Enhanced High-Temperature Cyclic Stability of Al-Doped Manganese Dioxide and Morphology Evolution Study Through in situ NMR under High Magnetic Field

In this work, Al-doped MnO₂ (Al-MO) nanoparticles have been synthesized by a simple chemical method with the aim to enhance cycling stability. At room temperature and 50 °C, the specific capacitances of Al-MO are well-maintained after 10 000 cycles. Compared with pure MnO₂ nanospheres (180.6 F g^-1 at 1 A g^-1), Al-MO also delivers an enhanced specific capacitance of 264.6 F g^-1 at 1 A g^-1. During the cycling test, Al-MO exhibited relatively stable structure initially and transformed to needlelike structures finally both at room temperature and high temperature. In order to reveal the morphology evolution process, in situ NMR under high magnetic field has been carried out to probe the dynamics of structural properties. The ²³Na spectra and the SEM observation suggest that the morphology evolution may follow pulverization/reassembling process. The Na⁺ intercalation/deintercalation induced pulverization, leading to the formation of tiny MnO₂ nanoparticles. After that, the pulverized tiny nanoparticles reassembled into new structures. In Al-MO electrodes, doping of Al³⁺ could slow down this structure evolution process, resulting in a better electrochemical stability. This work deepens the understanding on the structural changes in faradic reaction of pseudocapacitive materials. It is also important for the practical applications of MnO₂-based supercapacitors.