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

• Complexing ability of B12C4 towards different lithium salts was investigated. • Only LiNO3 was complexed by B12C4 in supercritical CO2. • [B12C4-Li]NO3 has limited solubility in supercritical CO2. • Supercritical CO2 soluble P(B12C4EMAAm-grad-FDA) copolymer was synthesized. • This copolymer achieved 83% Li recovery yield from LiNO3 in supercritical CO2 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 (LiNO 3 , LiOAc, Li 2 SO 4 and Li 2 CO 3 ) in supercritical carbon dioxide (scCO 2 ) was investigated. Among these salts, only Li + from LiNO 3 was effectively complexed by B12C4 in scCO 2 . As both B12C4 and the [B12C4-Li]NO 3 complex are poorly soluble in scCO 2 , a scCO 2 -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 CO 2 -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]/[LiNO 3 ]=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 scCO 2 conditions.