One‐Step Biphasic Interfacial Engineering Stabilizes Single‐Crystal Ultrahigh‐Nickel Cathodes

High‐nickel layered transition metal oxides are among the most promising cathode candidates for next‐generation high‐energy lithium‐ion batteries. However, their practical implementation is hindered by limited rate performance and poor cycling stability, primarily due to intrinsic structural and interfacial degradation. Here, a one‐step LiPO 2 F 2 ‐mediated interfacial engineering strategy for stabilizing single‐crystal ultrahigh‐nickel LiNi 0.93 Co 0.07 O 2 cathodes is reported. This process involves a secondary annealing step that simultaneously eliminates residual lithium species and facilitates the in situ formation of a conformal LiF‐Li 3 PO 4 biphasic shell. The LiF component contributes to a robust and ultrathin cathode/electrolyte interphase, while Li 3 PO 4 enhances Li + surface transport, promoting homogeneous lithium distribution and relieving stress accumulation during cycling. Furthermore, the composite coating increases the interfacial oxygen vacancy formation energy, effectively suppressing lattice oxygen release and detrimental surface phase transitions. As a result, the optimized cathode retains 86.5% of its initial capacity after 200 cycles at 1 C (2.7–4.4 V), delivers 148.7 mAh g −1 at 5 C, and exhibits reduced voltage decay and improved energy retention. This one‐step biphasic interfacial engineering strategy provides a viable route toward high‐performance ultrahigh‐nickel cathodes for advanced lithium‐ion battery systems.