Simultaneous Nb ⁵⁺ ‐Driven Defect Repair and Oxygen Stabilization of Spent LiNi _0.88 Co _0.09 Al _0.03 O ₂ Toward High‐Voltage and Fast‐Charging Cathode

ABSTRACT The rapid growth of electric vehicles and portable electronics has led to an escalating accumulation of spent lithium‐ion batteries, creating urgent demands for efficient and sustainable recycling technologies. Direct regeneration offers a promising approach by repairing and restoring cathode materials with minimal environmental impact, preserving their intrinsic structure and electrochemical properties. However, regenerating Ni‐rich cathodes like LiNi 1‐x‐y Co x Al y O 2 (NCA) remains challenging due to intrinsic structural instabilities, including oxygen loss and irreversible phase transitions that cause capacity fading. This study develops a one‐step molten salt‐assisted strategy that integrates single‐crystal reconstruction with in situ Nb 5+ bulk doping. Nb 5+ incorporation forms strong Nb─O bonds, enhancing lattice oxygen retention, suppressing oxygen vacancy formation, and mitigating irreversible strain caused by the H2–H3 phase transition. The regenerated Nb‐doped cathode (R‐NCA@2Nb) demonstrates outstanding cycling stability, retaining ∼80% capacity after 800 cycles at 1 C under a high cutoff voltage of 4.5 V and 84.1% retention after 300 cycles at 3 C, highlighting significant improvements in durability and rate capability. This scalable Nb 5+ doping strategy demonstrates significant promise for improving the stability and rate capability of recycled Ni‐rich cathodes, providing a practical pathway toward durable, high‐energy lithium‐ion batteries.