Recently, Jiang Liwei, a doctoral student in Group E01 of the Clean Energy Key Laboratory of the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, successfully constructed an aqueous potassium-ion full battery under the guidance of researcher Hu Yongsheng and associate researcher Lu Yaxiang, proposing to use Fe Manganese-rich potassium-based Prussian blue KxFeyMn1-y[Fe(CN)6]w·zH2O, which partially replaces Mn, is the positive electrode, and the organic dye perylene brilliant purple red 29 (PTCDI) (CAS: 81-33-4) is the negative electrode, 22 mol/L The aqueous solution of potassium trifluoromethanesulfonate is the electrolyte. The research results were recently published in Nature Energy (Nature Energy, 2019, doi:10.1038/s41560-019-0388-0), and the article is titled Building Aqueous K-Ion Batteries for Energy Storage. Aqueous alkali metal ion (Li+/Na+/K+) batteries have become one of the emerging candidate systems for grid energy storage due to their inherent safety. Among aqueous alkali metal ion batteries, aqueous potassium ion batteries have more prominent advantages. The main reasons are: first, the content of potassium in the earth's crust is 1290 times that of lithium, which has a great cost advantage; second, the standard electrode of potassium The potential is 0.22V lower than that of sodium, which means that the potassium cathode of the same type of structure has a higher voltage, resulting in a higher energy density for the full battery; thirdly, in an aqueous solution with the same anions and the same concentration, the potassium salt solution The ionic conductivity of lithium salt is much higher than that of lithium salt and sodium salt, which means that using potassium salt solution as electrolyte can make the full battery have faster charge and discharge capabilities. However, due to the dissolution phenomenon of many electrode materials in water and the narrow voltage window of traditional aqueous electrolytes (less than 2V), the choice of electrode materials in aqueous batteries is greatly limited. Therefore, exploring high-performance potassium-based positive electrodes, negative electrodes, and wide voltage window electrolytes has become a core issue that needs to be solved in the field of aqueous potassium-ion batteries. For the cathode, the manganese-rich potassium-based Prussian blue material with the P21/n space group has become the first choice for the cathode material of aqueous potassium-ion batteries because it is stable to water and has the advantages of high voltage and high capacity. However, manganese-rich potassium-based Prussian blue materials have serious dissolution problems when circulating in low-salt concentration electrolytes. The author found that after using a 22 mol/L potassium trifluoromethanesulfonate aqueous electrolyte with a high salt concentration, the dissolution of the electrode was greatly reduced but voltage and cycle attenuation still existed. The authors further found that by replacing part of Mn with Fe, material dissolution can be reduced and cycle performance can be greatly improved. Through further optimization, the K1.85Fe0.33Mn0.67[Fe(CN)6]0.98·0.77H2O (KFeMnHCF-3565) cathode material showed almost no voltage and capacity fading during the first 40 cycles. Subsequently, the author used ex-situ X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES) and first-principles calculations to reveal the mechanism by which Fe replaces Mn to improve the cycle and rate performance of the manganese-rich Prussian blue cathode. On the one hand, the replacement of part of Mn by Fe not only reduces the content of Mn3+ with the Ginger-Taylor effect in the positive electrode lattice after charging, but also changes the price change characteristics of the Mn2+/Mn3+ redox couple in the charge and discharge curve (Mn2+/Mn3+ redox couple). Not only does the order of valence change with the Fe2+/Fe3+ redox couple change during the charging process, but it also becomes asymmetrical during the charging and discharging processes), thus causing the positive electrode to transform from the original two phases into a solid solution during the structural evolution. Reaction plus a two-phase reaction. That is to say, the phase transition (cubic phase to tetragonal phase) related to the Mn3+ Jiang-Taylor effect is suppressed, greatly improving the cycle performance of the positive electrode. On the other hand, first-principles calculations show that replacing part of Mn with Fe can reduce the energy band gap of the cathode and the diffusion activation energy of potassium ions, thereby improving the electron and ion conductivity of the cathode material, making the material have higher rate performance. As for the negative electrode, there are very few negative electrode materials that can be used in aqueous potassium ion batteries. The author used the organic dye PTCDI as the negative electrode for the first time and found that PTCDI has a high potassium storage capacity in 22mol/L potassium trifluoromethanesulfonate electrolyte ( 125mAh/g) and good rate performance. In addition, as an electrolyte, the high-salt concentration aqueous solution of 22mol/L potassium trifluoromethanesulfonate not only has a wide voltage window (3V) and high conductivity (76mScm-1 at 25°C and 10mScm-1 at -20°C), but also Since there is very little free water in the high-salt concentration electrolyte (Figure 3d), it can inhibit the dissolution of the positive and negative electrode materials, so that the full battery has the characteristics of high voltage, wide temperature range, high power, and long life. The developed positive and negative electrode materials and high-salt concentration electrolyte were assembled into an aqueous potassium ion full battery, and it was found that it can operate in the voltage range of 0 to 2.6V, with a theoretical energy density of up to 80Wh/kg and a lifespan of more than 2,000 times. (Retention rate 73%).

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