Carbothermic reduction is a high-temperature pyrometallurgical process used to extract metals from spent batteries by reducing their oxides using carbon as a reducing agent. The process leverages the thermodynamic stability of carbon oxides (CO and CO₂) to drive the reduction of metal oxides into their metallic forms. This method is particularly effective for recovering valuable metals such as cobalt (Co), nickel (Ni), and manganese (Mn) from lithium-ion battery cathodes, where these metals exist as complex oxides (e.g., LiCoO₂, LiNiMnCoO₂). The process typically operates between 900°C and 1600°C, depending on the target metal and the composition of the feed material.

The fundamental reaction mechanism involves the reduction of metal oxides (MO) by carbon (C) or carbon monoxide (CO), producing metal (M) and carbon monoxide or carbon dioxide. The general reactions can be represented as:
MO + C → M + CO
MO + CO → M + CO₂

The choice between solid carbon (coke, coal) or gaseous CO as the reductant depends on the temperature and the thermodynamics of the system. At higher temperatures, solid carbon becomes more effective due to the Boudouard reaction, where carbon reacts with CO₂ to form CO:
C + CO₂ → 2CO

This reaction is critical because it ensures a continuous supply of CO, which acts as the primary reducing agent in many carbothermic processes. The overall feasibility of the reduction is governed by the Gibbs free energy change (ΔG) of the reactions, which can be analyzed using Ellingham diagrams. These diagrams plot the standard Gibbs free energy of formation of metal oxides versus temperature, allowing comparison of the relative stability of different oxides.

For battery metal recovery, the Ellingham diagram reveals that cobalt oxide (CoO), nickel oxide (NiO), and manganese oxide (MnO) can be reduced by carbon at achievable industrial temperatures. The reduction potentials follow the order NiO > CoO > MnO, meaning nickel oxide is the easiest to reduce, followed by cobalt and then manganese. This hierarchy enables selective recovery if process parameters are carefully controlled. For example, at 1000°C, NiO is readily reduced to metallic nickel, while MnO requires higher temperatures (above 1400°C) for complete reduction.

In industrial applications, spent lithium-ion batteries are first pretreated to remove organic components (electrolytes, separators) and then processed to produce a black mass containing metal oxides. This black mass is mixed with a carbonaceous reductant (coke or coal) and fed into a high-temperature furnace, such as an electric arc furnace or a rotary kiln. The furnace atmosphere is carefully controlled to optimize CO/CO₂ ratios and prevent re-oxidation of the metals.

A real-world example is the recovery of cobalt from LiCoO₂ cathodes. The process involves heating the black mass with coke at approximately 1200°C, where the following reaction dominates:
2LiCoO₂ + 3C → 2Co + Li₂O + 3CO

The lithium oxide (Li₂O) remains in the slag phase and can be further processed for lithium recovery, while metallic cobalt is tapped from the furnace. Industrial operations report cobalt recovery efficiencies of 90-95% under optimal conditions.

Nickel recovery follows a similar pathway, with NiO reduction occurring at slightly lower temperatures (900-1100°C). In mixed cathode systems (e.g., NMC cathodes), selective reduction can be achieved by staging the temperature. For instance, holding the temperature at 1000°C preferentially reduces nickel and cobalt, leaving manganese in the oxide form. This selectivity is exploited in processes like the Sumitomo Metal Mining approach, where NMC black mass is treated in a reducing atmosphere to produce a Ni-Co alloy and a Mn-rich slag.

Manganese recovery is more energy-intensive due to its higher stability. Temperatures exceeding 1400°C are required for complete reduction:
MnO + C → Mn + CO

However, in practice, many operations opt to recover manganese as a lower-value ferroalloy (e.g., ferromanganese) by co-reducing with iron oxides, which lowers the energy requirement.

The slag phase from carbothermic reduction contains lithium, aluminum, and other impurities. Hydrometallurgical methods are often employed to recover lithium as lithium carbonate or hydroxide, with typical recovery rates of 70-80%.

Despite its advantages, carbothermic reduction has challenges. The high energy demand and greenhouse gas emissions (from carbon consumption) are significant drawbacks. Innovations like using biomass-derived carbon or integrating renewable energy into furnace operations are being explored to improve sustainability.

In summary, carbothermic reduction is a robust method for recovering metals from spent batteries, leveraging well-established thermodynamics and scalable furnace technologies. By understanding and controlling the reduction potentials of different metal oxides, high-purity cobalt, nickel, and manganese can be selectively recovered, contributing to the circular economy for battery materials. Industrial implementations demonstrate recovery efficiencies exceeding 90% for cobalt and nickel, though manganese recovery remains less efficient due to its refractory nature. Continued optimization of temperature profiles and reductant selection will further enhance the viability of this process.