Protons Enhance Conductivities in Lithium Halide Hydroxide/Lithium Oxyhalide Solid Electrolytes by Forming Rotating Hydroxy Groups

Abstract

Li‐halide hydroxides (Li2OHX) and Li‐oxyhalides (Li3OX) have emerged as new classes of low‐cost, lightweight solid state electrolytes (SSE) showing promising Li‐ion conductivities. The similarity in the lattice parameters between them, careless synthesis, and insufficient rigor in characterization often lead to erroneous interpretations of their compositions. Finally, moisture remaining in the synthesis or cell assembling environment and variability in the equivalent circuit models additionally contribute to significant errors in their properties. Thus, there remains a controversy about the real values of Li‐ion conductivities in such SSEs. Here an ultra‐fast synthesis and comprehensive material characterization is utilized to report on the ionic conductivities of contaminant‐free Li2+xOH1−xCl (x=0‐0.7), and Li2OHBr not exceeding 10‐4 S cm‐1 at 110 °C. Using powerful combination of experimental and numerical approaches, it is demonstrated that the presence of H in these SSEs yields significantly higher Li+ ‐ionic conductivity. Born‐Oppenheimer molecular dynamics simulations show excellent agreement with experimental results and reveal an unexpected mechanism for faster Li+ transport. It involves rotation of a short OH‐group in SSEs, which opens lower‐energy pathways for the formation of Frenkel defects and highly‐correlated Li+ jumps. These findings will reduce the existing confusions and show new avenues for tuning SSE compositions for further improved Li‐ion conductivities.

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

“…Upon heating to 40 °C, only a single 7 Li MAS peak at 0.697 ppm remains, whereas the left flank peak disappears. The occurrence of one single peak corresponds to two near equivalent Li sites per unit cell with similar LiO bond lengths of 1.96 Å (Figure a), which is consistent with the cubic symmetry obtained from XRD . The change of Li environment from orthorhombic to tetragonal/cubic structure is associated with H + and Li + dynamics.…”

“…Second, we analyze the temperature‐dependent H + dynamics and their major impact on the Li + dynamics and report that there is no long‐range H‐ion diffusion. Finally, we shed light on the relationship between Li + , H + , and the crystal‐dependent ion dynamics that explains the impact of the temperature‐dependent crystal structure of Li 2 OHCl on its ionic conductivity . This validates the MD simulation results that the translational motion of Li + correlates with the motion of H + …”

“…Finally, we shed light on the relationship between Li + , H + , and the crystal-dependent ion dynamics that explains the impact of the temperature-dependent crystal structure of Li 2 OHCl on its ionic conductivity. [12,19] This validates the MD simulation results that the translational motion of Li + correlates with the motion of H + . [12,13] This paper addresses the influence of proton on Li + ionic dynamics in ultrahigh purity Li 2 OHCl by recording 1 H and 7 Li NMR data.…”

“…Li + translational motions result in a sharp increase in ionic conductivities . The Li‐ion conductivities measured by electrochemical impedance spectroscopy (EIS) are shown in Figure 4 a inset.…”

“…While Li 2 OHCl has a similar crystal structure to Li 3 OCl, Li vacancies and OH groups are present in Li 2 OHCl . The OH groups in Li 2 OHCl is believed to boost Li‐ion motion based on the recently published the molecular dynamic (MD) simulations . Unfortunately, it is challenging to validate the mechanism of Li‐ion transport in this compound with existing experimental data and by means of conventional techniques, such as X‐ray diffraction (XRD), and common electrochemical measurements.…”