Effect of element composition of cathode on the carbothermic reduction of spent lithium-ion electrode materials

<title>Abstract</title> <p>To clarify differences in thermal kinetic and transition-metal phase evolution during the carbothermic reduction of spent Ni-rich lithium-ion electrode materials, spent NCM523, NCM811 and NC101 electrode materials were investigated. Cathode powders were mixed with spent anode graphite at the mass ratio of 7:3 and analyzed by TG-DSC, multi-heating-rate KAS isoconversional kinetics, thermodynamic calculations, XRD and carbothermic reduction followed by water leaching. The main carbothermic reduction reactions occurred at 500°C-900°C and involved coupled processes including decomposition of the layered cathode structure, lattice oxygen release, oxygen consumption by graphite, reduction of transition-metal oxides and secondary phase transformation. Cathode composition strongly affected the reaction pathway and kinetic stability. NC101 showed stable KAS fitting over a broad conversion range, indicating a relatively concentrated reaction pathway. NCM811 exhibited staged kinetic behavior, whereas NCM523 showed greater fitting dispersion and stronger coupling among reaction channels. The kinetic stability followed NC101 &gt; NCM811 &gt; NCM523, while reaction complexity followed NC101 &lt; NCM811 &lt; NCM523. Thermodynamic analysis indicated that the increase of Ni and Co contents and decrease of Mn content lowered the Gibbs free energy of reduction and weakened the layered structure. Water-leaching experiments showed optimum lithium leaching efficiencies at 700°C, reaching 83.01%, 89.76% and 91.31% for NCM523, NCM811 and NC101, respectively.</p>