Molten salt synthesis of Al‐substituted single‐crystal NCMA cathodes: synergistic mitigation of structural degradation and enhanced electrochemical stability

Abstract Nickel‐rich single‐crystal cathode materials are considered optimal candidates for next‐generation lithium‐ion batteries (LIBs) due to their combination of high‐energy density and crack‐resistant, grain boundary‐free structures. However, cyclic stress induces repetitive surface slip in single crystals, promoting the propagation of microcracks and exposing fresh crystal planes to the electrolyte. This accelerates interfacial reactions and corrosion, increasing charge transfer resistance and active material loss, ultimately degrading electrochemical performance. To address the slip‐related issues in single‐crystal materials, we have synthesized a single‐crystal quaternary cathode: LiNi 0.8 Co 0.08 Mn 0.06 Al 0.06 O 2 (SC‐NCMA), via a molten salt method. The effects of Al incorporation on suppressing lamellar slip and enhancing electrochemical performance are systematically investigated by comparing the characteristics of SC‐NCMA and ternary SC‐NCM. The experimental results show that SC‐NCMA retains 83.0% of its capacity after 200 cycles (2.7–4.3 V), significantly higher than the 66.5% capacity retention observed for SC‐NCM. In situ X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) analyses reveal that Al incorporation expands the (003) layer spacing, suppresses Li + /Ni 2+ mixing, and mitigates lattice distortion induced by the H2–H3 phase transition by reinforcing the Al–O bonds. Furthermore, Al 3+ not only optimizes Li + diffusion kinetics and reduces surface lattice oxygen loss but also facilitates the formation of a moderate‐thickness cathode‐electrolyte interphase (CEI) layer, enabling the pouch cell to achieve 89.1% capacity retention after 400 cycles. Overall, this work validates molten salt synthesis as a viable route for producing single‐crystal quaternary cathodes and elucidates Al’s dual stabilization mechanism—through both crystal structure modulation and interfacial passivation—offering strategic insights into the development of advanced nickel‐rich cathode systems.