Mechanisms of Thermal Decomposition in Spent NCM Lithium-Ion Battery Cathode Materials with Carbon Defects and Oxygen...

Resource recovery from retired electric vehicle lithium-ion batteries (LIBs) is a key to sustainable supply of technology-critical metals. However, the mainstream pyrometallurgical recycling approach requires high temperature and high energy consumption. Our study proposes a novel mechanochemical processing combined with hydrogen (H₂) reduction strategy to accelerate the breakdown of ternary nickel cobalt manganese oxide (NCM) cathode materials at a significantly lower temperature (450 °C). Particle refinement, material amorphization, and internal energy storage are considered critical success factors for the accelerated decomposition of NCM cathode materials. In our proposed approach, NCM cathode materials can develop active sites with carbon defects (C_v) and oxygen vacancies (O_v), which improve the reduction and breakdown of H₂. The adsorbed H₂ on the surface of NCM decomposes into H* and combines with oxygen to form OH species, which can be facilitated by O_v via the enhanced charge transfer. The introduced C_v can enhance H₂ cracking and generate *C-H species to promote the thermal decomposition of NCM. The presence of defects proves to foster the preferential reduction of Mn(IV) by H₂, leading to a lower activation energy for the NCM decomposition (from 139 to 110 kJ/mol) with less H₂ consumption. Life cycle assessment suggests a reduction of 4.42 kg CO₂ eq for the recycling of every 1.0 kg of retired batteries. This study can promote material circularity and minimize the environmental burden of mining technology-critical metals for a low-carbon transition.