| Summary: | Solid-state reaction was used for Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> material synthesis from Li<sub>2</sub>CO<sub>3</sub>, La<sub>2</sub>O<sub>3</sub> and ZrO<sub>2</sub> powders. Phase investigation of Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> was carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) methods. The thermodynamic characteristics were investigated by calorimetry measurements. The molar heat capacity (<i>C<sub>p,m</sub></i>), the standard enthalpy of formation from binary compounds (Δ<i><sub>ox</sub>H<sub>LLZO</sub></i>) and from elements (Δ<i><sub>f</sub>H<sub>LLZO</sub></i>), entropy (<i>S</i><sup>0</sup><sub>298</sub>), the Gibbs free energy of the Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> formation (∆<i><sub>f</sub></i> <i>G</i><sup>0</sup><sub>298</sub>) and the Gibbs free energy of the LLZO reaction with metallic Li (∆<i><sub>r</sub>G<sub>LLZO/Li</sub></i>) were determined. The corresponding values are <i>C<sub>p,m</sub></i> = 518.135 + 0.599 × T − 8.339 × T<sup>−2</sup>, (temperature range is 298–800 K), Δ<i><sub>ox</sub>H<sub>LLZO</sub></i> = −186.4 kJ·mol<sup>−1</sup>, Δ<i><sub>f</sub>H<sub>LLZO</sub></i> = −9327.65 ± 7.9 kJ·mol<sup>−1</sup>, <i>S</i><sup>0</sup><sub>298</sub> = 362.3 J·mol<sup>−1</sup>·K<sup>−1</sup>, ∆<i><sub>f</sub></i> <i>G</i><sup>0</sup><sub>298</sub> = −9435.6 kJ·mol<sup>−1</sup>, and ∆<i><sub>r</sub>G<sub>LLZO/Li</sub></i> = 8.2 kJ·mol<sup>−1</sup>, respectively. Thermodynamic performance shows the possibility of Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> usage in lithium-ion batteries.
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