Hydrometallurgical recycling of lithium-ion batteries, particularly those with high manganese content such as NMC111 (LiNi1/3Mn1/3Co1/3O2), presents unique challenges and opportunities. The process involves leaching, purification, and selective recovery of valuable metals, but the presence of manganese complicates separation due to its chemical similarity to nickel and cobalt. Additionally, manganese's lower economic value compared to nickel and cobalt raises questions about the economic viability of its recovery. However, innovative strategies for selective separation and valorization pathways, such as converting manganese into MnO2 for reuse in new batteries, can improve the sustainability and economics of recycling.
The first step in hydrometallurgical recycling is leaching, where cathode materials are dissolved in acidic or alkaline solutions. For NMC111, sulfuric acid is commonly used with hydrogen peroxide as a reducing agent to enhance metal dissolution. Leaching efficiency depends on factors such as acid concentration, temperature, and solid-to-liquid ratio. Under optimal conditions, over 95% of lithium, nickel, cobalt, and manganese can be extracted. However, the simultaneous dissolution of these metals creates a complex solution requiring precise separation techniques.
Selective recovery of manganese from nickel and cobalt is challenging due to their similar chemical properties. Solvent extraction is a widely used method, where organic extractants selectively bind to specific metals. For example, phosphinic acid derivatives like Cyanex 272 preferentially extract cobalt over nickel and manganese at controlled pH levels. Nickel can then be separated using extractants such as Versatic 10 or LIX 84, leaving manganese in the aqueous phase. However, achieving high purity for all metals requires multiple extraction stages and careful pH adjustment.
An alternative approach is precipitation. By controlling pH and oxidation conditions, manganese can be selectively precipitated as manganese carbonate or hydroxide while keeping nickel and cobalt in solution. For instance, at pH 8-9, manganese hydroxide precipitates while nickel and cobalt remain soluble. Subsequent steps can then recover nickel and cobalt through sulfide or hydroxide precipitation. Although effective, this method generates large volumes of wastewater, necessitating additional treatment.
The lower economic value of manganese compared to nickel and cobalt raises concerns about the cost-effectiveness of its recovery. While cobalt and nickel command high prices due to their use in high-performance batteries, manganese is more abundant and less expensive. This disparity makes it less attractive for recyclers to invest in manganese recovery unless valorization pathways are established. One promising approach is converting recovered manganese into MnO2, a material used in primary alkaline batteries and some lithium-ion batteries. MnO2 production involves oxidizing manganese ions in solution, followed by thermal treatment to achieve the desired crystal structure. This creates a circular economy loop where manganese from spent batteries re-enters the supply chain.
Another valorization pathway is using recovered manganese in new NMC formulations. While high-nickel NMC cathodes (e.g., NMC811) dominate the electric vehicle market, manganese-rich cathodes like NMC111 and LMNO (LiMn2O4) remain relevant for cost-sensitive applications. By reintegrating recycled manganese into these cathodes, recyclers can offset some of the processing costs. However, strict quality control is necessary to ensure the recycled material meets battery-grade purity standards, as impurities can degrade electrochemical performance.
Environmental considerations also play a role in hydrometallurgical recycling. Manganese, while less toxic than cobalt or nickel, still poses environmental risks if not managed properly. Effluents containing manganese must be treated to meet regulatory discharge limits, typically below 2 mg/L. Lime or sodium hydroxide can precipitate manganese as Mn(OH)2, which is then filtered and disposed of safely. Alternatively, advanced oxidation processes can oxidize manganese to MnO2, which is easier to filter and recover.
Energy consumption is another critical factor. Hydrometallurgical processes require significant energy for leaching, solvent extraction, and precipitation. Optimizing reagent use and recycling process streams can reduce energy demands. For example, regenerating and reusing sulfuric acid from leaching solutions lowers both costs and environmental impact. Similarly, recovering lithium as lithium carbonate or phosphate late in the process ensures high purity while minimizing waste.
The economic viability of recycling NMC111 hinges on balancing metal recovery rates with processing costs. A typical cost breakdown might include:
- Leaching reagents: 30-40%
- Solvent extraction chemicals: 20-30%
- Energy and labor: 20-25%
- Waste treatment: 10-15%
Given that manganese contributes only a small fraction of the total metal value in NMC111, recyclers must prioritize nickel and cobalt recovery to remain profitable. However, as regulatory pressures increase and landfill disposal becomes more expensive, finding value in manganese will grow in importance.
Future advancements may improve manganese recovery economics. Researchers are exploring biohydrometallurgy, where microorganisms selectively leach metals, reducing chemical use. Electrochemical methods are also being developed to directly recover metals from leach solutions with high selectivity. These innovations could lower costs and enhance sustainability.
In summary, hydrometallurgical recycling of high-manganese batteries like NMC111 requires sophisticated separation techniques to recover manganese alongside more valuable metals. While manganese's low economic value poses challenges, valorization through MnO2 production or reuse in new cathodes can enhance sustainability. Process optimization, environmental management, and emerging technologies will play key roles in making manganese recovery economically viable. As the battery industry evolves, integrating recycling into material supply chains will be essential for a sustainable energy future.