Lithium halides with general formula Li₃MX₆ (M = metals such as Y, In, Yb or Sc; X = Cl or Br) have attracted significant attention because of their promising room temperature conductivities and high oxidative stability^[1]. Li-ion mobility in these halides is strongly affected by the synthesis method and parameters. In Li₃YCl₆ with a hexagonal-close-packed (hcp) anion framework, the ionic conductivity is commonly observed to decrease with increased crystallinity. While in Li₃YBr₆ with a face-centered-cubic (fcc) anion framework, the ionic conductivity increases with increased crystallinity. So far there is no clear explanation for such opposite trends in these two otherwise very similar crystal structures. Ab initio molecular dynamics (AIMD) simulation^[2] suggested higher ionic conductivity and lower activation energy in Li₃YCl₆ than inLi₃YBr₆, which also does not agree well with the experimental observation that Li₃YBr₆ commonly shows higher conductivity than Li₃YCl₆. Above-mentioned unexplained observations and inconsistencies imply that likely some critical structural features of these halides that strongly impact ionic diffusion are not yet explored nor taken into consideration. Here, we report our recent findings in the crystal structure of Li₃YCl₆ and Li₃YBr₆ and the design, synthesis and electrochemical testing of a few new halide compounds designed based on the new findings. The crystal structures are investigated with using ex situ and in situ synchrotron X-ray diffraction (XRD) and neutron diffraction (ND). Our study^[3] demonstrated that the discrepancies between simulated and experimental results in Li₃YBr₆-type structure is partly caused by the grain boundary. Through PDF data analysis, we provide experimental evidences of cation local ordering in ab plane and stacking fault along c-direction in Li₃YBr₆-type structure. The lower ionic conductivity in low crystallinity is owing to the blockage of Li diffusion channel along c-direction caused by the stacking fault. Moreover, a new tetrahedral Li-site is identified, which may be critical in understanding the Li diffusion behaviors. Via rationally tuning Li occupancies at the tetrahedral and octahedral sites, low Li diffusion barriers can be achieved, which is in consistence with theoretical calculation results.

[1] T. Asano, A. Sakai, S. Ouchi, M. Sakaida, A. Miyazaki, S. Hasegawa, Advanced Materials 2018, 30, 1803075.

[2] S. Wang, Q. Bai, A. M. Nolan, Y. Liu, S. Gong, Q. Sun, Y. Mo, Angewandte Chemie International Edition 2019, 58, 8039-8043.

[3] Z. Liu, S. Ma, J. Liu, S. Xiong, Y. Ma, H. Chen, ACS Energy Letters 2021, 6, 298-304.