The effect of electrolyte cation on electrochemically induced activation and capacitive performance of Mn3O4 electrodes

Effect of Electrolytic Cations on a 3D Cd-MOF for Supercapacitive Electrodes

A cadmium-based metal-organic framework (Cd-MOF) is synthesized in a facile manner at ambient temperature by an easy slow diffusion process. The three-dimensional (3D) structure of Cd-MOF is authenticated by single-crystal X-ray diffraction studies and exhibits a cuboid-shaped morphology with an average edge length of ∼1.13 μm. The prepared Cd-MOF was found to be electroactive in nature, which resulted in a specific capacitance of 647 F g^-1 at 4 A g^-1 by maintaining a retention of ∼78% over 10,000 successive cycles in the absence of any binder. Further, to distinguish the efficiency of Cd-MOF electrodes, different electrolytes (NaOH, KOH, and LiOH) were explored, wherein NaOH revealed a higher capacitive response due to its combined effect of ionic and hydrated ionic radii. To investigate the practical applicability, an asymmetric supercapacitor (ASC) device is fabricated by employing Cd-MOF as the positive electrode and activated carbon (AC) as the negative electrode, enabling it to light a commercial light-emitting diode (LED) bulb (∼1.8 V). The as-fabricated ASC device delivers comparable energy density and power density.

Tracking Electrochemical-Cycle-Induced Surface Structure Evolutions of Cathode Material LiMn2O4 with Improved Operando Raman Spectroscopy

Linking surface structure evolution to the capacity fading of cathode materials has been a problem in lithium ion batteries. Most of the strategies used to solve this problem are focused on the differences between the unaged and aged materials, leading to the loss of intermediate dynamic change information during cycling. Raman spectroscopy is a convenient, nondestructive, and highly sensitive tool for characterizing the surface/near-surface region structure. In this work, we improved an operando Raman system, which is able to record in situ and in real time a series of Raman spectra during charging/discharging cycles and is even able to record very weak Raman peaks without the use of SRES enhancement, which facilitates sample preparation. These series of Raman spectra revealed an inherent correlation between the electrode potential/Li content and the surface structure changes of the as-prepared pure LiMn₂O₄ film, including the biphase reaction, the evolution of the peroxo O-O bond, and the formation of the Mn₃O₄ surface phase. They were the first to show that the number of peroxo O-O bonds was decreased with an increasing number of cycles and that this decrease was accompanied by an increase in the Mn₃O₄ phase. With the help of the data measured by XPS, c-AFM, electrochemical testing equipment, and the calculation based on density functional theory, the causes of the capacity fading of the material are discussed. This work not only showed a direct correlation between the surface structure evolution and the capacity fading of the LiMn₂O₄ but also could provide an alternative operando Raman system that could be widely used for the in situ characterization of battery electrode materials.