Mechanism of Li-Ion Migration in the Superionic Conducting Open-Framework Structure Li₆B₁₈(Li₃N)_1–x(Li₂O)_x (0 ≤ x ≤ 1)

Abstract

Solid lithium-ion conductors are important components for allsolid-state batteries and the knowledge of the mechanism of Li diffusion is an important step in improving known materials and developing new materials. For the Li-ion conductor α-Li 3 N, the lithium diffusion process has been intensively investigated. We report here on the Li-ion diffusion in the open-framework structure Li 6 B 18 (Li 3 N) 1−x (Li 2 O) in which Li 3 N and/or Li 2 O serve as a guest. Whereas vacancy formation for α-Li 3 N is not possible by forming a solid solution with Li 2 O, the solid solution of Li 6 B 18 (Li 3 N) 1−x (Li 2 O) x exists over the whole composition range with an increasing number of Li vacancies with x and samples for x = 0, 0.25, 0.5, 0.75, and 1 are investigated. A variety of solid-state NMR approaches, including 7 Li T 1 relaxation NMR, temperature-dependent 6 Li-magic angle spinning (MAS)-NMR, and 6 Li-{ 7 Li}-cross-polarization (CP)-MAS 2Dexchange NMR, and a detailed 7 Li line shape analysis are combined with quantum chemical calculations of Li migration pathways to unravel the mechanism of Li diffusion in the open-framework structures Li 6 B 18 (Li 3 N) 1−x (Li 2 O) x , hosting three different Li sites. The combined results indicate an anisotropic Li diffusion process, in which the motion along the crystallographic c-direction seems to be strongly hindered (Li2 ↔ Li3). On the other hand, the diffusion pathway in the ab-plane is characterized by a two-step motional process that combines Li1 ↔ Li2 and Li2 ↔ Li2 jumps with very low activation energies in the range of 30−40 kJ/mol for Li1 ↔ Li2 and 5�15 kJ/mol for Li2 ↔ Li2. Thus, lithium migration within the title compound bears strong similarities to the Li diffusion processes present in the well-known Li-ion conductor α-Li 3 N.

Lithium‐ion Mobility in Li6B18(Li3N) and Li Vacancy Tuning in the Solid Solution Li6B18(Li3N)1−x(Li2O)x

“…A study of the solid solution including theoretical and experimental determination of Li ion migration pathways is presented in a forthcoming paper. [33]…”

Determining the chemical composition of diamagnetic mixed solids via measurements of their magnetic susceptibility

“…Replacement of a fraction of the cations A and B or anions X of solid compounds of the types AX or ABX by a different element of the same valency A′, B′ or X′, respectively, can dramatically alter the physical and chemical properties of the system. 1–5 The ions can also be replaced by the same element but with a different isotope. 6 During the synthesis process of a sample with such mixed concentrations, often the chemical composition of the final product is different than the molar ratios of the starting solution.…”