Comparison of Carbothermal and Hydrogen Reduction for Recycling Valuable Materials from Cathode Black Mass of Spent Lithium-ion Batteries

The extensive use of lithium-ion batteries (LiBs) for different energy storage applications generates a significant amount of spent LiB waste annually. For sustainable use of the critical materials present in the LiBs, the recycling of spent LiBs is required. The combined pyro/hydro-metallurgy process for recycling spent LiBs is receiving considerable attention over traditional methods because it operates at lower temperatures (&lt; 1000 °C) during the pyro stage and utilizes less harmful reagents in the hydro stage. It includes the thermal reduction of Li_1-αNi_xMn_yCo_zO₂ (NMC) cathode material, followed by multi-step leaching for metal extraction [1]. Thermal reduction is a crucial step that helps avoid harmful inorganic acids during multi-step leaching.<br/><br/>Thermal reduction is usually performed in the presence of reducing agents such as carbon (carbothermal reduction or CR), H₂(g), SO₄^2-, NO₃^2-, and Cl^-. The CR or H₂(g) reduction has received considerable attention because (a) it avoids anionic interferences like SO₄^2-, NO₃^2-, and Cl^-during the multi-step leaching and (b) it produces valuable gases which aid in lithium carbonate or hydroxide production.<br/><br/>The decomposition temperatures of cathode black mass (CBM) components, i.e., binder (PVDF), acetylene black, and NMC, are 380 – 480 °C, 500 - 600 °C, and 850 – 1200 °C, respectively [2]. Thermal reduction in the H₂(g) environment occurs effectively between 400 - 600°C. It is observed that the thermal reduction of NMC at 400°C with 100% H₂(g) and at 500 °C with 10% H₂(g) yields LiOH, MnO, and Ni-Co alloy [3]. Existing literature shows that the product obtained after conventional CR of CBM at 400 - 1000 °C contains undecomposed NMC, Mn₃O₄, MnO₂, MnO, CoO, NiO, Co, Ni, and Li₂CO₃. The reported optimal operating temperature of the CR process for NMC is 600 to 900 °C. Different reaction parameters such as reaction medium, CBM to reductant ratio, catalyst, particle size, and holding period can influence the optimal operating temperature. For example, adding NaOH as a catalyst to the feed mixture (NMC + C) helps lower the reduction temperature to below 600 °C [4]. This literature confirms the feasibility of NMC thermal reduction at less than 600 °C; therefore, a systematic study of thermal reduction at a lower temperature range is required.<br/><br/>In this study, CR of NMC with and without NaOH catalyst is performed at 400 – 600 °C and compared with the result of H₂(g) reduction of NMC. The thermal heating of NMC without C and NaOH at 480 °C and 600 °C yields disordered and spinel NMC, respectively. The thermal heating with 20 wt.% C at 480 °C produces disordered NMC. At 520 – 600 °C, new phases (NiO and Li₂CO₃) apart from undecomposed NMC are obtained. With the addition of 20 wt.% NaOH catalyst with 20 wt.% C, the formation of NiO and Li₂CO₃ is observed even at lower temperatures, i.e., 420 – 480 °C. Further, the analysis of the same system at higher temperatures (520 – 600 °C) results in a strong reduction, yielding Ni-Co alloy in addition to NiO and Li₂CO₃. Therefore, it is essential to note that 480 °C to 520 °C is critical for forming new NMC phases during CR with C or C + NaOH. Thermal reduction of NMC performed with 10% H₂(g) at 380 - 450 °C gives comparable results as CR with 20 wt.% C at 480 °C. However, 100% H₂(g) strongly reduces NMC into Co-Ni alloy and Li₂O/LiOH at 450 °C.<br/><br/>This study demonstrates the feasibility of varying degrees of reduction of NMC at lower temperatures. Further research is needed to investigate thermal reduction using 10 – 100 % H₂(g) at 480 – 600 °C and compare it with CR products to obtain a comprehensive understanding.<br/><br/>References:<br/><br/>[1] He et al., 2023. Green Chemistry, 25, 6561-6580.<br/>[2] Meng et al., 2019. Waste Management, 84, 54–63.<br/>[3] Bhandari et al., 2023, Process Safety and Environmental Protection, 172, 523–534.<br/>[4] Zhang et al., 2023. Rare Metals, 42, 2688–2699.