The widespread adoption of lithium-ion batteries (LIBs), particularly in electric vehicles, has raised growing concerns regarding their end-of-life management. As demand for electric mobility increases, so does the volume of spent batteries requiring effective recycling strategies. A central challenge in LIB recycling lies in the recovery of valuable metals such as lithium, cobalt, and nickel, which are essential for battery production.
One commonly employed method for extracting these metals is hydrometallurgy, which involves leaching the battery "black mass"—a concentrated residue of active materials—with acid to dissolve the metals into solution. This is followed by separation and purification processes to isolate each metal. Among various leaching agents, sulfuric acid is widely used due to its effectiveness in dissolving transition metals. However, achieving high leaching yields often requires a large excess of sulfuric acid, far beyond the stoichiometric demand.
The use of excess acid presents a significant environmental and economic challenge. After the leaching process, the surplus sulfuric acid must be neutralized—typically using agents like sodium hydroxide or calcium carbonate—resulting in the formation of waste by-products such as sodium sulfate or gypsum. These neutralization residues are usually discarded, contributing to resource loss and waste management issues.
To address this inefficiency, recovering and reusing the surplus sulfuric acid becomes a valuable strategy. This approach not only minimizes the need for neutralization agents but also enables closed-loop recycling within the hydrometallurgical process. Depending on its purity and concentration, the regenerated acid can be reused either in the leaching step or in subsequent solvent extraction (SX) processes.
In this context, a promising and commercially available method for sulfuric acid recovery is solvent extraction using tri-n-octylamine (TOA). It is a commercially available liquid anion exchanger used in the extraction of zinc from chloride media. Experimental studies have shown that TOA can extract up to 97.6% of the sulfuric acid under optimized conditions—specifically, at an aqueous-to-organic phase ratio of 4 and an organic phase composition of 40 vol-% TOA. The addition of 20 vol-% 2-ethylhexanol as a modifier was found to stabilize the organic phase by preventing phase separation, although it did not influence the extraction yield.
The extraction of leachate exhibits an identical extraction behavior as the sulfuric acid /TOA system. Furthermore, no co-extraction could be detected for any metal. Investigations on the leachate /TOA system shows an increasing aqueous metal concentration with an increasing phase ratio However, it was noted that increasing the phase ratio led to an increase in the aqueous metal concentration. This phenomenon is attributed not to metal extraction but to water co-extraction, which dilutes the aqueous phase as the organic layer becomes saturated with water.
While the extraction step using TOA has shown great potential, the stripping process—the recovery of sulfuric acid from the organic phase—remains a critical bottleneck. The strong interaction between TOA and the sulfate ion hinders efficient back-extraction, making it difficult to regenerate the acid in a concentrated form. As a result, further research has focused on identifying alternative ion exchangers and investigating their equilibrium behavior to enable more efficient acid stripping.
The stripping process was investigated using various cross-flow configurations at the laboratory scale. These configurations aim to enhance mass transfer and promote phase disengagement, thereby achieving higher stripping yields and generating more concentrated sulfuric acid solutions. The outcome of these investigations is crucial for improving the overall sustainability and economic feasibility of hydrometallurgical LIB recycling.
In conclusion, recovering excess sulfuric acid from LIB leachates using TOA represents a significant step toward circular process design in battery recycling. While the extraction efficiency is already promising, ongoing efforts to overcome the stripping limitations will determine the viability of this approach at an industrial scale. Future work will continue to refine these techniques and explore their integration into full-scale recycling plants, contributing to more resource-efficient and environmentally friendly battery lifecycle management.