Density functional theory studies of the structural, electronic, and phonon properties ofLi2OandLi2CO3: Application toCO2capture reaction

The structural, electronic, and phonon properties of ${\text{Li}}_{2}\text{O}$ and ${\text{Li}}_{2}{\text{CO}}_{3}$ solids are investigated using density functional theory (DFT) and their thermodynamic properties for ${\text{CO}}_{2}$ absorption and desorption reactions are analyzed. The calculated bulk properties for both the ambient- and the high-pressure phases of ${\text{Li}}_{2}\text{O}$ and ${\text{Li}}_{2}{\text{CO}}_{3}$ are in good agreement with available experimental measurements. The calculated band gap of the high-pressure phase of ${\text{Li}}_{2}\text{O}$ (8.37 eV, indirect) is about 3 eV larger than the one corresponding to the ambient ${\text{Li}}_{2}\text{O}$ phase (5.39 eV, direct), whereas the calculated band gap for the high-pressure phase of ${\text{Li}}_{2}{\text{CO}}_{3}$ (3.55 eV, indirect) is about 1.6 eV smaller than that for the ambient phase of ${\text{Li}}_{2}{\text{CO}}_{3}$ (5.10 eV, direct). The oxygen atoms in the ambient phase of the ${\text{Li}}_{2}{\text{CO}}_{3}$ crystal are not equivalent as reflected by two different sets of C-O bond lengths (1.28 and $1.31\text{ }\text{\AA{}}$) and they form two different groups. When ${\text{Li}}_{2}{\text{CO}}_{3}$ dissociates, one group of O forms ${\text{Li}}_{2}\text{O}$, while the other group of O forms ${\text{CO}}_{2}$. The calculated phonon dispersion and density of states for the ambient phases of ${\text{Li}}_{2}\text{O}$ and ${\text{Li}}_{2}{\text{CO}}_{3}$ are in good agreement with experimental measurements and other available theoretical results. ${\text{Li}}_{2}\text{O}(s)+{\text{CO}}_{2}(g)\ensuremath{\leftrightarrow}{\text{Li}}_{2}{\text{CO}}_{3}(s)$ is the key reaction of lithium salt sorbents (such as lithium silicates and lithium zircornates) for ${\text{CO}}_{2}$ capture. The energy change and the chemical potential of this reaction have been calculated by combining DFT with lattice dynamics. Our results indicate that although pure ${\text{Li}}_{2}\text{O}$ can absorb ${\text{CO}}_{2}$ efficiently, it is not a good solid sorbent for ${\text{CO}}_{2}$ capture because the reverse reaction, corresponding to ${\text{Li}}_{2}{\text{CO}}_{3}$ releasing ${\text{CO}}_{2}$, can only occur at very low ${\text{CO}}_{2}$ pressure and/or at very high temperature when ${\text{Li}}_{2}{\text{CO}}_{3}$ is in liquid phase.