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Title of the thesis: Leaching behaviour of battery metals and impurities during black mass recycling

Thesis defender: Yuanmin Zou
Opponent: Professor D.-Ing. Bernd Friedrich, RWTH Aachen University, Germany
Custos: Professor Mari Lundström, Aalto University School of Chemical Engineering

As the global transition toward sustainable energy systems, lithium-ion batteries (LIBs) are indispensable in portable electronics, electric vehicles, and renewable energy storage. However, the rapid accumulation of spent LIBs can also present environmental, safety, and resource challenges. Efficient recycling and recovery of the related critical materials like lithium, nickel, cobalt, manganese, and graphite are therefore essential to a circular and sustainable energy economy. 

This doctoral research investigates the leaching and solution purification of industrial LIB black mass. The study focuses on optimizing selected parts of hydrometallurgical recycling process, understanding the dissolution behaviour of key impurities such as silicon and aluminum, improving characterization and purification of graphite residue, and enhancing the removal of iron and aluminum from pregnant leach solutions (PLS).

Results shows that leaching efficiency depends on acid type and reductant use. In sulfuric acid, a reductant such as hydrogen peroxide, lithium iron phosphate or copper is required. By contrast, in 4 mol/L hydrochloric acid, high extraction of nickel, cobalt, and manganese was achieved without external reductants due to in-situ reduction by chloride ions. The impurities behaviour is highly sensitive to process parameters: hydrochloric acid minimized silicon dissolution, whereas sulfuric acid required controlled operating conditions: low acidity (2 mol/L), low temperature, short leaching duration, and avoiding excess hydrogen peroxide to suppress silicon and aluminum leaching. 

Strategies were also developed to graphite leach residue. Both sulfuric and hydrochloric acids enabled graphite purification, with hydrochloric acid yielding the highest purity, while phosphoric acid decreased graphite quality due to unwanted precipitation of manganese phosphate monohydrate. No significant difference was observed in graphitization degree between black mass and graphite residue. For PLS purification, phosphate precipitation at pH 2.0 enabled selective precipitation of iron phosphate, whereas at pH 4.5 both iron and aluminum were precipitated, though with some valuable metal losses. 
These findings provide actionable insights for industrial-scale battery recycling, particularly in coping with rising impurity levels and the growing share of lithium iron phosphate batteries in waste streams. The work advances more efficient and sustainable hydrometallurgical recycling processes.

Keywords: LIBs, Graphite, Valorisation, Hydrometallurgy, Reductant, Precipitation.

Thesis available for public display 7 days prior to the defence at .

Contact information: yuanmin.zou@aalto.fi