Untangling the Structure and Dynamics of Lithium-Rich Anti-Perovskites Envisaged as Solid Electrolytes for Batteries

Abstract

Lithium-rich anti-perovskites (LiRAPs) have attracted a great deal of attention as they have been praised as another superior group of solid electrolytes that can be used to realize all-solid-state batteries free of flammable liquids. Despite several studies that have reported on the properties of LiRAPs, many questions remain unanswered. In particular, these include fundamental ones concerning the structure, stability, and Li-ion conductivity and diffusivity. Moreover, it is not clear whether some of the previously reported compounds do really exist. To untangle the current picture of LiRAPs, we synthesized "Li 3 OCl" and Li 2 OHCl polymorphs and applied a wide spectrum of methods, such as powder X-ray diffraction (PXRD), powder neutron diffraction (PND), nuclear magnetic resonance spectroscopy, and impedance spectroscopy to carefully shed some light on LiRAPs. Here we self-critically conclude that the cubic polymorph of the two compounds cannot be easily distinguished by PXRD alone as the lattice metrics and the lattice parameters are very similar. Furthermore, PXRD suffers from the difficulty of detecting H and Li. Even Rietveld refinement of our PND data turned out to be complicated and not easily interpreted in a straightforward way. Nevertheless, here we report the first structural models for the cubic and a new orthorhombic polymorph containing also structural information about the H atoms. In situ PXRD of "Li 3 OCl", intentionally exposed to air, revealed rapid degradation into Li 2 CO 3 and amorphous LiCl•xH 2 O. Most likely, the instability of "Li 3 OCl" explains earlier findings about the unusually high ion conductivities as the decomposition product LiCl•xH 2 O offers an electrical conductivity that is good enough for some applications, excluding, of course, those that need aprotic conditions or electrolytes free of any moisture. Considering "H-free Li 3 OCl" as well as Li 5 (OH) 3 Cl 2 , Li 5 (OH) 2 Cl 3 , Li 3 (OH) 2 Cl, and Li 3 (OH)Cl 2 , we are confident that Li 4 (OH) 3 Cl and variants of Li 3−x (OH x )Cl, where x > 0, are, from a practical point of view, so far the only stable lithium-rich anti-perovskites.

Understanding Li‐Ion Dynamics in Lithium Hydroxychloride (Li2OHCl) Solid State Electrolyte via Addressing the Role of Protons

“…For both 7 Li and 1 H nuclei, the spin‐lattice relaxation rate R 1 , which is the inverse of T 1 , was recorded between −40 and +80 °C ( Figure 2 ). Notably, we observed at most temperatures a nonexponential T 1, which is contrast to previous studies, suggesting inhomogeneous spatial structure of the Li 2 OHCl with different mobilities, likely associated with spatial fluctuation induced by the defect sites. To describe it and provide a good fit for measured data we had to use bi‐exponential function with two different relaxation times T 1 (fast) and T 1 (slow) for both 1 H and 7 Li (Figure S5, Supporting Information), Equation …”

“…A slight reduction in the linewidth still occurs above 40 °C (Stage IV) due to the change in the crystal structure, which eliminates the remaining minor dipolar coupling with the onset of Li + translational motions . In the high temperature (HT) cubic structure, the Li + translation motion is consistent with what has been found in previous studies . However, a remaining open question is why Li + translational motion appears to occur only above 40 °C, while a significant amount of line‐narrowing and associated rotational motion occurs below 40 °C.…”

“…[23] In the high temperature (HT) cubic structure, the Li + translation motion is consistent with what has been found in previous studies. [17,18] However, a remaining open question is why Li + translational motion appears to occur only above 40 °C, while a significant amount of line-narrowing and associated rotational motion occurs below 40 °C. 7 Li-{ 1 H} decoupling experiments effectively removed the heteronuclear 1 H- 7 Li dipolar coupling, resulting in the reduced the linewidth of 7 Li NMR spectra ( Figure S3, Supporting Information), thus suggesting that H + dynamics in the crystallites of OH groups may impact the long-range Li + diffusion.…”

“…Solid-state nuclear magnetic resonance (NMR) is uniquely positioned to study fundamentals of ionic motions in SSEs. [14,15] While several previous NMR studies conducted for Li-antiperovskites have founded a reduction of the linewidth in 7 Li spectra with Li-ionic conductivity, [16][17][18] there is no comprehensive examination of Li-ion diffusion. Moreover, there is no experimental evidence of proton dynamics, which prohibits our understanding of the possible scope of Li 2 OHCl use.…”

Lithium and Chlorine-Rich Preparation of Mechanochemically Activated Antiperovskite Composites for Solid-State Batteries

“…For x = 0.5 and 1, observed major peaks display cubic symmetry in the Pm3m space group, suggesting the formation of an antiperovskite Li 3 OCl phase. However, hydrated variants of Li 3 OCl can also crystallize in the cubic symmetry with similar lattice parameters (Schwering et al, 2003 ; Hanghofer et al, 2018 ). As their XRD patterns are indistinguishable, we denote this cubic phase as Li 3−y (OH y )Cl (0 ≤ y < 1).…”