(531d) Reusable DEEP Eutectic Solvents for Critical Metal Recycling of Lithium-ION Batteries

Due to the increase demand of lithium (Li) and cobalt (Co) for lithium-ion batteries, the recycling technique for the spent lithium-ion battery cathodes has been urgently needed. However, conventional hydrometallurgical cathode recycling technique has been suffering from the environmental impacts of harmful inorganic acids and organic solvents for the dissolution and extraction of the critical metals. Here we present an environmentally friendly cathode recycling technique using hydrophobic eutectic solvents (HESs) as the alternative to inorganic acids and organic solvents in conventional hydrometallurgical recycling.

Method

The HES composed of 2:1 molar ratio of benzoyltrifluoroacetone (HBTA) and trioctylphosphine oxide (TOPO) was prepared and employed as the leaching medium for a model cathode material lithium cobalt oxide (LCO). The effect of reducing agents, water content, and leaching temperature were investigated. After leaching, the metal-loaded HES was contacted with oxalic acid solution for the stripping and separating Li and Co. The effect of oxalic acid concentration on the stripping efficiency was studied. Finally, the 3-cycle reusability test was performed.

Results

The HES performed excellent liquid-liquid extraction performance of Li, Co, and other transition metals. However, the pure HES performed negligible leaching of LCO. Interestingly, the addition of a small amount (< 2.5%) of water improved the LCO leaching. Moreover, the LCO leaching was significantly enhanced by further adding an organic reducing agent ascorbic acid in the HES. The optimal leaching condition was 2.5% water and 0.1 mol dm^-3 ascorbic acid in HBTA/TOPO, achieving leaching efficiency of more than 90% for both Li and Co in 3 h at 343 K.

The stripping of Li and Co from the metal-loaded HES was conducted by contacting with a 0.5 mol dm^-3 oxalic acid solution. Owing to the poor solubility of Co oxalate salt, Co was recovered as a precipitate and Li was recovered in the aqueous phase after stripping.

Finally, the reusability of the HES was examined by repeating the leaching and the stripping steps. The leaching and the stripping performance were comparable in 3 cycles of the experiments.

Conclusions

In this study, HES has been firstly applied for the dissolution of metal oxides. With the aid of a small amount of water and ascorbic acid, highly efficient leaching of LCO into the HES was realized. The fundamental of the environmentally friendly recycling protocol based on reusable HES has been successfully developed.