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

Solid lithium-ion conductors are important components for all-solid-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-Li₃N, the lithium diffusion process has been intensively investigated. We report here on the Li-ion diffusion in the open-framework structure Li₆B₁₈(Li₃N)_1-x(Li₂O) in which Li₃N and/or Li₂O serve as a guest. Whereas vacancy formation for α-Li₃N is not possible by forming a solid solution with Li₂O, the solid solution of Li₆B₁₈(Li₃N)_1-x(Li₂O)_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 ⁷Li T₁ relaxation NMR, temperature-dependent ⁶Li-magic angle spinning (MAS)-NMR, and ⁶Li-{⁷Li}-cross-polarization (CP)-MAS 2D-exchange NMR, and a detailed ⁷Li 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 Li₆B₁₈(Li₃N)_1-x(Li₂O)_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-Li₃N.