Stability, Elastic Properties, and the Li Transport Mechanism of the Protonated and Fluorinated Antiperovskite Lithium Conductors

Abstract

Lithium-rich antiperovskites (APs) have attracted significant research attention due to their ionic conductivity above 1 mS cm −1 at room temperature. However, recent experimental reports suggest that proton-free lithium-rich APs, such as Li 3 OCl, may not be synthesized using conventional methods. While Li 2 OHCl has a lower conductivity of about 0.1 mS cm −1 at 100 °C, its partially fluorinated counterpart, Li 2 (OH) 0.9 F 0.1 Cl, is a significantly better ionic conductor. In this article, using density functional theory simulations, we show that it is easier to synthesize Li 2 OHCl and two of its fluorinated variants, i.e., Li 2 (OH) 0.9 F 0.1 Cl and Li 2 OHF 0.1 Cl 0.9 , than Li 3 OCl. The transport properties and electrochemical windows of Li 2 OHCl and the fluorinated variants are also studied. The ab initio molecular dynamics simulations suggest that the greater conductivity of Li 2 (OH) 0.9 F 0.1 Cl is due to structural distortion of the lattice and correspondingly faster OH reorientation dynamics. Partially fluorinating the Cl site to obtain Li 2 OHF 0.1 Cl 0.9 leads to an even greater ionic conductivity without impacting the electrochemical window and synthesizability of the materials. This study motivates further research on the correlation between local structure distortion, OH dynamics, and increased Li mobility. Furthermore, it introduces Li 2 OHF 0.1 Cl 0.9 as a novel Li conductor.

Anti‐perovskite materials for energy storage batteries

“…30 And theoretical calculations also demonstrated that the formation energy of Li 2 OHCl from LiOH + LiCl is significantly lower than that of Li 3 OCl. 65 However, the calculated value of formation energy of Li 2 OHCl is positive, meaning that Li 2 OHCl is also thermodynamically unstable. It is easy to obtain the Li 2 OHCl with LiCl-deficient experimentally, and hence it is more stable than the ideal Li 2 OHCl crystal.…”

“…In addition, the mismatch of the ionic sizes between F À , OH À , and Cl À (1.33 Å, 1.37 Å, and 1.81 Å, respectively) causes the local structural distortions that enlarged the Li + migration channels close to the F atom. 65…”

“…80,81 As shown in Figure 6E, the electrochemical window of Li 2 OHCl and fluorinated Li 2 OHCl was estimated to be 0.82-3.15 V versus Li/Li + . 65 The lithiation plateau at 0.82 V corresponds to the H + reduction reaction with the possible decomposition products LiH, Li 2 O, and LiCl. The upper limit of the electrochemical stability window at 3.15 V corresponds to the Cl À oxidation reaction, which is much lower than the reported 9 V by conventional CV measurement.…”

Paradigms of frustration in superionic solid electrolytes

“…Other studies investigated alloying as a way to induce site distortion in anti-perovskites. For instance, Effat et al attributed the faster conduction of fluorinated Li 2 OHCl in part to distortion of sites upon substitution of F − for Cl − , which flattens out the energy minima in the landscape [65]. Chen et al found the same distortion-induced frustration effect could be prompted by incorporating Br − on the Cl − site in Li 3 OCl 1−x Br x [66].…”

“…For instance, Effat et al. attributed the faster conduction of fluorinated in part to distortion of sites upon substitution of for , which flattens out the energy minima in the landscape [ 65 ]. Chen et al.…”