Density functional theory studies of the structural, electronic, and phonon properties of Li2O and Li2CO3: Application to CO2 capture reaction

The structural, electronic, and phonon properties of Li2O and Li2CO3 solids are investigated using density functional theory (DFT) and their thermodynamic properties for CO2 absorption and desorption reactions are analyzed. The calculated bulk properties for both the ambient- and the high-pressure phases of Li2O and Li2CO3 are in good agreement with available experimental measurements. The calculated band gap of the high-pressure phase of Li2O (8.37 eV, indirect) is about 3 eV larger than the one corresponding to the ambient Li2O phase (5.39 eV, direct), whereas the calculated band gap for the high-pressure phase of Li2CO3 (3.55 eV, indirect) is about 1.6 eV smaller than that for the ambient phase of Li2CO3 (5.10 eV, direct). The oxygen atoms in the ambient phase of the Li2CO3 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 Li2CO3 dissociates, one group of O forms Li2O, while the other group of O forms CO2. The calculated phonon dispersion and density of states for the ambient phases of Li2O and Li2CO3 are in good agreement with experimental measurements and other available theoretical results. Li2O(s)+CO2(g)<-> Li2CO3(s) is the key reaction of lithium salt sorbents (such as lithium silicates and lithium zircornates) for CO2 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 Li2O can absorb CO2 efficiently, it is not a good solid sorbent for CO2 capture because the reverse reaction, corresponding to Li2CO3 releasing CO2, can only occur at very low CO2 pressure and/or at very high temperature when Li2CO3 is in liquid phase.