Thermodynamic properties of solid electrolyte Li7La3Zr2O12

The heat capacity temperature dependence of the high-potential solid electrolyte Li7La3Zr2O12 was experimentally determined using differential scanning calorimetry. Besides, the heat capacity in the temperature range 298-800 K was calculated according to the additive Neumann-Kopp rule. The experimental and calculated values of heat capacities are close to each other. The correctness of the Neumann-Kopp assumption for zirconates was also confirmed by comparing the available literature for other similar systems. The molar heat capacity of Li7La3Zr2O12 at constant pressure in the temperature range 298-800 K should be calculated as Cp,m = 533.725 + 0.128 × T - 9.537·106 × T−2, where T is absolute temperature. Other thermodynamic characteristics of tetragonal Li7La3Zr2O12 were determined: entropy (S○298 = 424.0 J mol−1 K−1), the standard Gibbs free energy of formation (ΔfG°298 = −6777.3 kJ mol−1), the enthalpy increment from 0 to 298 K (H○298-H○0 = 72.64 kJ mol−1), the enthalpy of formation at 0 K (ΔfH○0 = −7109.7 kJ mol−1). The performed thermodynamic calculations confirm that solid electrolyte Li7La3Zr2O12 is stable against metallic lithium at room temperature.