Conditions of stability for (Li₂CO₃ + Li ₂O) melts in air

Characterizing the equilibrium between molten (Li₂CO₃ + Li₂O) and CO₂ is important for a number of applications ranging from carbonate fuel cells to Li industrial production. The equilibrium pressure of CO₂ was measured above (Li₂CO₃ + Li₂O) mixtures containing mole fraction of lithium oxide, xLi ₂O, between 0.01 and 0.06 and heated within the temperature range (1073 to 1248) K. These data were used to evaluate the enthalpy and entropy of thermal decomposition of Li₂CO₃. For 0.02≤xLi ₂O≤0.06, both values remain constant to within experimental uncertainty: ΔH = (275 ± 5) kJ · mol^-1 and ΔS = (179 ± 4) J · mol^-1 · K^-1. For mole fraction of lithium oxide 0.01≤xLi₂O≤0.02, both ΔH and ΔS decrease considerably with decreasing concentration of the oxide. Nevertheless, they remain much larger than the values calculated for the decomposition of Li₂CO₃ based on thermodynamic quantities reported for the formation of CO₂, Li₂O, and Li ₂CO₃: ΔH_s = 148 kJ · mol ^-1 and ΔS_s = 79 J · K^-1 · mol^-1. We attribute this discrepancy to the solubilization of Li ₂O by Li₂CO₃. Using our derived thermodynamic parameters, we can predict the existence of a range of temperatures and concentrations of Li₂O in the Li₂CO₃ melt that are in equilibrium with atmospheric CO₂ or are capable of absorbing CO₂ from air. Experimentally, it was verified that, following melting at 1008 K, a Li₂CO₃ melt is stable in air at 998 K for at least 30 h without signs of Li₂O precipitation. The stability of the melt is attributed to partial decomposition of Li₂CO₃ into Li₂O, which decreases the liquidus temperature and reduces the equilibrium partial pressure of CO₂. Our findings prompt a revised view of the thermal stability of Li₂CO₃ melts in air.