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. 2025 Dec 13;18(24):e202501306.
doi: 10.1002/cssc.202501306. Epub 2025 Sep 26.

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

Boren Xu  1 , M Luisa Ferrer  1   2 , Francisco Del Monte  1 , María C Gutiérrez  1
Affiliations

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

Boren Xu et al. ChemSusChem. .

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.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
A) Pictures and UV‐Vis absorption bands of the five different leachates prepared by dilution of 1 mg of LCO per gram of ChCl:2EG:0.2PA:7.3H2O mixture and subsequent dilution to 0.011, 0.033, 0.065, 0.086, and 0.1 mg of LCO per gram of ChCl:2EG:0.2PA:7.3H2O. B) Calibration curve obtained by linear fitting between absorption intensity and mg of LCO per gram of liquid mixture. C) Pictures and UV‐Vis absorption bands of the leachates obtained by dilution of 40 mg of LCO per gram of ChCl:2EG:0.2PA:7.3H2O mixture. These leachates were prepared by triplicate and, after filtration, each one was diluted to obtain 0.13, 0.093 and 0.061 mg per gram of liquid mixture. D) Deviation between theoretical values and real ones obtained from the calibration curve depicted in (B).
Figure 2
Figure 2
DSC scans of A) ChCl:2EG (pink line), ChCl:2EG:7.3H2O (brown line), ChCl:2EG:0.2PA:7.3H2O (violet line), ChCl:2EG:0.2PA:1.2TMG:7.3H2O (green line), and ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO (dark grey line) and B) ChCl:2EG:0.2PA:7.3H2O:LCO (light grey line), ChCl:2EG:0.2PA:1.2TMG:7.3H2O:LCO (blue line), ChCl:2EG:0.2PA:1.2TMG:7.3H2O:7.3DMSO:LCO (cyan line), ChCl:2EG:0.2PA:1.2TMG:14.6H2O:LCO (orange line), and ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO:LCO (dark yellow line).
Figure 3
Figure 3
1H NMR spectra of A) ChCl:2EG, B) ChCl:2EG:7.3H2O, C) ChCl:2EG:0.2PA:7.3H2O, and D) ChCl:2EG:0.2PA:1.2TMG:7.3H2O.
Figure 4
Figure 4
FTIR spectra of EG (black line in A), PA:36.6H2O (red line in A), ChCl:2EG (pink line in A), ChCl:2EG:0.2PA:7.3H2O (purple line in A), ChCl:2EG:0.2PA:7.3H2O:LCO (grey line in A), ChCl:2EG:0.2PA:1.2TMG:7.3H2O:LCO (blue lines in A and B), ChCl:2EG:0.2PA:1.2TMG:14.6H2O:LCO (orange line in B), ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO:LCO (dark yellow line in B), and ChCl:2EG:0.2PA:1.2TMG:7.3H2O:7.3DMSO:LCO (cyan line in B).
Figure 5
Figure 5
CA curves obtained in a three‐electrode configuration for ChCl:2EG:0.2PA:7.3H2O A,B), ChCl:2EG:0.2PA:7.3H2O:LCO C,D), ChCl:2EG:0.2PA:1.2TMG:7.3H2O:LCO E,F), ChCl:2EG:0.2PA:1.2TMG:14.6H2O:LCO G,H), ChCl:2EG:0.2PA:1.2TMG:7.3H2O:7.3DMSO:LCO I,J), and ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO:LCO K,L). Different potentials were studied for the negative (A, C, E, G, I, and K) and the positive (B, D, F, H, J, and L) electrodes depending on the mixture.
Figure 6
Figure 6
A) CV curves, B) GCD curves, C) the Nyquist plot, and D) the Ragone plot obtained with ChCl:2EG:0.2PA:7.3H2O (purple lines), ChCl:2EG:0.2PA:7.3H2O:LCO (grey lines), and ChCl:2EG:0.2PA:1.2TMG:7.3H2O:LCO (blue lines) acting as electrolytes in SCs operating at 1.4, 1.7, and 2.0 V, respectively. For comparison, the Ragone plot includes data for 21 m LiTFSI (red line) and TCENMC/G70 (black line; see ref.  for nomenclature) obtained in SC cells operating at 2.0 and 1.7 V, respectively.
Figure 7
Figure 7
Evolution of the capacitance retention (in%) and the coulombic efficiency during 10 000 cycles carried out 4 A g−1 for A) ChCl:2EG:0.2PA:7.3H2O:LCO (grey line and symbols) and ChCl:2EG:PA:1.2TMG:7.3H2O:LCO (blue line and symbols) and B) ChCl:2EG:0.2PA:1.2TMG:14.6H2O:LCO (orange line and symbols), ChCl:2EG:0.2PA:1.2TMG:7.3H2O:7.3DMSO:LCO (cyan line and symbols) and ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO:LCO (dark yellow line and symbols), acting as electrolytes in SCs operating at 2.0 V.
Figure 8
Figure 8
A) The Nyquist plot, B) the GCD curves, C) the CV curves, and D) the Ragone plot obtained with ChCl:2EG:0.2PA:1.2TMG:14.6H2O:LCO (orange line and symbols), ChCl:2EG:0.2PA:1.2TMG:7.3H2O:7.3DMSO:LCO (dark yellow line and symbols), and ChCl:2EG:0.2PA:1.2TMG:7.3DMSO:LCO (cyan line and symbols) acting as electrolytes in SCs operating at 2.0 V. For comparison, the Ragone plot also includes data for 21 m LiTFSI (red line and symbols) obtained in SC cells operating with our electrodes at 2.0 V.
Figure 9
Figure 9
Total cost (in euros) of the electrolytes used in this work a) in comparison with some other electrolytes described in previous papers; b) 21 M LiTFSI in H2O, c) 1.6 M TEABF4 in CH3CN, d) 1.6 M TEABF4 in propylene carbonate, and e) 1 M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (see refs. [53,69,70,71] in main text). Data was taken from https://www.sigmaaldrich.com/ES/es on September 2023.
Figure 10
Figure 10
A) The CV curves, B) the GCD curves, C) the Nyquist plot, D) the Ragone plot, and E) the evolution of the capacitance retention (in%) and the coulombic efficiency during 10 000 cycles carried out 4 A g−1 obtained with ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO:LCO (orange line and symbols), ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO:NMC‐1 (black line and symbols), and ChCl:2EG:0.2PA:1.2TMG:14.6H2O:7.3DMSO:NMC‐2 (red line and symbols) acting as electrolytes in SCs operating at 2.0 V.
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