Investigation of new recycling strategies for spent Li-ion batteries based on early Li recovery via selective leaching

Doctoral thesis, 2025

In response to our society's environmental challenges, great efforts are being made worldwide to achieve climate neutrality: one of the actions is the electrification of transport, with significant help from rechargeable batteries mostly based on Li-ion chemistries. However, with this increasing demand for Li-ion batteries, many concerns have arisen regarding their life cycle, from raw materials supply to waste management. Recycling is necessary not only to dispose of the waste safely, but also to recover the critical metals such as Li, Co, Ni, Cu, and Mn. Hydrometallurgical techniques have been favoured in the last decades, as they enable the recovery of most elements (including Li) at a high rate and purity. This thesis investigates two new recycling strategies based on early Li recovery, which aim to increase the Li yield and allow the recovery of cathode metals together as precursors to directly re-synthesize the electrode material. The first strategy is based on the pyrolysis of the black mass, followed by water leaching. The second approach is based on using oxalic acid for the selective recovery of Li. In the first route, it was shown that during pyrolysis, the cathode material was reduced and transformed to more leachable forms: a mixture of Co, CoO, Ni, NiO, MnO, Mn2O3, and Li2CO3. Since Li2CO3 is sparingly soluble in water, water leaching was applied, and up to 70% of Li was recovered from the sample pyrolyzed at 600°C. Li recovery limitations could be connected to the formation of 3 distinct by-products of the pyrolysis: LiF, Li3PO4, and LiAlO2. The presence of Al and fluorine in the final leachate solution was considered to be unavoidable. However, the pyrolysis positively impacted the transition metal leachability, and they could be recovered with sulfuric acid without a reducing agent. The second strategy investigated the direct use of oxalic acid without any thermal pretreatment. The best operating conditions were determined as a temperature of 60°C, a solid to liquid ratio of 50 g/L, and an acid concentration of 0.6 M, corresponding to a molar ratio of 1:2.5 (cathode material metals: acid). This leads to more than 95% recovery of Li and 100% of Al. It was determined that the Li dissolution rate is chemically controlled, and the activation energy was estimated to be 76 kJ/mol. The leaching residue comprises a mix of graphite and a disordered (Co,Ni,Mn)C2O4 ·2H2O phase. This residue was then leached with sulfuric acid to dissolve the metal oxalates and separate them from the graphite. A one-stage leaching (2 M H2SO4, 65°C, 120 min, S/L = 20 g/L) results in more than 95% recovery of Ni, Co, and Mn and about 70% of Cu. Solvent extraction was used for Cu removal, and a 30 vol% Acorga M5640 in ESCAID was applied for 30 min at 25°C, with θ = 4 and 4 stages. The resulting recycled solution, free from Al, Li, and Cu, represents a promising feedstock for producing NMC 111 (LiNi1/3Mn1/3Co1/3O2).