Highly Efficient p‑Toluenesulfonic Acid-Based Deep-Eutectic Solvents for Cathode Recycling of Li-Ion Batteries

The common use of Li-ion batteries (LIBs) in portable devices and electric vehicles is promoting an exponential growth of the rechargeable batteries market. Their use in electric vehicles while not yet so common will be favored in the near future by government incentives aiming to mitigate global warming effects coming from combustion engines. Despite the remote risk of cobalt (Co) and lithium (Li) supplies running low, metals recovery from “postconsumer” scrap is a rapidly expanding topic in the political agenda within the context of the circular economy. Among other recycling technologies, processes using mild conditions and providing high recycling efficiencies are gaining increased interest. For instance, organic acids are particularly interesting for LiCoO₂ dissolution even though they typically need the additional use of reducing agents. Herein, we explore the Co (and Li) recovery from spent LIBs using deep eutectic solvents (DESs) at temperatures as low as 90 °C, at times of dissolution as short as 15 min and without the use of any additional reducing agent. In particular, we used the eutectic mixture composed of p-toluenesulfonic acid (PTSA) monohydrate and choline chloride (ChCl) (e.g., PTSA·H₂O·ChCl_DES with a 1:1:1 molar ratio) and aqueous dilutions thereof (e.g., PTSA·2H₂O·ChCl_DES and PTSA·3H₂O·ChCl_DES with 1:2:1 and 1:3:1 molar ratios, respectively). Compared to organic acids, PTSAChCl-based DESs offered a significant reduction of the solute to solvent ratio (in g/L) needed for full Co dissolution. This brings benefits both in economic and sustainable terms that handling low solvent volumes has in industrial processes. Interestingly, Co recovery efficiencies from spent LIBs reached figures of up to 94% over the whole process relating to the extraction from spent LIBs using PTSAChCl-based DESs, the precipitation with either Na₂CO₃ or (NH₄)₂CO₃, and the final calcination to obtain Co₃O₄.