N–S Codoped Strategy Enables Ultramicroporous Chitosan Activated Carbon for Selective Recovery of Li+/Mg2+ via Capacitive Deionization

Abstract

Biomass-derived activated carbon electrodes for capacitive deionization (CDI) enable the sustainable extraction of lithium from salt lakes, thereby avoiding the generation of chemical pollutants. However, the insufficient ion selectivity and limited charge storage capacity of conventional activated carbon electrodes hinder their practical application in industry. Herein, we develop a uniformly N–S codoped ultra-microporous chitosan-based activated carbon (NSAC) as a CDI electrode via a green synthesis method for selective recovery of Li⁺/Mg²⁺ from salt lakes. Specifically, NSAC prepared at 800 °C for 1 h exhibits an ultra-microporous structure (average pore size: 0.349 nm via CO₂@273 K) with N and S doping ratios of 11.12% and 1.57%, respectively. The unique ultra-microporous structure of NSAC can filter out large-radius ions and thereby selectively adsorb small-radius ions such as Li⁺ and Mg²⁺. Moreover, adsorption energy calculations demonstrate that N–S codoped carbon layers can significantly enhance the adsorption of Li⁺, Mg²⁺, and Na⁺ while greatly inhibiting the adsorption of Ca²⁺ and K⁺. Owing to the synergistic effect between its ultra-microporous structure and N–S codoping, the NSAC electrode can achieve selective adsorption of Li⁺/Mg²⁺ in salt lakes. Consequently, the NSAC electrode exhibits a Li⁺ adsorption capacity of 53.59 mg g^–1 and an ion removal rate of 18.89% in natural salt lakes. This work develops an eco-friendly CDI electrode material exhibiting excellent Li⁺/Mg²⁺ selectivity and homogeneous heteroatom distribution, which demonstrates promising potential for sustainable lithium extraction applications.