Pseudocapacitive Energy Storage in Schiff Base Polymer with Salphen-Type Ligands

Salphen-type nickel Schiff bases Ni­(salphen), Ni­(CH₃-salphen), and Ni­(CH₃O-salphen) are synthesized and electropolymerized on stable ITO electrode, respectively. The morphologies of the three polymer electrodes were evaluated by field emission scanning electron microscopy (FESEM). X-ray photoelectron spectroscopy (XPS) measurements were carried out to shed light on the polymerization mode and energy storage mechanism. Meanwhile, kinetic analysis of the redox reactions was used to verify the pseudocapacitive mechanisms of charge storage. The result signals that the polymerization mode and the mechanism of energy storage are related to the reversible conversion of the azomethine nitrogen group (−NCH−) in the six-membered ring of Schiff base instead of the Ni²⁺/Ni³⁺ process. Meanwhile, the azomethine nitrogen group was found to be directly affected by the addition of the electron-donating group methyl and methoxy so that additional peaks of the CV curve are generated, making polyNi­(CH₃-salphen) and polyNi­(CH₃O-salphen) have higher doping level, charge transfer ability, and better pseudocapacitive energy storage property than the pristine polyNi­(salphen) polymer. At the current density of 0.05 mA cm^–2, the specific capacity of the polyNi­(CH₃O-salphen) electrode was about 216 F g^–1, higher than the specific capacity of 85 F g^–1 for polyNi­(salphen) and 133 F g^–1 for polyNi­(CH₃-salphen). In the meantime, the conductivity of polyNi­(CH₃O-salphen) is 108.7 S cm^–1 higher than that of the other two polymers. Therefore, the addition of the stronger methoxy group for electron-donating substituents makes polyNi­(CH₃O-salphen) have more excellent electrochemical kinetics and pseudocapacitive characteristics

Surface Heterostructure Induced by PrPO₄ Modification in Li_1.2[Mn_0.54Ni_0.13Co_0.13]O₂ Cathode Material for High-Performance Lithium-Ion Batteries with Mitigating Voltage Decay

Lithium-rich layered oxides (LLOs) have been attractive cathode materials for lithium-ion batteries because of their high reversible capacity. However, they suffer from low initial Coulombic efficiency and capacity/voltage decay upon cycling. Herein, facile surface modification of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material is designed to overcome these defects by the protective effect of a surface heterostructure composed of an induced spinel layer and a PrPO₄ modification layer. As anticipated, a sample modified with 3 wt % PrPO₄ (PrP3) shows an enhanced initial Coulombic efficiency of 90% compared to 81.8% for the pristine one, more excellent cycling stability with a capacity retention of 89.3% after 100 cycles compared to only 71.7% for the pristine one, and less average discharge voltage fading from 0.6353 to 0.2881 V. These results can be attributed to the fact that the modification nanolayers have moved amounts of oxygen and lithium from the lattice in the bulk crystal structure, leading to a chemical activation of the Li₂MnO₃ component previously and forming a spinel interphase with a 3D fast Li⁺ diffusion channel and stable structure. Moreover, the elaborate surface heterostructure on a lithium-rich cathode material can effectively curb the undesired side reactions with the electrolyte and may also extend to other layered oxides to improve their cycling stability at high voltage