New Thermal Transport Regime for Partial-Crystalline Partial-Liquid Materials

Materials in partial-crystalline partial-liquid (PCPL) state are now widely used as thermoelectrics and battery electrodes, due to their low thermal conductivity and high ionic conductivity, respectively. However, the well-developed computational methods for pure crystalline materials such as anharmonic lattice dynamics coupled with Boltzmann transport equation cannot be used to study such systems. By performing first-principles and molecular dynamics simulations, we give a robust explanation of the thermal transport mechanism in PCPL material Li₂S. At the temperature range where the system can be regarded as a solid, the large hopping of Li is found to be responsible for phonon thermal conductivity’s deviation from the traditional 1/T relationship. At high temperatures, the contribution of convection and liquid-phonon interaction increase significantly due to the fluidization of Li ions. Furthermore, there is an interplay between the enhanced phonon scattering and the increased force hopping between neighboring atoms as temperature arises, which results in a dip in the evolution of the virial term around 1200K. When the temperature is even higher, the virial term increases with temperature due to the contribution of vibrations with extremely short mean free path (diffusons). This point is validated by the evolution of the accumulative thermal conductivity with mean free path. At 1300 K, more than 46% of the heat carried by the S sublattice is contributed by the carriers with mean free path smaller than a few angstroms, which is the typical hopping distance. Our study [1] provides a clear physical map of the heat transport in PCPL materials and describes the key mechanism to guide the design of future thermoelectric materials and battery electrodes.

[1] Yanguang Zhou, Shiyun Xiong, Xiaoliang Zhang, Sebastian Volz, and Ming Hu, "Thermal Transport Crossover from Crystalline to Partial-Crystalline Partial-Liquid State", Nature Communications, 9, 4712 (2018).