Ion Dynamics and CO2 Absorption Properties of Nb-, Ta-, and Y-Doped Li2ZrO3 Studied by Solid-State NMR, Thermogravimetry, and First-Principles Calculations

Among the many different processes proposed for large-scale carbon capture and storage (CCS), high-temperature CO₂ looping has emerged as a favorable candidate due to the low theoretical energy penalties that can be achieved. Many different materials have been proposed for use in such a process, the process requiring fast CO₂ absorption reaction kinetics as well as being able to cycle the material for multiple cycles without loss of capacity. Lithium ternary oxide materials, and in particular Li₂ZrO₃, have displayed promising performance, but further modifications are needed to improve their rate of reaction with CO₂. Previous studies have linked rates of lithium ionic conduction with CO₂ absorption in similar materials, and in this work we present work aimed at exploring the effect of aliovalent doping on the efficacy of Li₂ZrO₃ as a CO₂ sorbent. Using a combination of X-ray powder diffraction, theoretical calculations, and solid-state nuclear magnetic resonance, we studied the impact of Nb, Ta, and Y doping on the structure, Li ionic motion, and CO₂ absorption properties of Li₂ZrO₃. These methods allowed us to characterize the theoretical and experimental doping limit into the pure material, suggesting that vacancies formed upon doping are not fully disordered but instead are correlated to the dopant atom positions, limiting the solubility range. Characterization of the lithium motion using variable-temperature solid-state nuclear magnetic resonance confirms that interstitial doping with Y retards the movement of Li ions in the structure, whereas vacancy doping with Nb or Ta results in a similar activation energy as observed for nominally pure Li₂ZrO₃. However, a marked reduction in the CO₂ absorption of the Nb- and Ta-doped samples suggests that doping also leads to a change in the carbonation equilibrium of Li₂ZrO₃, disfavoring the CO₂ absorption at the reaction temperature. This study shows that a complex mixture of structural, kinetic, and dynamic factors can influence the performance of Li-based materials for CCS and underscores the importance of balancing these different factors in order to optimize the process.