Ion-Trapping Binder for Ceramic-Coated Separators to Suppress Transition Metal Crossover in NCM811 Batteries

High-nickel layered oxide cathodes, such as LiNi0.8Co0.1Mn0.1O2 (NCM811), suffer from transition metal ion (Ni, Co, and Mn) dissolution during cycling, which degrades the solid electrolyte interphase (SEI) and reduces cell performance. Here, we introduce a malonamide poly(ether imide) (MPEI)-based binder for ceramic-coated separators (CCSs) that chelates dissolved metal ions before reaching the anode. Chelation between the malonamide carbonyl groups and metal ions effectively suppresses ion migration, reducing side reactions and stabilizing the SEI. As a result, half-cells with the MPEI-based CCS retain 35.1% greater capacity after 300 cycles than those with conventional polyethylene separators. Inductively coupled plasma mass spectrometry (ICP-MS) and depth-profiling X-ray photoelectron spectroscopy (XPS) provide supporting evidence that the MPEI-based CCS enhances transition metal retention within the separator and mitigates transition-metal-ion-associated SEI degradation at the anode side. This strategy demonstrates that ion-trapping separators can effectively mitigate metal-ion-induced degradation and improve the long-term stability of Ni-rich cathodes.