Unveiling Bidentate Nitrile-Driven Structural Degradation in High-Nickel Cathodes

Ultra-high-Ni layered oxides are promising cathodes for high-energy lithium–ion batteries (LIBs), however, their stability is compromised by interfacial degradation. Understanding molecular-level interactions at the cathode–electrolyte interface, especially involving transition-metal ions, is crucial. Here, we investigate dinitrile additives, focusing on succinonitrile (CN4) and its effects on Ni3+ ions in LiNi0.91Co0.08Al0.01O2 (NCA91). CN4 coordinates with Ni3+ via rigid bidentate binding through its two −CN groups, inducing ligand-field-driven electronic reconfiguration and Jahn–Teller distortion. This interaction enhances lattice strain, promotes π* back-donation, reduces surface electron density, and accelerates electrolyte oxidation, causing rapid deterioration in NCA91|graphite–Si full-cells. By contrast, longer-chain dinitriles (CN–[CH2]x–CN, x ≥ 3) exhibit weaker interactions due to internal backbone strain, which limits bidentate coordination. These findings explain the limited effectiveness of current dinitrile additives in mitigating structural degradation of ultra-high-Ni cathodes and provide molecular-level design principles for next-generation electrolyte additives to stabilize these materials.