Dynamics of Hydroxyl Anions Promotes Lithium Ion Conduction in Antiperovskite Li₂OHCl

Abstract

Li 2 OHCl is an exemplar of the antiperovskite family of ionic conductors, for which high ionic conductivities have been reported, but in which the atomic-level mechanism of ion migration is unclear. The stable phase is both crystallographically defective and disordered, having ∼1/3 of the Li sites vacant, while the presence of the OH − anion introduces the possibility of rotational disorder that may be coupled to cation migration. Here, complementary experimental and computational methods are applied to understand the relationship between the crystal chemistry and ionic conductivity in Li 2 OHCl, which undergoes an orthorhombic to cubic phase transition near 311 K (≈38 °C) and coincides with the more than a factor of 10 change in ionic conductivity (from 1.2 × 10 −5 mS/cm at 37 °C to 1.4 × 10 −3 mS/cm at 39 °C). X-ray and neutron experiments conducted over the temperature range 20−200 °C, including diffraction, quasi-elastic neutron scattering (QENS), the maximum entropy method (MEM) analysis, and ab initio molecular dynamics (AIMD) simulations, together show conclusively that the high lithium ion conductivity of cubic Li 2 OHCl is correlated to "paddlewheel" rotation of the dynamic OH − anion. The present results suggest that in antiperovskites and derivative structures a high cation vacancy concentration combined with the presence of disordered molecular anions can lead to high cation mobility.

Anti‐perovskite materials for energy storage batteries

“…Copyright 2020, American Chemical Society. 63 SSEs, solid-state electrolytes phase, as shown in Figure 3C. It indicated that the H movement in the orthorhombic phase is much slower than that in the cubic phase.…”

“…The MD simulations showed that the lower coordination of oxygen with Li corresponds to the faster the movement of the proton due to the larger space left for the movement of the proton. 63 Additionally, the dynamics behavior of the proton and Li in the cubic Li 2 OHCl has been studied by Dawson et al 64 The MD simulations demonstrated that the protons can just locally move around the O atoms in the center of the octahedron, thus they do not participate in the long-distance diffusion. In contrast, the Li + can diffuse in the whole lattice.…”

“…Furthermore, the tracing positions of the motional Li and H during MD simulations (Figure 3B) shows that the OH rotation can create more Li interstitials and facilitate Li + diffusion. 34 Moreover, Wang et al 63 reported that the mean square displacement (MSD) for Li and H in the cubic Li 2 OHCl is much larger than that in the orthorhombic 34 (C) Mean-squared displacement (MSD) of species in cubic, orthorhombic, and cubic (with fixed H) phases at 550 K, evaluated from ionic trajectories calculated over 100 ps ab initio molecular dynamics (AIMD). Reproduced with permission.…”

Paradigms of frustration in superionic solid electrolytes

“…Finally, we point out that anion rotational disorder and highly concerted local cation mobility operate in tandem within the so-called ‘paddlewheel mechanism’, which has been widely discussed in the literature [ 72 , 88 , 93 , 95 , 105 – 112 ]. Although this term is sometimes invoked to describe any type of anion rotation dynamical frustration, in our view this definition is too liberal.…”

Antiperovskite Superionic Conductors: A Critical Review

“…Theoretical and experimental investigations into Li 2 OHCl have provided solid evidence of a conductivity jump related to a paddlewheel mechanism. Li 2 OHCl is stable in the orthorhombic phase at room temperature and transitions to the cubic phase at 38 °C. ,,, The phase transition is accompanied by an increase in ionic conductivity from 1.2 × 10 –8 S cm –1 at 37 °C to 1.4 × 10 –6 S cm –1 at 39 °C . Above the transition, its ionic conductivity is similar to Li 2 OHBr, which is cubic at all temperatures. , Eilbracht et al establish with neutron powder diffraction of Li 2 ODCl that OD is static in the low-temperature orthorhombic phase but can point to different octahedral faces in the cubic phase .…”