Efficient Regeneration of Degraded LiNi0.9Mn0.1O2 by Acid Etching–Hydrothermal Relithiation Coupled with Li4Ti5O12 Coating

With the growing global demand for sustainable resources, recycling spent lithium-ion batteries has become a strategic priority. Conventional pyrometallurgical and hydrometallurgical methods suffer from high energy consumption, severe pollution, and structural destruction, making them unsuitable for regenerating high-nickel cathodes. In this work, spent polycrystalline high-nickel LiNi0.9Mn0.1O2 cathodes were selected, and an upcycling strategy integrating acid etching, hydrothermal relithiation, short-time annealing, and simultaneous Li4Ti5O12 (LTO) coating was developed. This process directly transformed degraded polycrystalline cathodes into single-crystal cathode materials with excellent structural stability and electrochemical performance. During regeneration, lithium compensation and lattice recrystallization effectively repaired lithium loss, reduced Li/Ni cation mixing, reactivated the degraded structure, and reconstructed a highly ordered layered single-crystal framework. The LTO coating further stabilized the cathode/electrolyte interface, suppressed side reactions, alleviated volume strain, and promoted Li+ transport kinetics. Electrochemical measurements showed that the regenerated single-crystal cathode exhibited superior structural integrity, strong resistance to crack propagation, low polarization, excellent rate capability, and long-term cycling stability. A capacity retention of 84.3% was achieved after 300 cycles at 1C, outperforming commercial polycrystalline cathodes. This strategy provides an efficient and promising route for the direct regeneration of spent high-nickel ternary cathodes.