Simultaneous Nb⁵⁺-Driven Defect Repair and Oxygen Stabilization of Spent LiNi_0.88Co_0.09Al_0.03O₂ Toward High-Voltage and Fast-Charging Cathode

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-yCo_xAl_yO₂ (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 Nb⁵⁺ bulk doping. Nb⁵⁺ 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 Nb⁵⁺ 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.