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

Abstract

A high nickel content of the cathode usually results in a large discharge capacity but causes structural collapse. Ni 2+ 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 0.9 Co 0.085 Mn 0.015 O 2 (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 R3̅ m 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.

Degradation mechanisms and modification strategies of nickel-rich NCM cathode in lithium-ion batteries

“…To mitigate any adverse effects on ionic and electronic conductivity, which could lead to increased cathode interface impedance after coating, researchers have developed surface layers with conductive properties. These include conductive polymers such as polyaniline (PANI) [106][107][108], poly(3,4-ethylenedioxythiophene) (PEDOT) [109,110], polypyrrole (PPy) [111,112], etc and fast ion-conductive materials like lithium phosphorous oxynitride (LiPON) [113], and Li 3 PO 4 [114][115][116][117].…”

A Perspective on the Requirements of Ni‐rich Cathode Surface Modifications for Application in Lithium‐ion Batteries and All‐Solid‐State Lithium‐ion Batteries

“…The current research trend in traditional lithium‐ion batteries (LIBs) surface modification focuses on addressing mechanical cracking issues while also enhancing or maintaining the physical barrier concept employed in traditional coating methods. Novel research has emerged in the form of self‐healing coating mechanisms, which involve the application of complex polymers to the cathode material [67,68] . These polymers aim to rapidly repair surface defects caused by mechanical strain.…”

“…Novel research has emerged in the form of selfhealing coating mechanisms, which involve the application of complex polymers to the cathode material. [67,68] These polymers aim to rapidly repair surface defects caused by mechanical strain. In their study, Yang et al introduced a self-adaptive polymer named polyrotaxane-co-poly(acrylic acid) (Figure 3a).…”

Cationic Covalent Organic Framework Coating of the Ni-Rich Cathode Surface for the Significant Improvement of the Interfacial Li+ Transport and Structural Stability

“…Several compounds have been investigated for coating materials, which lead to notable improvements in electrochemical performances. Metal oxides, such as Al 2 O 3 , SiO 2 , ZrO, and TiO 2 , serve as physical barriers to obstruct the contact between the cathode and electrolyte. − Fluorides, such as LiF, AlF 3 , and PrF 3 , are incorporated to suppress the degradation of Li salts on the cathode surface. − Li-containing oxides, such as Li 2 ZrO 3 , LiTiO 4 , and Li 2 SiO 4 , provide a fast Li + transport channel through the coating layer. − Polymer materials with ionic/electronic conductivity polymers reduce the interfacial resistance. − …”