The escalating global consumption of electronic devices has resulted in a significant surge in waste batteries, particularly spent lithium-ion batteries (LIBs). These batteries contain valuable critical elements such as lithium, cobalt, nickel, and manganese, the demand for which is projected to outstrip identified reserves. This creates an urgent need for sustainable recycling strategies to recover these critical materials and reduce reliance on environmentally and politically problematic primary mining operations. Recycling multi-metallic cathodes, such as lithium nickel manganese cobalt oxide (NMC), is particularly important due to their high content of valuable d-block elements. Current hydrometallurgical processes involve multiple pretreatment steps (discharging, dismantling, separation, and active material harvesting) followed by leaching to transfer elements into a liquid phase. However, selective separation of cobalt and nickel from the resulting solution is challenging due to their similar physicochemical properties. State-of-the-art techniques, such as solvent extraction and precipitation, often suffer from high chemical costs, waste generation, and complexities in solution chemistry. Electrochemical methods, especially electrodeposition, offer a potentially more sustainable alternative. While electrodeposition is versatile and simple, achieving selectivity between cobalt and nickel in aqueous electrolytes is difficult due to their similar standard reduction potentials. This research addresses this challenge by exploring a novel approach to tune the selectivity of cobalt and nickel electrodeposition.

Existing cobalt/nickel separation methods for NMC cathode recycling include solvent extraction, precipitation, adsorption, intercalation, and dialysis. While some exhibit high selectivity, they often involve substantial chemical costs or waste. Electrochemical methods, particularly electrodeposition, have been proposed as promising alternatives. Previous studies investigated selective electrochemical deposition of cobalt and nickel in high-temperature molten salts, but these methods are not ideal due to their high energy requirements. Low-temperature aqueous-based electrodeposition is more desirable, but the similar reduction potentials of cobalt and nickel pose a significant challenge to selectivity. This work aims to overcome this limitation by exploring the synergistic effects of electrolyte control and interfacial design for selective electrodeposition.

The study investigates the selective electrodeposition of cobalt and nickel using a combination of electrolyte engineering and interfacial design. Initially, the effect of electrolyte composition was examined using copper foil as a substrate. Linear sweep voltammetry (LSV) and chronoamperometry were used to characterize the electrodeposition behavior of cobalt and nickel in various electrolytes (0.1 M Li₂SO₄, 0.1 M LiCl, and 10 M LiCl). The speciation of cobalt and nickel ions in these electrolytes was analyzed. The 10 M LiCl electrolyte, promoting the formation of an anionic cobalt complex (CoCl₄²⁻) and a cationic nickel complex ([Ni(H₂O)₅Cl]+), showed improved potential separation between the two metals. Electrochemical quartz crystal microbalance (EQCM) measurements were conducted to investigate the faradaic efficiencies and reaction mechanisms during electrodeposition. Subsequently, the influence of a positively charged polyelectrolyte, poly(diallyldimethylammonium chloride) (PDADMA), was studied. PDADMA was coated onto the copper substrates at various loadings. The impact of PDADMA loading on the selectivity of cobalt and nickel electrodeposition was examined through LSV, chronoamperometry, and surface compositional analysis (ICP-OES, XRF, EDS, XPS). The electrochemical behavior of CoCl₄²⁻ and Ni(II) in the presence and absence of PDADMA was investigated via LSV at various scan rates to determine the diffusion coefficients. Finally, the developed methodology was applied to recover cobalt and nickel from commercially sourced spent NMC cathodes. The cathodes were pretreated through discharging, dismantling, N-methylpyrrolidine (NMP) treatment, and leaching in 10 M HCl. Sequential electrodeposition and stripping steps were conducted to achieve high purity cobalt and nickel recovery, with final purity analyzed via ICP-OES. A technoeconomic analysis was performed to assess the economic viability of the proposed process.

The study revealed that concentrated chloride electrolytes (10 M LiCl) significantly improved the selectivity of cobalt and nickel electrodeposition compared to low-to-moderate chloride concentrations. This is attributed to the formation of distinct anionic cobalt chloride complexes (CoCl₄²⁻) and cationic nickel complexes ([Ni(H₂O)₅Cl]+) enabling a more significant difference in their reduction potentials. EQCM analysis revealed differences in faradaic efficiencies for cobalt and nickel deposition and the formation of cobalt hydroxide at the early stage of electrodeposition, further enhancing cobalt selectivity. The introduction of PDADMA further refined the selectivity. Low PDADMA loadings (≤0.075 mg cm⁻²) enhanced cobalt selectivity, while higher loadings (>0.0375 mg cm⁻²) suppressed cobalt deposition leading to nickel selectivity. This effect is ascribed to the electrostatic interaction between the positively charged PDADMA and negatively charged CoCl₄²⁻, limiting its mobility near the electrode surface. The synergistic combination of concentrated chloride electrolyte and PDADMA coating led to significant enhancement in Co/Ni selectivity, achieving the highest surface Co/Ni ratio of 16.73 at -0.725 V vs Ag/AgCl in 100 mM Co(II) and Ni(II). Surface characterization techniques (XRF, EDS, XPS) corroborated these findings, confirming high cobalt and nickel purity in the electrodeposits. The developed process successfully recovered cobalt and nickel from spent NMC cathodes achieving high purities (96.4 ± 3.1% for cobalt and 94.1 ± 2.3% for nickel). Technoeconomic analysis indicated that the cost of PDADMA is a minor expense, while the main cost arises from lithium hydroxide used for pH adjustment after HCl leaching. Recycling lithium hydroxide as LiCl is suggested as a potential cost reduction strategy.

The results demonstrate the feasibility of achieving highly selective electrodeposition of cobalt and nickel through the synergistic use of electrolyte engineering and interfacial control. The concentrated chloride electrolyte enables distinct speciation of cobalt and nickel ions, creating a separation window. The PDADMA coating further enhances selectivity by modulating the mobility of CoCl₄²⁻. This approach allows for potential-dependent tuning of selectivity, achieving both cobalt-rich and nickel-rich deposits. The success of the method in recovering high-purity cobalt and nickel from spent NMC cathodes validates its potential for sustainable battery recycling. The technoeconomic analysis highlights the importance of optimizing the electrolyte recycling strategy to reduce costs. The observed morphology control suggests potential applications in materials processing beyond metal separation.

This study demonstrates a novel approach for selective electrodeposition of cobalt and nickel, crucial for efficient lithium-ion battery recycling. The synergistic combination of concentrated chloride electrolyte and a positively charged polyelectrolyte coating enables tunable selectivity based on applied potential and polymer loading. High-purity cobalt and nickel were successfully recovered from spent NMC cathodes, showcasing the potential of this electrochemical method for sustainable battery recycling. Future work should focus on optimizing electrolyte recycling and scaling up the process for industrial applications. Further investigation into the precise mechanisms of dendrite control through polymer coating would also be valuable.

The current study uses a laboratory-scale setup and may not fully reflect the challenges of large-scale industrial implementation. While the technoeconomic analysis suggests viability, further optimization of the process parameters (such as polymer loading, electrode design, and electrolyte recycling) is necessary to achieve greater cost-effectiveness. The long-term stability of the PDADMA coating under industrial conditions requires further investigation. The study focuses on NMC cathodes, and the applicability to other battery chemistries needs to be explored.