Abstract
The accelerating demand for lithium in energy storage technologies is straining conventional mining and evaporation methods, driving interest in selective recovery from low-grade and chemically complex brines. This thesis evaluates spinel lithium titanium oxide (Li4Ti5O12, LTO) as a lithium-ion sieve using density functional theory (DFT) to provide atomistic insight into its thermodynamic stability, structural evolution, and ion-exchange kinetics. Two objectives guided this work: (i) establishing the phase stability and electrochemical response of LTO during lithium extraction with proton incorporation, and (ii) identifying and quantifying lithium and proton migration pathways that govern ion-exchange rates.
DFT thermodynamics show that proton substitution stabilizes delithiated frameworks relative to vacancy states, shifting the voltage profile to lower potentials and yielding solid-solution behavior with near-zero-strain structural evolution. Nudged elastic band (NEB) analysis revealed a kinetic hierarchy: lithium diffuses efficiently via the 8a-16c-8a pathway, where the 16c site acts as a kinetic bridge, while octahedral 16d lithium remains kinetically trapped. In contrast, proton migration exhibits substantially higher barriers (at least 1.9 eV) that scale with geometric path length, indicating that H+ mobility is the likely rate-limiting step in the exchange cycle.
These findings reconcile experimental observations of structural robustness yet sluggish adsorption-desorption kinetics in LTO sieves. They highlight three design levers for improving performance: enhancing 16c accessibility to accelerate Li+ diffusion, engineering shorter or lower-barrier proton pathways, and maintaining homogeneous lithiation for optimal sieving rates. Together, this study establishes a basis for rational improvements to spinel LTO sieves and sets a computational-experimental agenda for advancing sustainable and economic lithium recovery from challenging brines.
