Halide sublattice dynamics drive Li-ion transport in antiperovskites

Abstract

Proton incorporation and halide mixing allow for lower synthesis temperatures and more rapid processing of cubic lithium antiperovskites. Halide mixing softens halide-sublattice dynamics for improved Li+ transport.

Halide Substitution Effects on Lithium-Ion Diffusion in Protonated Antiperovskites

“…, which have a span of ±500 ppm for Li 2 OHCl at room temperature, 6 not observable in our Li 2 OHCl sample at room temperature before the transition to cubic from orthorhombic, in conjunction with the facilitated Li + ion mobility. In addition, we confirmed that the observed line widths of the peaks are primarily due to the homonuclear dipole−dipole interactions (i.e., 7 Li− 7 Li), rather than heteronuclear dipolar interactions (such as 7 Li− 1 H), on the basis of the 7 Li NMR spectral line width obtained at 353 K, with and without 1 H decoupling (see Figure S3) and the separation of the rotational correlation times between Li + ions and OH − groups. 7 Consequently, the line widths of the 7 Li NMR spectra in these samples are strongly correlated with the mobility of Li + ions.…”

“…The static 7 Li NMR spectra of the series of samples are shown in Figure 1. The peak at ∼0 ppm corresponds to the central transition (m…”

“…Samples that are poorly sintered, having excess grain boundaries, tend to have σ activation energies between 0.5 and 0.7 eV, while dense samples with large grains have values between 0.2 and 0.4 eV. In this work, we were able to determine the Li + ion diffusion coefficients in the grain, D G , and grain boundary, D GB , for pLiAPs directly from the two main components of the echo profiles in 7 Li pulsed-field gradient nuclear magnetic resonance (PFG-NMR). The results showed that the two main components could be discriminated at the diffusion length of ∼1.7 μm, which is approximately the average size of grains.…”

“…All nuclear magnetic resonance (NMR) experiments were performed on a 600 MHz NMR spectrometer (Agilent). Lithium-ion diffusion coefficients (D G and D GB ) for the series of samples were determined at 353 K by using 7 Li pulsed-field gradient nuclear magnetic resonance (PFG-NMR) with a 5-mm diffusion probe (Doty Scientific) equipped with a maximum z-gradient strength of ∼31 T/m. The 13-interval bipolar-gradient stimulated-echo sequence ("Dbppste," VNMRJ, Varian) was used to record the gradient strength-dependent echo intensities (i.e., integrated area) to determine the diffusion coefficients of Li + ions following the Stejskal−Tanner equation 13…”

“…7 Here, it would be noteworthy to mention that the T 1 (and T 2 ) relaxations are dominated by short-range interactions (with distances on the order of a few angstroms) due to their dependence on the inverse sixth power of distance (r −6 ) between the interacting spins, 20 and 7 Li (I = 3/2) has multiple T 1 transition probabilities between the states −1/2 ↔ +1/2, −3/2 ↔ −1/ 2, and +1/2 ↔ +3/2. However, on account of the cubic symmetry of the sites, the 7 Li relaxation occurs by the dipole− dipole interactions instead of quadrupolar interaction, as evidenced by the fact that all 7 Li T 1 decays observed in this study were single-exponential decays, especially owing to the fast exchange of Li + ions. 17 It should be noted that the line width of the narrow component of the PFG echo is the same as that of the single-pulse excitation (Figure S2), suggesting all lithium ions with higher mobility have the similar T 2 values.…”