Tuning Site Energy by XO6 Units in LiX2(PO4)3 Enables High Li Ion Conductivity and Improved Stability.

Solid-state electrolytes (SSEs) with high ion conductivity are necessary for all-solid-sate lithium ion batteries. Here, a less studied NASICON-type LiZr2(PO4)3 (LZP) is screened out from seven LXP compounds (LiX2(PO4)3, X = Si, Ge, Sn, Ti, Zr, Hf, and Mo), which combines the electrochemical stability with high Li conductivity. The bond valence site energy (BVSE), climbing image nudged elastic band (Cl-NEB) method, and electrochemical phase diagram prove LZP has a lower Li migration barrier and the largest electrochemical stability window. The underlying reason for high Li conductivity is analyzed from the structural features to the electronic structures. Furthermore, the XO6 unit mixed frameworks Li1.667Ca0.333Zr1.667(PO4)3 (LCZP) and Li1.667Mg0.333Zr1.667(PO4)3 (LMZP) exhibit high Li ion conductivity associated with a very low Li migration barrier (∼0.20 eV). This work opens a new avenue of broad compositional spaces in LXP for SSEs.