On the\nThermal Conductivity and Local Lattice Dynamical\nProperties of NASICON Solid Electrolytes

The recent development\nof solid-state batteries brings them closer\nto commercialization and raises the need for heat management. The\nNASICON material class (Na<sub>1+<i>x</i></sub>Zr<sub>2</sub>P<sub><i>x</i></sub>Si<sub>3–<i>x</i></sub>O<sub>12</sub> with 0 ≤ <i>x</i> ≤ 3) is\none of the most promising families of solid electrolytes for sodium\nsolid-state batteries. While extensive research has been conducted\nto improve the ionic conductivity of this material class, knowledge\nof thermal conductivity is scarce. At the same time, the material’s\nability to dissipate heat is expected to play a pivotal role in determining\nefficiency and safety, both on a battery pack and local component\nlevel. Dissipation of heat, which was, for instance, generated during\nbattery operation, is important to keep the battery at its optimal\noperating temperature and avoid accelerated degradation of battery\nmaterials at interfaces. In this study, the thermal conductivity of\nNaZr<sub>2</sub>P<sub>3</sub>O<sub>12</sub> and Na<sub>4</sub>Zr<sub>2</sub>Si<sub>3</sub>O<sub>12</sub> is investigated in a wide temperature\nrange from 2 to 773 K accompanied by in-depth lattice dynamical characterizations\nto understand underlying mechanisms and the striking difference in\ntheir low-temperature thermal conductivity. Consistently low thermal\nconductivities are observed, which can be explained by the strong\nsuppression of propagating phonon transport through the structural\ncomplexity and the intrinsic anharmonicity of NASICONs. The associated\nlow-frequency sodium ion vibrations lead to the emergence of local\nrandom-walk heat transport contributions via so-called diffusons.\nIn addition, the importance of lattice dynamics in the discussion\nof ionic transport as well as the relevance of bonding characteristics\ntypical for mobile ions on thermal transport, is highlighted.