Electrochemical recovery of cobalt from battery recycling leachates is a critical process in the sustainable reclamation of valuable metals from spent lithium-ion batteries. Among the various techniques available, electrowinning, electrodeposition, and redox-targeting processes stand out due to their efficiency, selectivity, and scalability. These methods leverage electrochemical principles to extract cobalt from complex leachate solutions, ensuring high purity and minimal environmental impact. The following discussion explores these techniques in detail, focusing on cell design, current efficiency, deposit morphology, and the challenges associated with impurities and energy consumption.

Electrowinning is a widely used electrochemical method for cobalt recovery, particularly effective in industrial-scale operations. The process involves the reduction of cobalt ions (Co²⁺) from the leachate onto a cathode, typically made of stainless steel or titanium, while oxygen evolution occurs at the anode. The cell design for electrowinning often employs a divided configuration to prevent re-oxidation of deposited cobalt and to minimize side reactions. Key parameters include current density, temperature, and pH, which significantly influence the quality of the cobalt deposit. Current densities in the range of 200–400 A/m² are commonly used, with higher densities favoring faster deposition but potentially leading to dendritic growth and reduced purity. Temperature control between 50–70°C enhances ion mobility and reduces energy consumption, while maintaining a pH of 3–5 prevents hydroxide precipitation and ensures stable operation. Current efficiency in cobalt electrowinning typically ranges from 80–95%, depending on the leachate composition and operating conditions. Impurities such as nickel, copper, and iron can adversely affect efficiency by competing for reduction sites or forming passive layers on the cathode. Pretreatment steps like solvent extraction or precipitation are often necessary to mitigate these effects. The morphology of the cobalt deposit is another critical factor, with smooth, dense deposits being preferred for ease of handling and further processing. Excessive current density or impurity content can lead to porous or powdery deposits, which are less desirable.

Electrodeposition shares similarities with electrowinning but is often employed for higher-purity cobalt recovery or for producing cobalt coatings. The process involves the reduction of cobalt ions onto a substrate under controlled potential or current. Unlike electrowinning, electrodeposition is typically performed in smaller-scale cells with precise control over deposition parameters. The choice of electrolyte is crucial, with sulfate or chloride-based solutions being common. Additives such as boric acid or organic brighteners are sometimes used to refine grain structure and improve deposit adhesion. Current efficiency in electrodeposition can exceed 90% under optimal conditions, but the presence of organic residues or metal impurities can degrade performance. Deposit morphology is highly sensitive to the applied potential, with lower potentials favoring finer grains and smoother surfaces. However, too low a potential may result in incomplete deposition, while excessively high potentials can cause hydrogen evolution and embrittlement. The cell design for electrodeposition often includes rotating cylinder or plate electrodes to ensure uniform mass transfer and deposit thickness. Energy consumption is a significant consideration, with typical values ranging from 2.5–4.0 kWh per kilogram of cobalt recovered. Innovations in pulse or reverse-current electrodeposition have shown promise in reducing energy use and improving deposit quality.

Redox-targeting processes represent a more recent development in cobalt recovery, leveraging mediated electrochemical reactions to enhance selectivity and efficiency. In this approach, a redox-active mediator shuttles electrons between the electrode and cobalt ions in solution, enabling selective reduction even in the presence of competing metals. The cell design for redox-targeting typically involves a three-electrode setup with a working electrode, counter electrode, and reference electrode, along with a compartment for the mediator solution. The mediator, often an organic molecule or metal complex, is chosen for its reversible redox behavior and compatibility with the leachate. Current efficiency in redox-targeting can approach 95%, with the mediator facilitating rapid charge transfer and minimizing side reactions. Deposit morphology is generally superior to conventional electrowinning, with fewer defects and higher purity due to the selective nature of the process. However, the stability of the mediator over repeated cycles remains a challenge, as degradation can lead to increased costs and reduced efficiency. Energy consumption in redox-targeting is comparable to or slightly lower than electrowinning, with the added benefit of reduced reliance on stringent leachate purification.

A major challenge across all electrochemical cobalt recovery methods is the impact of impurities on process performance. Leachates from battery recycling often contain residual lithium, aluminum, manganese, and organic compounds, which can interfere with cobalt deposition. For example, manganese ions may co-deposit with cobalt, reducing purity, while organic residues can form passivating films on electrodes. Pretreatment steps such as pH adjustment, filtration, or solvent extraction are essential to minimize these effects. Energy consumption is another critical issue, particularly for large-scale operations. The theoretical minimum energy required to reduce Co²⁺ to cobalt metal is approximately 1.2 kWh per kilogram, but practical systems often consume 2–5 times this value due to overpotentials, ohmic losses, and side reactions. Advances in electrode materials, such as catalytic anodes or high-surface-area cathodes, along with optimized cell designs, can help reduce these losses.

In summary, electrochemical methods for cobalt recovery from battery recycling leachates offer a combination of efficiency, selectivity, and scalability. Electrowinning remains the industrial workhorse, while electrodeposition and redox-targeting provide avenues for higher purity and reduced energy use. The choice of method depends on factors such as leachate composition, desired product quality, and economic considerations. Continued research into electrode materials, mediator stability, and process optimization will further enhance the viability of these techniques, supporting the transition to a circular economy for battery materials.