Progress in electrochemical cycle stability of supercapacitor electrode material doped with manganese oxide

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Recently, Yan Jian, a professor at the School of Materials Science and Engineering, Hefei University of Technology, and Mao Wenping, a researcher at the Strong Magnetic Field Science Center of the Hefei Institute of Material Science, Chinese Academy of Sciences, collaborated to study the electrochemical cycle stability of Al 3+ doped manganese dioxide. The results were published in the ACS Appl. Mater. Interfaces magazine.
Supercapacitors have the characteristics of high specific capacity, long cycle life and environmental friendliness, and they play the role of green energy in electronic products and hybrid systems. Supercapacitor electrode materials are the key factors affecting the electrochemical performance of supercapacitors. Manganese dioxide (MnO 2 ) is not only theoretically high in specific capacity, but also rich in raw materials. It is an electrode material with good application prospects. However, due to its poor conductivity and cycle stability, the capacity retention rate during electrochemical cycling needs to be improved. The electrochemical performance and cycle stability of manganese dioxide can be improved by doping metal ions.

The researchers prepared Al 3+ doped MnO 2 (Al-MO) and pure MnO 2 (MO) electrode materials by chemical precipitation method and analyzed their electrochemical properties. The test found that the Al-MO electrode has a specific capacity of 264.6F/g at a current density of 1A/g, which is higher than the MO electrode (180.6F/g), and has good cycle stability at room temperature and 50°C. Sex. The microscopic morphology of the electrode after different cycles was observed by field emission scanning electron microscope. It was found that the Al-MO electrode gradually changed from granular to needle-like structure, but the crystal form did not change, and the MO electrode was in the process of cycling. At the same time, changes in morphology and crystal form occurred.

To further understand the relationship between electrode morphology evolution and electrochemical stability, the in-situ solid-state NMR (NMR) observation of the insertion/deintercalation of Na + in Al-MO and MO positive electrodes in different charge-discharge cycles revealed During the charge and discharge process, the 23Na peak of the MO electrode showed significant changes at different potentials and different periods, indicating that the MO electrode undergoes structural changes during the cycle; the 23Na peak did not change significantly during the charge and discharge of the Al-MO electrode. Even in the first cycle, there is no change, indicating that Na + has a fast reversible intercalation/deintercalation reaction on the surface of the Al-MO electrode, which also indicates that the Al-MO electrode structure is stable.

Based on the above test results, the researchers speculated that the morphology evolution of the MO electrode during the cycle may follow the "powdering-self-assembly" process. The intercalation/deintercalation of Na + causes a change in the volume of the MnO 2 nanoparticles to cause surface pulverization, and these powdered nanoparticles exhibit a morphological change upon reassembly. In the case of weak bond bonding, the reassembled fine particles may be separated from the matrix and dissolved in the electrolyte. As the active electrode material is lost, the capacitance will gradually decrease. Al 3+ doping of MnO 2 can enhance the bonding between the powdered particles, thereby facilitating the structural stability of MnO 2 .

Some of the experiments in this study were performed on a 600 MHz solid-state NMR spectrometer at the Center for Large-scale Instrumentation of Hefei's Strategic Energy and Material Science Center of the Chinese Academy of Sciences.

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Figure: 23Na spectra, micromorphology and specific capacity at room temperature and 50 °C during charge and discharge of Al 3+ doped MnO 2


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