Functional nanocellulose hydrogel with amino acid integration for enhanced Li/Fe separation in LiFePO₄ batteries

With the rising prevalence of lithium-ion batteries, efficient recovery of metal ions, particularly those potentially released from LiFePO₄ anodes, has become critical. Given that both Fe³⁺ and Li⁺ ions can form electrostatic adsorptive interactions, achieving effective separation of conventional adsorbent materials becomes challenging. This study presents an amino acid-functionalized nanocellulose hydrogel (ANH) synthesized by incorporating L-threonine, which significantly enhances the selective adsorption of Fe³⁺ in a mixed-ion environment by leveraging coordination differences between Li⁺ and Fe³⁺. The morphology, functional groups, and pore structure of ANH were extensively characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, and mercury intrusion porosimetry techniques. Through batch experiments, the adsorption thermodynamics and isotherms of ANH for Fe³⁺ were examined. Furthermore, the adsorption selectivity of ANH for Li⁺ and Fe³⁺ was evaluated in a mixed-ion system, revealing that the adsorption capacity for Fe³⁺ was four times higher than for Li⁺ at elevated concentrations. The adsorption mechanism of Li⁺/Fe³⁺ was elucidated through multi-scale simulations, and the influence of varying amino acid grafting degree on adsorption metrics, such as solvent-accessible area and hydrogen bonding numbers, was investigated. The combination of experimental and theoretical results demonstrates the potential of ANH to inform the development of high-performance, selective adsorbents.