Preparation of Highly Stable Hydrophilic Electrodes Based on Thermally Crosslinked SPEEK and Their Application in Lithium Extraction from Salt Lake Brine by MCDI

Abstract: Lithium is a core strategic resource for the new energy industry. Extracting lithium from salt lake brine is of great significance for ensuring a stable supply of lithium resources and meeting the growing demand for lithium. Membrane capacitive deionization (MCDI) technology, with its advantages of low energy consumption and high selectivity, has shown promising potential for application in the field of salt lake lithium extraction. The physicochemical properties of the electrode binder are critical factors determining the structural stability and electrochemical performance of the electrode. In this paper, layered hydrous titanium oxide (HTO) was used as the active material, a titanate coupling agent as the interface modifier, and conductive carbon black as the conductive agent. Sulfonated poly(ether ether ketone) (SPEEK) with a sulfonation degree of 70% was employed as the binder, replacing traditional polyvinylidene fluoride (PVDF), and dissolved in N-methyl-2-pyrrolidone (NMP) to fabricate MCDI electrodes for lithium extraction. The titanate coupling agent, through chemical bonding with hydroxyl groups on the HTO surface and good compatibility with the organic phase, improves the dispersion of the active material while enhancing interfacial bonding, effectively inhibiting electrode swelling. Heat treatment induces a thermal crosslinking reaction in SPEEK, enabling precise control over the electrode's crosslinking degree and hydrophilicity. Simultaneously, the sulfonic acid groups (-SO3H) in the SPEEK molecular chains provide additional adsorption sites for lithium ions. This study systematically investigates the microstructure, hydrophilicity, and electrochemical performance of electrodes prepared with different binders, as well as their lithium extraction behavior in simulated salt lake brine. The results indicate that the SPEEK binder significantly improves electrode hydrophilicity, drastically reducing the water contact angle from 129° for PVDF-based electrodes to 40°. Electrochemical tests show that the specific capacitance of the optimized SPEEK-based electrode is 39% higher than that of the PVDF-based electrode. In simulated salt lake brine systems, this electrode achieves a high equilibrium lithium adsorption capacity of 44.8 mg·g-1 and a Li/Mg separation factor α(Li/Mg) of 102.8. Remarkably, after 250 galvanostatic charge-discharge cycles, the adsorption capacity retention remains above 99%. The enhanced overall performance of the electrode is attributed to the synergistic effects of the hydrophilic interface constructed by SPEEK, the enhanced structural stability provided by the coupling agent, and the additional adsorption sites offered by the sulfonic acid groups. This study provides a new strategy for binder design and interface engineering for high-performance MCDI lithium extraction electrodes.