Lithium-ion batteries (LIBs) recycling is an urgent need to address the mass generation of spent LIBs. However, the large-scale regeneration for spent LiNi_xCo_yMn_zO₂ (0 < x,y,z <1, x + y + z = 1, or NCM) cathode materials is hampered by economic and safety issues mainly due to high-temperature processing (>200^oC). Here, we are the first to demonstrate a safe and energy efficient direct regeneration process based on low-temperature hydrothermal relithiation (LTHR) at 100^oC (or below) and at ambient pressure for spent NCM cathode materials. A low concentration of low-cost redox mediator is employed to reduce the activation barrier for relithiation of spent NCM materials, paving the way for low-temperature relithiation. Specifically, three NCM materials, including cycled (degraded) NCM111, delithiated NCM111, and cycled NCM622, were successfully regenerated with complete recovery of crystal structure, composition, and electrochemical performance. Meanwhile, the total energy consumption of spent cell recycling was reduced from 5.28 MJ kg⁻¹ for traditional regeneration processes to only 4.22 MJ kg⁻¹ for LTHR, and the greenhouse gas emission was reduced by ≈ 0.03 kg kg⁻¹. This work provides a facile and scalable way to move LIBs direct recycling closer to practical applications.