Significant regulation of stress on the contribution of optical phonons to thermal conductivity in layered Li₂ZrCl₆: First-principles calculations combined with the machine-learning potential approach

The layered solid electrolyte Li₂ZrCl₆ and Li metal electrodes have a very good contact stability, but the thermal transport properties of Li₂ZrCl₆ are still unclear. Here, we systematically study the intrinsic lattice thermal conductivity (κ_p) of Li₂ZrCl₆ using the machine-learning potential approach based on first-principles calculations combined with the Boltzmann transport theory. The results show that the κ_p of Li₂ZrCl₆ at room temperature is 3.94 W/mK along the in-plane (IP) direction and 1.05 W/mK along the out-plane (OP) direction, which means that the κ_p is significantly anisotropic. In addition, under the compressive stress in the OP direction, the κ_p evolution along the IP and OP directions exhibits completely different trends, because the stress has a significant regulatory effect on the contribution of optical phonons to κ_p. With the increase in stress, the κ_p in the IP direction monotonically decreases, while the κ_p in the OP direction increases by a factor of 2.2 under a compressive strain of 13%. This is because the contribution of low-frequency optical phonons to κ_p in the IP direction is as high as 58% when no stress is applied, and this contribution is significantly suppressed with increasing compressive strain. However, the contribution of optical phonons in the OP direction to the κ_p increases with the increase in stress. Our results reveal the thermal transport properties of Li₂ZrCl₆ and the effect of the compressive strain on the κ_p of Li₂ZrCl₆, thereby providing a reference for the use of Li₂ZrCl₆ in Li-metal batteries.