Ammonium Ionic Liquid-Functionalized Phenothiazine as a New Redox Mediator for High Chemical Stability on the Anode Surface in Lithium–Air Batteries

The application of redox mediators (RMs) as soluble catalysts can address the problem of insufficient contact between conventional solid catalysts for lithium–air batteries (LABs). However, oxidized RM molecules migrate to the lithium anode and react with lithium, which results in the accumulation of surface corrosion products that weaken the redox activity of the RM. This paper presents a new combination of phenothiazine (PTZ) as an RM and an ammonium–based ionic liquid (IL) source as a protective agent to prevent the side reactions with lithium and to enhance the electrochemical performance of LABs. IL-functionalized PTZ (IL-PTZ) was successfully synthesized through N-alkylation, quaternization, and anion–exchange reactions. IL-PTZ improved the chemical stability of the RM molecules on the lithium surface as well as the electrochemical performance. A microstructural analysis revealed that the IL group in the IL-PTZ molecules facilitated smooth lithium stripping/plating by blocking the side reactions between the RM and lithium. Compared with the LAB with the PTZ electrolyte, that with the IL-PTZ electrolyte exhibited a significantly higher discharge capacity (2500 mA h/g vs 1500 mA h/g) and a cycle life that was 2 times longer. The IL-PTZ molecule was demonstrated to exhibit great potential as a novel soluble catalyst for application in high-performance LABs

Fundamental Understanding of the Effect of a Polyaniline Coating Layer on Cation Mixing and Chemical States of LiNi_0.9Co_0.085Mn_0.015O₂ for Li-Ion Batteries

A high nickel content of the cathode usually results in a large discharge capacity but causes structural collapse. Ni2+ ions move to the Li layer when Li+ ions are deintercalated during discharge, resulting in irreversible phase transition, cation mixing, dissolution of transition metal ions, and side reactions. A protective barrier is essential for maintaining the layered structures of cathode materials, even after several charge/discharge cycles of Li-ion batteries. Polyaniline (PANi) is an organic coating material with high conductivity and flexibility. PANi-coated cathodes have been widely reported for improving electrochemical performances. However, it is insufficient to prove the correlation between the PANi coating layer and structural stability through further analysis after an electrochemical test. Therefore, we focused on the structural stability and chemical states of the PANi-coated cathode after a cycle test by observing the morphology, lattice patterns, and chemical states of the surface. PANi-coated LiNi0.9Co0.085Mn0.015O2 (NCM; PANi@NCM) exhibited an initial discharge capacity of 221 mAh g–1 and a capacity retention of 81% after 50 cycles at 45 °C, which corresponded to an improved performance compared to pristine NCM. The cycled PANi@NCM showed an identical morphology to that of the cathode before the test. The R3̅m layered structure of PANi@NCM was maintained even after 50 cycles, as confirmed by transmission electron microscopy analysis with fast Fourier transform patterns and high-angle annular dark-field images. In addition, PANi@NCM maintains a thinner passivation layer (8 nm) compared with that of pristine NCM (27 nm). According to the X-ray photoelectron spectroscopy results, the surface chemical state of PANi@NCM showed that side reactions between the cathode and the electrolyte were suppressed during the cycle test. Therefore, it is demonstrated that the PANi coating layer prevents cation mixing and side reactions