Boosting the active sites and kinetics of VO2 by Mn pre-intercalated and PVP modified nanostructure to improve the cycle stability for aqueous zinc batteries
Graphical abstract
The VO2 after Mn-doped and PVP-modified as aqueous zinc ion battery cathode materials. Both embedding of Mn and the modification of PVP can improve the morphology and internal structure of the material to improve performance.
Introduction
At present, people generally believe that the two main limiting factors of the energy crisis and environmental pollution are restricting social development. In order to better promote social development and cut down these two obstacles, it is necessary to burgeon clean energy [1], [2], [3], [4], [5]. At the same time, it is also urgent to seek more efficient energy storage equipment for portable smart devices and new energy vehicles [6], [7]. As a new species of green products and environmental protection battery, the high-power density, low cost, simple preparation and high safety are achieved by water-based zinc ion secondary battery. In large-scale energy storage applications, zinc ion batteries(ZIBs) are expected to become a new generation of green batteries with potential application value and development prospects [8]. In addition, compared with alkaline batteries, water-based zinc ion batteries have the advantage of energy storage, because it can use zinc foil as a negative electrode, which greatly increases the energy density. Nevertheless, their development has been hindered by limited cathode selection, which usually showing poor rate capability and insufficient cycle life [9]. Therefore, the selection and structural optimization of cathode materials have become the main research content recently. So far, ZIBs mainly researched cathode materials including manganese-based compounds [10], vanadium-based compounds [11], [12], [13], Prussian blue and its analogues [14], transition metal oxides [15] and polymers [16]. Among them, vanadium-based materials not only have multiple oxidation states but also have layered and tunnel-like open structures, so they usually have excellent zinc storage capacity and cycle capacity [4].
As a result of the low diffusion rate of the divalent cation Zn2+ in the crystal lattice, the cathode material needs more open interspace structure to promote the diffusion of Zn2+ in the diffusion control process. One major challenge in the practical application of ZIBs is short of the positive electrode materials with select structural stability during the Zn2+ intercalation/deintercalation process [17]. To stabilize the framework and promote the intercalation/deintercalation of Zn2+ ions, inserting ions in the layered or tunnel-like structure of vanadium-based materials as pillars (such as Mn2+ [18], [19], NH4+ [20], [21], Al3+ [22], K+ [23], Na+ [24], PANI [25]) is an accepted strategy. Moreover, the morphology of the material is also very important to the cycle performance of the electrode. Surfactants play a role in controlling morphology in the process of material synthesis. Among them, polyvinylpyrrolidone (PVP) can be used as material growth regulator and surface stabilizer [26], [27], [28]. Recently, Qin’s group [29] have explored the influence of the amount of PVP on the material properties. The synthesized Zn2V2O7 was used as the electrode material of the zinc ion battery, which proved that PVP can cause the phase change of the sample. Under 3 A g−1, after the electrode activation process, the electrode showed a maximum capacity of 114 mA h g−1. In addition, Yuan's team [30] used PVP as the soft template and carbon source, and SiO2 as the hard template to synthesize Mn3O4 nanocrystalline@3D honeycomb hierarchical porous network scaffold carbon. The capacity of the synthesized material at 10 A g−1 can reach 15.7 mAh g−1. Therefore, inserting Mn ions in the VO2 structure and modifying the morphology by PVP can improve the storage performance of zinc ions.
Herein, we embed Mn ions as pillar in the VO2 layered structure and reduce the “dead area” of the material through PVP modification, which is denoted as MnVO2-PVP. Furthermore, the cycling stability and diffusion ability of electrode material are improved by PVP affecting on morphology control. The MnVO2-PVP nanoprisms/nanosheet composite structure still has a capacity of 176.5 mAh g−1 after being cycled for 5000 times at a high current density of 10 A g−1. As the morphology of VO2 changes from flake to rod in the synthesis process, the agglomeration of materials is reduced. Therefore, this work will provide an insightful method produce a type of ZIBs cathode material with potential value.
Section snippets
Synthesis of VO2, MnVO2, and MnVO2-PVP:
First, a mixture of 0.2 g polyvinylpyrrolidone (PVP) and 2 mmol NH4VO3 were added to 15 mL deionized water at 80 °C with stir until the solution was bright yellow, which was labeled as solution A. Meanwhile, 3 mmol H2C2O4·2H2O and 1 mmol MnSO4·H2O were dissolved in 15 mL deionized water and ultrasonicated for 30 mins to prepare solution B. After that, solution A was poured into solution B and kept stirring for 30 min. The mixed solution was put into a 50 mL Teflon-lined stainless steel
Characterization of samples
Fig. 1 illustrates the synthesis mechanism of MnVO2-PVP and zinc ion insertion and extraction mechanism. Firstly, Mn ions are embedded in the VO2 layered structure to improve the ion diffusion ability of the material; secondly, PVP acts as a capping [28] agent to adjust the microstructure of the material having a larger specific surface area and more stable nanostructure. The tunnel structure supported by strong V-O chemical bonds, facilitates the repeated insertion/extraction of Zn2+ and
Conclusion
In summary, we reasonably designed the MnVO2-PVP nanoprisms/nanosheet composite structure as the advanced cathode of ZIB, which has excellent capacity and outstanding cycle performance. The intercalation of Mn ions can appropriately change the morphology of the material, increase the diffusion coefficient of Zn ions, ultimately benefit electrochemical kinetics and improve cycle performance. The surfactant PVP further optimizes the morphology of the material and increases the specific surface
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Key R&D Program of China (2020YFB1505603).
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