Phytic Acid-Based Deep Eutectic Solvents for Metal Extraction from Lithium Cobalt Oxide and Nickel Manganese Cobalt and the Use of the Resulting Leachates as Electrolytes for 2.0 V Supercapacitors

Abstract

Li-ion batteries (LIBs) are essential in modern society but raise environmental concerns due to the intensive use of metals in cathodes and the challenges of end-of-life disposal. Besides traditional pyrometallurgical and hydrometallurgical processes used for metal recovery, deep eutectic solvents (DESs) have recently emerged as greener alternatives for leaching metals from spent cathodes of LIBs. A key drawback is, however, the unresolved recovery of the DES, whose cost can represent 30-60% of the leachate, thereby reducing the overall sustainability of the process. Herein, we used the leachates as electrolytes for supercapacitors. DESs based on phytic acid provided leachates with mass loadings of 40 mg of lithium cobalt oxide (LCO) or lithium nickel manganese cobalt oxide (NMC) per gram of DES. The typically poor performance of acidic leachates as electrolytes was addressed through chemical and solvent treatments. Neutralization with tetramethylguanidine expanded the electrochemical window, while dilution with water and/or water-dimethyl sulfoxide mixtures enhanced ionic mobility and rate capability. As a result, the processed leachates delivered energy densities of ≈17.9 Wh kg^-1 at 488.35 W kg^-1 and 5.77 Wh kg^-1 at 4343.72 W kg^-1, in the range of those provided by much less cost-efficient electrolytes such as 21 m LiTFSI.

Keywords: deep eutectic solvents; electrolytes; metal recovery and reutilization; spent cathodes of LIBs; supercapacitors.