Lithium Antiperovskite-Derived Glass Solid Electrolytes

Abstract

In this paper, we report the synthesis of Li₂OHX (X = Br, Cl)-based glasses. These glasses were found to be challenging to synthesize, requiring extreme cooling rates achievable only by a twin-roll quench process. As has been speculated for antiperovskite-derived glasses, indications of improved lithium-ion dynamics are observed. Notably, spin-lattice relaxation nuclear magnetic resonance spectroscopy reveals a higher hopping frequency and significantly lower activation energy for Li₂OHBr glasses (0.29 eV) compared to the crystalline Li₂OHBr (0.39 eV). This may be attributable to the increased free volume in the glass samples (ρ_glass/ρ_cryst = 0.83) and a reduced ionic interaction of lithium ions with the glass structure. Despite these promising findings, the glasses were found to be unstable under pressure and crystallized in attempts to produce bulk samples for impedance measurements.

Figure 1

Proof of glass synthesis. a) Photographs of transparent “glassy” Li₂OHBr flakes (i) held by tweezers and (ii) stored in a vial. b) Similar photographs, this time showing opaque examples of flakes. c) XRD patterns of Li₂OHBr and Li₂OHCl glasses showing the absence of the characteristic Bragg peaks seen in the crystalline homologues. d) DSC measurements of crystalline and glassy Li₂OHBr and Li₂OHCl on the first heating cycle. The insets (i–iv) show glass transition events are present only in the glassy samples.

Figure 2

Structural and chemical characterization. a) Surface SEM imaged of a Li₂OHBr glass flake. The rolling direction of the twin-roll quench process can be seen to go from bottom left to top right. Small cracks can be seen in various locations. Small amounts of surface contamination appear as bright specs. b) SEM image of a cross-section of a Li₂OHBr glass flake prepared using a plasma focused-ion beam. Small isolated pores can be seen in various locations. c) Raman spectroscopy of glassy and crystalline Li₂OHBr measured between 3500 and 3650 cm^–1. d) ⁷Li MAS NMR line shape obtained from glass-ceramic Li₂OHBr at 393 K, showing a glass peak centered on 2.9 ppm and a crystalline peak at −1.6 ppm. e) Static ⁷Li NMR of glass and crystalline Li₂OHBr. Satellites are only observed in the crystalline material due to its high symmetry local structure.

Figure 3

Lithium ion mobility. SLR-NMR R_1ρ measurements for a) glass and b) crystalline Li₂OHBr. Two rate peaks can be seen in each instance. The high-temperature flank activation energies and temperature of the peak maximum are both lower in the glass sample, indicating better lithium dynamics.