Synthesis, characterization and Li adsorption study of a new thermoresponsive crown-ether bearing copolymer

With the ever-growing sales of electric vehicles (EVs) in the world and uncontrolled consumption of nonrenewable lithium resources for lithium-ion batteries (LIBs), new materials were proposed to reinforce the recycling technologies that are currently in use. For such purpose, the design and synthesis of a new crown etherbearing monomer, namely 2-(benzo-12-crown-4-ether)ethyl methacrylamide (BCEEM), its copolymerization with N-isopropylacrylamide (NiPAAm) and its adsorption properties were studied. While crown ethers (CEs) are well known for their metal adsorption properties, NiPAAm brought thermoresponsive properties to the copolymer, allowing for sorption at certain temperatures, and great processability through precipitation at other temperatures. With varied BCEEM/NiPAAm ratios, Lower Critical Solution Temperatures (LCST) ranged from 6 to 26.5 degrees C. Regarding the sorption performances by the CEs, several parameters were varied, such as pH, temperature and Li/CE ratio. Under optimized experimental conditions, the lithium sorption isotherm highlighted a maximum adsorption capacity of 0.075 mmol/g polymer (0.55 mg.g- 1), which is in the lower limit compared to similar CE studies reported in the literature. Moreover, competitive sorption studied in multicomponent solutions (Li, Co, Ni, Mn) simulating lithium-ion batteries (LIB) leachates (containing mostly lithium and cobalt), as well as in solutions with increased amounts of nickel and manganese, showed that selectivity towards lithium (and to a lesser extent cobalt) appeared to depend on the relative concentration of these two elements compared to Ni and Mn. In leachate-like conditions, the copolymer showed selectivity towards Li and Co whereas the selectivity evolved towards Ni and Mn as the initial concentration of Ni and Mn increased. DLS and ITC measurements were undergone to comprehend the obtained sorption capacities and selectivity from a mechanistic and thermodynamic point of view. While DLS measurements indicated potential salting-out effect of the ions causing degradation of water and metal interactions with the CEs, ITC experiments shed light on the potential culprit: the rigidity and hydrophobicity of the benzo-group.