CO₂ capture properties of lithium silicates with different ratios of Li₂O/SiO₂: An ab initio thermodynamic and experimental approach

The lithium silicates have attracted scientific interest due to their potential use as high-temperature sorbents for CO₂ capture. The electronic properties and thermodynamic stabilities of lithium silicates with different Li₂O/SiO₂ ratios (Li₂O, Li ₈SiO₆, Li₄SiO₄, Li ₆Si₂O₇, Li₂SiO₃, Li ₂Si₂O₅, Li₂Si₃O ₇, and α-SiO₂) have been investigated by combining first-principles density functional theory with lattice phonon dynamics. All these lithium silicates examined are insulators with band-gaps larger than 4.5 eV. By decreasing the Li₂O/SiO₂ ratio, the first valence bandwidth of the corresponding lithium silicate increases. Additionally, by decreasing the Li₂O/SiO₂ ratio, the vibrational frequencies of the corresponding lithium silicates shift to higher frequencies. Based on the calculated energetic information, their CO₂ absorption capabilities were extensively analyzed through thermodynamic investigations on these absorption reactions. We found that by increasing the Li ₂O/SiO₂ ratio when going from Li₂Si ₃O₇ to Li₈SiO₆, the corresponding lithium silicates have higher CO₂ capture capacity, higher turnover temperatures and heats of reaction, and require higher energy inputs for regeneration. Based on our experimentally measured isotherms of the CO ₂ chemisorption by lithium silicates, we found that the CO ₂ capture reactions are two-stage processes: (1) a superficial reaction to form the external shell composed of Li₂CO₃ and a metal oxide or lithium silicate secondary phase and (2) lithium diffusion from bulk to the surface with a simultaneous diffusion of CO₂ into the shell to continue the CO₂ chemisorption process. The second stage is the rate determining step for the capture process. By changing the mixing ratio of Li₂O and SiO₂, we can obtain different lithium silicate solids which exhibit different thermodynamic behaviors. Based on our results, three mixing scenarios are discussed to provide general guidelines for designing new CO₂ sorbents to fit practical needs.