Recycling and Reutilization of Metals Aided by Deep Eutectic Solvents: from NMC Cathodes of Spent Li-ion Batteries to Electrolytes for Supercapacitors

Abstract

With the rapidly increasing demand for lithium ion batteries (LIBs), recycling the metals found in spent cathodes is mandatory to both alleviate shortages resulting from the mining of natural metal ores and manage the disposal of spent LIBs. The use of deep eutectic solvents (DESs) for metals recovery from spent cathodes of LIBs (e. g., LCO and NMC types) offers a sustainable yet efficient alternative to conventional hydrometallurgical processes. Nonetheless, g efforts are required to use milder temperatures and higher mass loadings, thus ensuring cost-effectiveness. In this latter regard, addressing the reutilization of DESs in subsequent stages of metal extraction, and streamlining or eliminating the chemical procedures employed for metal separation, is even more crucial to guarantee the economic feasibility of the recycling process. Herein, we have prepared a DES that provides extraction efficiencies of ca. 100 % for every metal of NMC cathodes even at mild experimental conditions (e. g., 60 °C) and for loadings as high as 70 mg_NMC/g_DES. Moreover, we have pioneered the direct use of leachates containing DESs and metals as electrolytes for supercapacitors. This approach enables the reintroduction of DESs and the recovered metals into the value chain with a minimal economic and environmental impact.

Keywords: NMC recycling; deep eutectic solvents; electrolytes; leachate reutilization; supercapacitors.

Figure 1

DSC (A) and TGA (B) scans of the ternary DESs (e. g., T05CE and T1CE) studied in this work. DSCs and TGAs of some binary DESs (e. g., C05E and T1 C), of T05CE dissolved with 70 wt % GBL (e. g., T05CE/G70) and of the leachate coming from T05CE with a loading of 50 mgNMC/gDES, before and after dissolution with 70 wt % GBL (e. g., T05CENMC and T05CENMC/G70, respectively) are also included for comparison.

Figure 2

(A) Cartoon representing the leaching process described in reference 15 for NMC dissolution (upper row), the leaching process followed in this work for T1 C (middle row) and the process followed in this work for T05CE and T1CE (lower row). (B) Pictures of the leachates obtained for T05CE (left) and T1 C (center and right) at the mild conditions used in this work (e. g., 60 °C and 360 min) and for NMC loadings of 50 mg per gram of T05CE, and of 25 and 20 mg per gram of T1 C.

Figure 3

UV‐Vis spectra of T05CENMC diluted with further T05CE (A, B), GBL (A, C) or DMSO (A). Dilutions with T05CE contents of 97.5 wt % (e. g., T05CENMC/T05CE97.5), GBL contents of 98.5 wt % (e. g., T05CENMC/GBL98.5) and DMSO contents of 97.6 wt % (e. g., T05CENMC/DMSO97.6) were used to avoid saturation of the absorption band in the region of 600–750 nm. The spectra in the region of 300–550 nm were recorded for (B) T05CE and (C) GBL contents of 70, 80 and 95 wt % (e. g., T05CENMC/T05CE70, T05CENMC/T05CE80 and T05CENMC/T05CE95, and T05CENMC/GBL70, T05CENMC/GBL80 and T05CENMC/GBL95). The respective T05CE and GBL solutions are shown in the pictures included as insets in B and C (from left to right, solvent contents of 95, 80 and 70).

Figure 4

(A) CV and (B) GCD curves of T05CE (purple line), T1CE (yellow line) and T05CE/G70 (grey line) at the ESW of 1.1 and of T05CENMC (red line) and T05CENMC/G70 (green line) at the ESW of 1.7 V. (C) IR drops (purple bars) and ESRs (orange bars) of T05CE, T05CE/G70, T05CENMC and T05CENMC/G70. (D) Capacitance (open symbols) and coulombic efficiency (solid symbols) retention of T05CE (purple line), T05CENMC (red line) and T05CENMC/G70 (green line) after 10000 charge‐discharge cycles carried out at a current density of 2 A/g and with a ESW of 1.7 V. (E) Walburg coefficient (σ) of T05CE, T05CE/G70, T05CENMC, and T05CENMC/G70. (F) Ragone plot and (G) plot of capacitance versus current density of T05CE (dark purple line), T05CE/G70 (grey line), T05CENMC (red line), and T05CENMC/G70 (green line) using SCU‐800 as the carbon for electrodes preparation, and of T05CENMC (light purple line), and T05CENMC/G70 (light red line) using CK‐900 as the carbon for electrodes preparation. (H) Nyquist plots of T05CE (purple line), T05CE/G70 (grey line), T05CENMC (red line), and T05CENMC/G70 (green line).

Figure 5

Deconvolution of ATR bands of T05CE, T05CENMC, T05CE/G70 and T05CENMC/G70 in the streching regions of the O−H (panel A) and the C=O (panel B) .

Figure 6

(A) Total cost (in euros, USD and RMB) of (a) T05CENMC/G70, in comparison with some other electrolytes described in previous papers; (b) 21 M LiTFSI in H₂O, (c) 17 M NaClO₄ in H₂O, (d) 1.6 M TEABF₄ in CH₃CN, (e) 1.6 M TEABF₄ in propylene carbonate, and (f) 13.4 M EMIMBF₄ in GBL. (B) Contribution of solvents (in yellow) and salts (in purple) to the total cost of the different electrolytes. Data was taking from https://www.sigmaaldrich.com/ES/es on September 2023.