Fundamental Understanding
of the Effect of a Polyaniline
Coating Layer on Cation Mixing and Chemical States of LiNi0.9Co0.085Mn0.015O2 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.