Microwave–Matter Interactions in Lithium‐Ion Battery Black Mass: Temperature‐Dependent Dielectric Properties and Carbothermal Reduction Pathways

ABSTRACT Microwave (MW) heating is a promising alternative to conventional pyrometallurgical processes for recovering strategic metals from spent lithium‐ion batteries (LIBs). However, the temperature‐dependent dielectric behavior of real waste remains poorly understood, limiting the optimization of MW‐assisted recycling processes. In this work, the interaction between MW radiation and black mass from LiCoO 2 (LCO) and LiNi 0 . 6 Co 0 . 2 Mn 0 . 2 O 2 (NMC) cathodes was investigated by combining dielectric characterization, thermal analysis, structural identification, and MW experiments. The complex permittivity was measured from room temperature to 900°C at 910 MHz and 2.45 GHz, using the cavity perturbation method, providing frequency‐resolved datasets linking dielectric evolution with phase transformation in real industrial LIBs black mass. Thermogravimetric (TGA) identified the main thermal events, while X‐ray diffraction (XRD) tracked MW‐induced phase transformations. The results showed that dielectric losses increased markedly in the temperature range 500°C–650°C, corresponding to the onset of carbothermal reduction reactions. In this interval, XRD analysis confirms the transformation of layered cathode phases into Li 2 CO 3 and transition‐metal oxides such as CoO, NiO, and MnO, while TGA reveals the beginning of significant mass loss associated with gas‐evolving reactions. At higher temperatures (> 700°C), further reduction leads to the formation of metallic Co and Ni. The comparison between the two frequencies indicates that 910 MHz promotes deeper and more uniform volumetric heating, whereas 2.45 GHz produces more localized surface heating. These findings show that MW absorption evolves with thermochemical transformations of black mass, highlighting the role of dielectric characterization in optimizing MW‐assisted carbothermal recycling of LIBs.