Leaching behaviour of battery metals and impurities during black mass recycling

Abstract

With the global transition toward sustainable energy systems, lithium-ion batteries (LIBs) have become indispensable in portable electronics, electric vehicles, and renewable energy storage. Nonetheless, the rapid accumulation of spent LIBs presents environmental, safety, and resource challenges. Efficient recycling and recovery of critical materials like Co, Li, Ni, Cu, and graphite are therefore vital for the advancement of a circular and sustainable energy economy. Furthermore, the behaviour of impurities such as Si and Al must be considered to ensure recycling process efficiency and product purity.

This study systematically investigates the leaching and purification behaviour of industrial LIB black mass, addressing: (i) extraction of battery metals under different leaching systems, (ii) dissolution of Si and Al, (iii) characterization and purification of graphite residues, and (iv) precipitation of Fe and Al from pregnant leach solutions (PLS). Among the leaching agents investigated, strong HCl (4 M) achieved the highest battery metals extraction efficiency in the absence of externally added reductants, attributed to in-situ reduction by Cl- ions. Here, addition of external reductant (H2O2) had only a limited impact on further improving battery metals leaching and graphite-rich residues suitable for valorisation were generated. At lower mineral acid concentrations, a reductant was needed to guarantee sufficient Ni, Mn, and Co extraction. When 2 M H2SO4 was used, synthesized LiFePO4 (LFP) was found to act as a reductant, however, this had insufficient reductive power to achieve high Ni and Mn dissolution. Addition of stronger reductants like H2O2 (2–6 vol.%) significantly improved battery metals leaching in H2SO4 - at 60 °C extraction increased to 93% for Ni, 99% for Mn, and 100% for Co, whereas complete Mn dissolution was achieved at 80 °C.

Dissolution of Si and Al was found to be solution dependent; HCl minimized Si dissolution (<20%) with low time dependence, whereas H2SO4 required controlled operating conditions; low acidity (2 M), low temperature, short leaching duration, and avoiding excess H2O2 was found to suppress Si and Al leaching. For high-purity graphite recovery H2SO4 leaching was preferred, optimal purification (i.e. high Si and Al dissolution) was achieved using 4 M H2SO4 with 2 vol.% H2O2 at 60 °C for ≥3 h, while HCl produced ~20% lower graphite purity under similar conditions. Both HCl and H2SO4 can support effective graphite purification, with 4 M HCl producing residues containing 54 wt.% graphite. Dominant impurities found in the graphite leach residue included P, F, Si, organic C, and Al. H3PO4 leaching was shown to result in the in-situ precipitation of MnPO4·H2O and FePO4 precipitates that negatively affected graphite purity. This highlights distinct effects of different leaching agents on residue composition.

Further, PLS purification via Fe and Al precipitation from NMC-LFP waste PLS was investigated and the results suggested nearly complete Fe (100%) and high Al (91%) removal is possible at pH 4.5, with notable losses of Li, Ni, Co, and Mn (9–26%). However, at pH 2.0, selective Fe removal (97.8%) could be realized via phosphate precipitation, with limited Al co-precipitation (22.9%) and minimal battery metal losses (~4–5%).

The results of the thesis govern industrial black mass leaching, impurity dissolution, leach residue graphite characterization as well as Fe–Al separation. The new knowledge of the work can help in building strategies in future for hydrometallurgical battery recycling, specifically taking into account increasing Si/Al concentrations in the black mass, emerging LFP battery waste entering the recycling market as well as need for improved graphite recovery.