How green are choline chloride-ethylene glycol based deep-eutectic solvents?

In a fruitful collaboration of SOLVOMET/SIM2 with the group of Prof. Dirk de Vos (KU Leuven), researchers have demonstrated that choline chloride-ethylene glycol based deep-eutectic solvents should not be used for high-temperature leaching processes for cobalt recovery from lithium-ion battery cathode materials. The reason is their limited stability under such conditions and the formation of toxic decomposition products. The work was published in Green Chemistry.

Many questions without answers

It is often claimed that deep-eutectic solvents (DESs) are safe, eco-friendly and non-toxic solvents. Because of this, there is a large increase in the number of publications and projects (including European Horizon 2020 & Horizon Europe projects) in which metals are recovered using DESs.

Recently, several lab studies have been published with adopt DESs to recover cobalt from lithium cobalt oxide (LiCoO2), which is a common cathode material in lithium-ion batteries. For this recovery process it is imperative that cobalt(III) is reduced to cobalt(II). However, most of the recent publications report on the high-temperature use of such DESs without the addition of a reducing agent.

One should ask the question how cobalt(III) is being reduced then? Is the DES, or a part of it, acting as a reducing agent? Is it a degradation product of the DES that operates as a reducing agent? If the DESs are degrading, can they be recycled? All these unanswered questions triggered the research that allowed us to better understand the chemistry behind these processes.

Poor stability and formation of toxic decomposition products

Extensive experimental and analytical work allowed us to demonstrate that one should be cautious when using the DES ChCl:EG (1:2). This DES is not stable at elevated temperatures (180 °C) and toxic decomposition products such as trimethylamine or 2-chloroethanol are formed.

Furthermore, in the recovery of cobalt from lithium cobalt oxide, the choline chloride acts as the reducing agent for cobalt(III). If DESs are decomposing during the recovery processes at high temperatures, then it cannot be claimed that these processes are eco-friendly and safe.

Introspection needed

We hope this work triggers a more (self-)critical stance towards the alleged advantages of neoteric solvents such as deep-eutectic solvents, which are still hugely popular in academic research. We believe such introspection should guide metallurgical researchers to choose more suitable solvents for the processes that will provide the energy transition metals of the future.

Full reference paper

Peeters, N., Janssens, K., de Vos, D., Binnemans, K., Riaño, S. (2022). Choline chloride–ethylene glycol based deep-eutectic solvents as lixiviants for cobalt recovery from lithium-ion battery cathode materials: are these solvents really green in high-temperature processes?. GREEN CHEMISTRY, 2022, 24, 6685 – 6695, doi.org/10.1039/D2GC02075K

Acknowledgements

This research project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Program: Grant Agreement 694078—Solvometallurgy for critical metals (SOLCRIMET). The authors thank Christine Wouters for the support in the GC/MS analysis and Galahad O’Rourke for the help with the TGA.

Author: Dr. Sofía Riaño Torres (SOLVOMET Group / SIM² KU Leuven)

 

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