Benzo-12-crown-4-ether-mediated lithium transport in supercritical CO2: A preliminary study for recycling lithium-ion battery cathode materials

The design of metal-complexing copolymer architectures is essential to enable solvent-free recovery of critical metals, and of interest for a large number of applications. In this study, the lithium transport efficiency of benzo-12-crown-4-ether (B12C4) from various salts (LiNO₃, LiOAc, Li₂SO₄ and Li₂CO₃) in supercritical carbon dioxide (scCO₂) was investigated. Among these salts, only Li⁺ from LiNO₃ was effectively complexed by B12C4 in scCO₂. As both B12C4 and the [B12C4-Li]NO₃ complex are poorly soluble in scCO₂, a scCO₂-philic gradient polymer, poly(B12C4 ethyl methacrylamide-grad-1,1,2,2-tetrahydroperfluorodecyl acrylate) [P(B12C4EMAAm-grad-FDA)] was synthesized by RAFT polymerization. In this copolymer, the FDA unit is CO₂-philic, while B12C4EMAAm acts as a metal-complexing group. The solubility of the copolymer was determined by cloud point measurement and compared to that of a PFDA homopolymer. The lithium recovery yield from lithium nitrate, quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, reached 83 % under supercritical conditions (40 °C and 250 bar) in the presence of a small amount of water (molar ratio [water]/[LiNO₃]=7.6), whereas only 25 % was recovered under dry conditions. The Li⁺ transport efficiency of the copolymer was also evaluated in the presence of cobalt ions. Using a mixture of lithium nitrate and cobalt nitrate hexahydrate (molar ratio [B12C4EMAAm]:[Li]:[Co]=3.6:1:1), recovery yields of 46 % and 84 % for lithium and cobalt were obtained, respectively. Despite its lack of selectivity toward lithium, P(B12C4EMAAm-grad-FDA) demonstrates strong potential as a complexing ligand for both lithium and cobalt under scCO₂ conditions.