Enhanced Long-Term Stability and Safety of LiNi _0.8 Co _0.1 Mn _0.1 O ₂ Cathode via Nanoscale Surface Modification with Sulfonyl-Functionalized Polyimide Layer

Nickel-rich layered oxide cathodes (LiNi_0.8Co_0.1Mn_0.1O₂, NCM811) have shown great promise in high-energy-density lithium-ion batteries due to their high specific capacity. However, their practical application is severely hampered by structural instability, interfacial side reactions, and transition metal dissolution. To address this issue, in the present work, a highly polar polyimide containing sulfonyl moiety (named PI (6FOS)) is designed and utilized as a functional interfacial coating layer on NCM811 by copolymerizing 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with 4,4'-oxidianiline (ODA) and 4,4'-diaminodiphenyl sulfone (DDS). The combination of high polar constant and superior interfacial stability of PI (6FOS) contributes to effective suppression of electrolyte decomposition and transition metal dissolution. The strong interactions between polar trifluoromethyl and sulfonyl groups in the molecular chain and NCM811 particles enhance interfacial stability and facilitate lithium-ion transport. As expected, the PI (6FOS)/NCM811 electrode exhibits excellent long-term cycling performance with a remarkable capacity retention of 94% over 100 cycles at 0.2 C and 83% over 300 cycles at 1 C within 2.5-4.3 V. Even under high-voltage conditions (4.7 V), it maintains 59% capacity retention after 300 cycles at 1 C. The PI (6FOS)/NCM811 electrode shows outstanding rate capability, with a discharge specific capacity of 140.3 mAh g^-1 at 5 C. Meanwhile, the presence of the polyimide coating layer increases the thermal decomposition temperature of the cathode material and reduces heat generation. The perspective of the coating design strategy based on sulfonyl-containing polyimide modification offers a novel path toward high-stability cathodes for high-energy-density and high-safety lithium-ion batteries.