Density functional theory studies of the structural, electronic, and phonon properties of Li_2O and Li_2CO_3: Application to CO_2 capture reaction

摘要

The structural, electronic, and phonon properties of Li_2O and Li_2CO_3 solids are investigated using density functional theory (DFT) and their thermodynamic properties for CO_2 absorption and desorption reactions are analyzed. The calculated bulk properties for both the ambient- and the high-pressure phases of Li_2O and Li_2CO_3 are in good agreement with available experimental measurements. The calculated band gap of the high-pressure phase of Li_2O (8.37 eV, indirect) is about 3 eV larger than the one corresponding to the ambient Li_2O phase (5.39 eV, direct), whereas the calculated band gap for the high-pressure phase of Li_2CO_3 (3.55 eV, indirect) is about 1.6 eV smaller than that for the ambient phase of Li_2CO_3 (5.10 eV, direct). The oxygen atoms in the ambient phase of the Li_2CO_3 crystal are not equivalent as reflected by two different sets of C-O bond lengths (1.28 and 1.31 A) and they form two different groups. When Li_2CO_3 dissociates, one group of O forms Li_2O, while the other group of O forms CO_2. The calculated phonon dispersion and density of states for the ambient phases of Li_2O and Li_2CO_3 are in good agreement with experimental measurements and other available theoretical results. Li_2O(.v)+CO_2(g)Li_2CO_3(.s) is the key reaction of lithium salt sorbents (such as lithium silicates and lithium zircornates) for 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 Li_2O can absorb CO_2 efficiently, it is not a good solid sorbent for CO_2 capture because the reverse reaction, corresponding to Li_2CO_3 releasing CO_2, can only occur at very low CO_2 pressure and/or at very high temperature when Li_2CO_3 is in liquid phase.