg2VO4 zinc ion battery positive electrode material based on ion exchange mechanism First author

Professor Lin He, JPS view: High-performance zinc ion battery positive electrode material based on innovative ion exchange mechanism

[Article information]

High-performance anti-spinel based on ion exchange mechanism Mg2VO4 zinc ion battery positive electrode material

High-performance reverse spinel based on ion exchange mechanism Mg2VO4 zinc ion battery positive electrode material

First author: Zhang Yu

Corresponding author: Lin He*

Unit: Xinjiang University

[Research background]

water-based zinc ion batteries (ZIBs) stand out among the next generation of large-scale energy storage equipment due to their advantages of high safety, low cost and environmental friendliness. However, due to its inherent slow diffusion kinetics, this energy storage technology is still troubled by the lack of high-quality positive electrode materials. The existing positive electrode materials of aqueous zinc ion batteries are mainly divided into two types in terms of structure, layered structure and tunnel structure. As a unique structure, spinel structure has a large internal three-dimensional spatial structure, showing great potential for zinc storage. Here, guided by theoretical calculation, we propose a new strategy in anti-spinel Mg2VO4 to promote Zn2+ migration based on ion exchange and improve Zn2+ diffusion kinetics. This work provides a new direction for the development of high-performance ZIBs positive electrode materials.

[Article Introduction]

In order to improve the diffusion dynamics of Zn2+, recently, Lin He's research group at Xinjiang University published an article titled "Ion-Exchange-Induced High-Performance of Inverse Spinel Mg2VO4 for Aqueous Zinc-Ion Batteries" in the journal Journal of Power Sources. This article proposes a high-performance zinc ion battery positive electrode material based on ion exchange mechanism - inverse spinel Mg2VO4. It also reveals the essential causes of this ion exchange phenomenon and its positive effect on the performance of electrochemical through molecular dynamics simulation (AIMD) and density functional theory calculation (DFT). It was verified through experiments.

[Big points of this article]

Key points of first principles molecular dynamics simulation (AIMD) reveals the trend of ion exchange in Mg2VO4

​ AIMD simulation was carried out by constructing the Zn2+ embedded Mg2VO4 model. The results show that the diffusion coefficient of Mg2+ in reverse spinel Mg2VO4 is much higher than that of Zn2+ (7.741×10−7 cm2 s−1). Due to the huge difference in diffusion coefficient, during the electrode cycle, Mg2+ with a higher diffusion coefficient preferentially breaks out of the Zn2+ body, and there is a tendency to exchange with Zn2+. Therefore, this reveals the underlying reason for the existence of Zn2+ and Mg2+ exchange trends in anti-spinel Mg2VO4. At the same time, Mg2+ produced vacancy when it was released from the Mg2VO4 frame, which also promoted the migration of Zn2+.

Figure 1. Molecular dynamics simulation of Mg2VO4

Point 2: Density functional theory calculation (DFT) reveals the positive effect of ion exchange on electrochemical performance

Through DFT calculation, the positive effect of ion exchange on electrochemical systems is further revealed. Density of state (DOS), differential charge, Bader charge analysis, and diffusion energy barrier show that due to the existence of this ion exchange, it not only enhances the conductivity of the material, but also alleviates the strong electrostatic interaction between Zn2+ and the vanadium oxygen frame, which is conducive to the improvement of diffusion kinetics. Further diffusion energy barrier calculations show that the diffusion energy barrier of Zn2+ in the system after ion exchange is only 0.313 eV, which is lower than 0.464 eV in the original Mg2VO4, indicating that Zn2+ is more likely to diffuse in the system after ion exchange.

Figure 2. DFT calculation of Mg2VO4

Key points three: Characterization of synthesis, morphology, structure and other properties of Mg2VO4

For the first time, the reverse spinel Mg2VO4 was prepared by a simple sol gel method and used for the positive electrode of the aqueous zinc ion battery. The XRD refining results show that the prepared material has a high purity and crystallinity, and the prepared material presents uniform microspheres at microscopic.

Figure 3. Characterization of the structure, morphology and other properties of Mg2VO4

Keypoint 4: Characterization of electrochemical properties of Mg2VO4 positive electrode material

In addition to pure Mg2VO4, the same method was used to introduce glucose as a carbon source to prepare the Mg2VO4 composite material. The prepared material was made into electrodes and matched with the zinc negative electrode to construct a zinc ion battery, and its zinc storage performance was evaluated. The results show that thanks to this special exchange action, the electrode undergoes a significant capacity increase in the second circle and maintains good stability, showing excellent rate performance. In addition, due to the introduction of glucose, the conductivity of Mg2VO4@C has been significantly improved, showing higher capacity and more stable cycling performance. After 1000 cycles under 1 A g−1, the capacity retention rates of Mg2VO4 and Mg2VO4@C are both 100%, and their retention capacity is 74.4 and 102 mAh g−1, respectively.

Figure 4. Characterization of zinc storage performance of Mg2VO4 and Mg2VO4@C

Key points 5: Research on energy storage mechanism of Mg2VO4 positive electrode material

The zinc storage mechanism during the process was investigated through non-in-situ XRD, SEM, XPS and ICP-AES. The results show that after the first complete charge, part of Zn2+ remained in Mg2VO4 and failed to escape, forming a Znx-Mg2VO4 solid solution as a new active material, showing a highly reversible Zn2+ embedding/exit behavior in the subsequent cycle. In addition, during the cycle, the SEM shows that while zinc is detected at the positive electrode, the signal of Mg is detected on the zinc metal of the negative electrode. ICP-AES also shows the decrease in Mg content in the positive electrode material (Mg2VO4) and the increase in the Zn content, which fully demonstrates the occurrence of ion exchange.

Figure 5. Exploration of zinc storage mechanism of Mg2VO4 electrode

Key points 6: Construction and application of quasi-solid state batteries

Use PVA hydrogel electrolyte to construct a flexible quasi- solid state battery and test its electrochemical performance. The assembled batteries all show a stable discharge curve under different bending conditions (0°, 90°, 180°), and maintain a stable open circuit voltage . As a potential application demonstration, we assemble different types of flexible quasi-solid state batteries that can supply stable power to LEDs under different bending conditions. At the same time, only one battery device can supply stable energy to the small hygrometer for more than 72 hours, showing broad application prospects.

Figure 6. Construction and application of Mg2VO4 quasi-solid state battery

This paper first discovered through molecular dynamics simulation that Mg2+ and Zn2+ have a tendency to ion exchange in anti-spinel Mg2VO4. Density functional theory calculations further show that this ion exchange can alleviate the electrostatic effect between Zn2+ and the host material and reduce the diffusion energy barrier of Zn2+. Under the guidance of theoretical calculation, we used the simple sol-gel method to synthesize inverse spinel Mg2VO4 as the positive electrode of zinc ion battery for the first time. The results are consistent with the predictions of theoretical calculations: the experiment verifies the exchange behavior of Zn2+ and Mg2+. After the exchange of Zn2+ and Mg2+, the Mg2VO4/Zn system exhibits good cycling stability and excellent rate performance. This study provides a new direction for the development of positive electrode materials for high-performance zinc ion batteries.

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【Article link】

Ion-Exchange-Induced High-Performance of Inverse Spinel Mg2VO4 for Aqueous Zinc-Ion Batteries

https://doi.org/10.1016/j.jpowsour.2022.232075

[ Corresponding Author Introduction]

​ Lin He , obtained a doctorate in materials chemistry from Milan, Bicoca University, Italy, and has worked in research at Princeton University in the United States. Mainly engaged in research on the design, preparation and application of new energy materials and functional carbon materials. He was selected as a national funding program for high-level overseas students to return to China, and presided over scientific research projects such as National Natural Science Foundation of China, provincial and ministerial Natural Science Foundation of China, etc.He has successively published high-quality academic papers in academic journals such as Small, Nano Energy, Chemical Science, Journal of Power Sources, ACS Applied Materials & Interfaces, Carbon, etc. He won the second prize in the Xinjiang Division of the Second National College Teacher Teaching Innovation Competition and the first prize in the first young teacher teaching competition of Xinjiang University.

[First author introduction]

​Zhang Yu , a doctoral student at the School of Chemistry, Xinjiang University, and his main research direction is the design and mechanism of high-performance positive electrode materials for zinc secondary batteries.

[Project Group Introduction]

​ The research team relies on the "provincial and ministerial joint construction of carbon-based energy resource chemistry and utilization National Key Laboratory " of Xinjiang University, and has excellent staffing and a good experimental environment. The members of the research team are committed to the design and preparation process of new energy materials and functional carbon materials. They have solid professional basic knowledge and rich practical experience in the design, synthesis, characterization of materials, and can reasonably and effectively evaluate the comprehensive physical and chemical properties and electrochemical properties of materials. On the one hand, the research team actively participates in front-line scientific research, and is also committed to the cultivation of outstanding graduate students. We welcome young people with aspirations to join our team.