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 LiNi_0.9Co_0.085Mn_0.015O₂ (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.