Interstitial Li⁺and Li⁺Migrations in the Li_{2+ x}C_{1- x}B_xO₃Solid Electrolyte

Herein, based on powder and single-crystal diffractions of the samples synthesized, and density functional theory (DFT) calculations, we investigated the crystal structure of the Li_2+xC_1-xB_xO₃(LCBO) solid solution ss1 (Li₂CO₃phase, 0 ≤ x < 0.35) electrolyte. Besides the Li1 site, three interstitial lithium sites [Li2 (8f), Li3 (8f), and Li4 (8f)] are observed. Most of the Li⁺are located at site Li1 with the occupancy (occ_Li1) > 0.80(1) and the occ_Li1decreases as x increases [occ_Li1= 0.92(1), 0.82(1), and 0.82(1) in Li_2.1C_0.9B_0.1O₃, Li_2.2C_0.8B_0.2O₃, and Li_2.3C_0.7B_0.3O₃, respectively]. The interstitial Li⁺occupy the site Li2 with occ_Li2+ occ_Li1≈ 1.0 and Li3, Li4 sites. We carried out theoretical calculations/optimizations to check the energy-preferred structural geometry; Li1 is the most energy-preferred site in both Li₂CO₃and Li₉C₃BO₁₂. We also investigated the ionic conductivity and Li⁺migrations of Li_2+xC_1-xB_xO₃ss1. The continuous -Li3-s1-Li2-s2-Li1-s3-Li3- one-dimensional (1D) pathway (through s1-s3 saddle sites) shows the lowest barrier energy (0.094-0.29 eV). The 1D paths are then connected by the saddle sites s4 (0.31 eV) and s5 (0.48 eV) with a higher barrier energy to form a curved two-dimensional (2D) migration pathway in the a-c plane, and are connected by Li4 through the saddle site s8 (0.49-0.55 eV) to form a three-dimensional (3D) framework migration pathway. The Li⁺migration is dominated by the -Li3-s1-Li2-s2-Li1-s3-Li3- 1D pathway at T ≤ 75 °C and by 1D, 2D, and 3D pathways at T ≥ 100 °C.