Electronic structural and electrochemical properties of lithium zirconates and their capabilities of CO2 capture: A first-principles density-functional theory and phonon dynamics approach

Lithium zirconates have attracted researchers' interests because they can also be used as solid sorbents for CO2 capture. The structural, electronic, and phonon properties of Li2ZrO3, Li6Zr2O7, and monoclinic phase ZrO2 are investigated by the density-functional theory and phonon dynamics. Their electrochemical properties and their thermodynamics of CO2 absorption/desorption are analyzed. The calculated results show that their optimized structures and calculated bulk moduli as well as cohesive energies are in good agreement with experimental measurements. The calculated band gaps are 3.90 eV (indirect), 3.98 eV (direct), and 3.76 eV (direct) for Li2ZrO3, Li6Zr2O7, and ZrO2, respectively. The calculated Li intercalation voltage and energy densities of Li2ZrO3 are higher than that of Li6Zr2O7, which indicates that as a cathode material Li2ZrO3 is better than Li6Zr2O7. The calculated phonon dispersions and density of states show that there is one soft mode in Li2ZrO3 and two soft modes in Li6Zr2O7. From the calculated thermodynamic properties of these two lithium zirconates reacting with CO2, we found that the performance of Li2ZrO3 as a CO2 sorbent is better than that of Li6Zr2O7. In the first half cycle, sorbents absorbing CO2 to form lithium carbonate, Li6Zr2O7 performs better than Li2ZrO3 because the former releases more heat of reaction and has a lower Gibbs free energy and a higher CO2 capture capacity. However, during the second half cycle, regenerating sorbent from carbonate and zirconia to release CO2, the main product is the thermodynamically favorable Li2ZrO3 rather than forming Li6Zr2O7. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3529427]