Thermal properties and lattice anharmonicity of Li-ion conducting garnet solid electrolyte Li6.5La3Zr1.5Ta0.5O12

Abstract

Garnet-type solid electrolytes have garnered significant interest for all-solid-state battery applications due to their high Li-ion conductivity and excellent compatibility with the metallic lithium anode. Although their ionic conductivity optimization and conduction mechanisms have been extensively studied, the thermal properties of garnet electrolytes remain poorly understood. The thermal conductivity, thermal expansion coefficient, and specific heat of solid electrolytes play a crucial role in determining the overall performance, safety, and reliability of battery devices. Understanding these properties is essential for enhanced thermal management and improved energy efficiency of batteries. Here, we investigate the thermal properties of Ta⁵⁺-doped garnet Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) polycrystals. The thermal conductivity measurement reveals a low-temperature peak at around 50 K. Above this temperature, the thermal conductivity shows a weak temperature dependence and approaches the calculated minimum thermal conductivity. The average phonon mean free path increases as the temperature decreases, reaching approximately 500 nm at 5 K. The linear thermal expansion coefficient obtained through the X-ray diffraction measurements is 1.71 ± 0.01 × 10⁻⁵ K⁻¹ at 323 K, larger than the reported value for undoped Li₇La₃Zr₂O₁₂. The Grüneisen parameter of LLZTO calculated using the measured thermal expansion coefficient and specific heat is 1.63 ± 0.04 at 300 K, indicating relatively strong phonon anharmonicity. Furthermore, the thermal conductivity of aged LLZTO is increased by 70–80% at 300 K compared to the pristine samples, which can be attributed to the formation of Li₂CO₃ with increased thermal conductivity or reduced thermal boundary resistance. Our results provide useful insights into the structure–thermal property relationships that are closely relevant to thermal management of battery systems based on garnet solid electrolytes.