Sustainable recovery of cobalt and lithium from lithium-ion battery cathode material by combining sulfate leachates and aqueous biphasic systems based on tetrabutylphosphonium-ionic liquids

The consistent expansion of the lithium-ion battery (LIB) market, coupled with their relatively brief lifespan, necessitates the development of efficient and sustainable LIB recycling strategies. Recycling is crucial not only for the recovery of critical metals like Co(II) and Li(I) from the cathode material as a secondary resource but also from an environmental perspective. This study explores the use of a series of aqueous biphasic systems (ABS) with synthesized tetrabutylphosphonium ionic liquids (ILs) and ammonium sulfate as extraction platforms for metals from LIB cathode. Firstly, liquid-liquid equilibrium phase diagrams for each ABS were established, and partitioning experiments were conducted to assess the Co(II), Ni(II), Mn(II), and Li(I) recovery efficiencies. We observed distinct partitioning behaviors for the metals, with tetrabutylphosphonium diethylenetriaminepentaacetate, [TBP][DTPA], showing recovery efficiencies exceeding 98% for Co(II), Ni(II), and Mn(II). At the same time, Li(I) was predominantly retained in the aqueous salt-rich phase. By fine-tuning ABS operational parameters such as pH, temperature, system composition, and phase ratio, we identified optimal conditions for extracting metals from the cathode material of lithium-cobalt-oxide (LCO) batteries using sulfate lixiviate. Introducing [TBP][DTPA] after the leaching process induced ABS, achieving remarkable recovery efficiency over 95% for Co(II) in the IL-rich phase, with all Li(I) remaining in the lower phase. Cobalt was subsequently extracted using oxalic acid to precipitate as Co-oxalate from concentrate, while Li(I) was isolated from the aqueous phase using ammonium carbonate. After the "cleaning" of the IL-rich phase, the [TBP][DTPA] was recovered and reused in four consecutive cycles, with small detected losses on the recovery efficiency of Co(II) and Li(I). Therefore, our innovative strategy combines sulfate-based lixiviants with IL-ABS technology, thereby enhancing selectivity and sustainability within one of the most efficient lixiviant systems widely employed in the industry. This technological advancement presents a promising pathway for the recycling of spent batteries, offering substantial environmental advantages within the well-established and extensively utilized realm of metal recovery technology in the industry.