Alkali-Etched NiCoAl-LDH with Improved Electrochemical Performance for Asymmetric Supercapacitors.

Hydrotalcite, first found in natural ores, has important applications in supercapacitors. NiCoAl-LDH, as a hydrotalcite-like compound with good crystallinity, is commonly synthesized by a hydrothermal method. Al3+ plays an important role in the crystallization of hydrotalcite and can provide stable trivalent cations, which is conducive to the formation of hydrotalcite. However, aluminum and its hydroxides are unstable in a strong alkaline electrolyte; therefore, a secondary alkali treatment is proposed in this work to produce cation vacancies. The hydrophilicity of the NiCoAl-OH surface with cation vacancy has been greatly improved, which is conducive to the wetting and infiltration of electrolyte in water-based supercapacitors. At the same time, cation vacancies generate a large number of defects as active sites for energy storage. As a result, the specific capacity of the NiCoAl-OH electrode after 10,000 cycles can be maintained at 94.1%, which is much better than the NiCoAl-LDH material of 74%.

High-Stability MnOx Nanowires@C@MnOx Nanosheet Core-Shell Heterostructure Pseudocapacitance Electrode Based on Reversible Phase Transition Mechanism.

A stable MnOx @C@MnOx core-shell heterostructure consisting of vertical MnOx nanosheets grown evenly on the surface of the MnOx @carbon nanowires are obtained by simple liquid phase method combined with thermal treatment. The hierarchical MnOx @C@MnOx heterostructure electrode possesses a high specific capacitance of 350 F g-1 and an excellent cycle performance owing to the existence of the pore structure among the ultrasmall MnOx nanoparticles and the rapid transmission of electrons between the active material and carbon coating layer. Particularly, according to the in situ Raman spectra analysis, no characteristic peaks corresponding to MnOOH are found during charging/discharging, indicating that pseudocapacitive behavior of the MnOx electrode have no relevance to the intercalation/deintercalation of protons (H+ ) in the electrolyte. Further combining in situ X-ray powder diffraction analysis, the diffraction peak of α-MnO2 can be detected in the process of charging, while Mn3 O4 phase is found in discharge products. Therefore, these results demonstrate that the MnOx undergoes a reversible phase transformation reaction of Mn3 O4 ↔α-MnO2 . Moreover, the assembled all-solid-state asymmetric supercapacitor with a MnOx @C@MnOx electrode delivers a high energy density of 23 Wh kg-1 , an acceptable power density of 2500 W kg-1 , and an excellent cyclic stability performance of 94% after 2000 cycles, showing the potential for practical application.