A nanoconfined thermoresponsive membrane composed of Ti3C2Tx MXene and hydroxypropyl cellulose (HPC) was developed for selective Li+ extraction. By integrating the electrothermal conductivity of MXenes and hydration-responsive gating of HPC, the membrane forms heterochannels with tunable spacing that regulate ion transport through nanoconfinement-enhanced mechanisms based on interaction energy and hydration radius. While density functional theory calculations predicted stronger sorption for Mg2+, experimental data revealed a clear preference for Li+ uptake from both simulated brine and battery black mass. This selectivity is attributed to favorable interactions of Li+ within the nanoconfined composite channels, where the subnanometer interlayer spacings promote partial dehydration and size-sieving effects. Li+ retention is governed not only by thermodynamic affinity but also by kinetic acceleration in nanoconfined pathways and hydration-based steric control. The membrane exhibits a reversible thermal response and maintains stable performance under Joule heating. It achieves >90% extraction efficiency from simulated Atacama brine and up to 98% Li+ recovery from black mass supplied by VGM Sustainability Solutions (SG3R, Pte. Ltd.).