Mechanical detection of ultraslow, Debye‐like Li‐ion motions in LiAlO single crystals

Abstract

Single crystalline LiAlO 2 is known as a very poor ion conductor. Thus, in its crystalline form it unequivocally disqualifies itself from being a powerful solid electrolyte in modern energy storage systems. On the other hand, its interesting crystal structure proves beneficial to sharpen our understanding of Li ion dynamics in solids which in return might influence application-oriented research. LiAlO 2 allows us to apply and test techniques that are sensitive to extremely slow Li ion dynamics. This helps us clarifying their diffusion behaviour from a fundamental point of view. Here, we combined two techniques to follow Li ion translational hopping in LiAlO 2 that can be described by the same physical formalism: dynamic mechanical relaxation and electrical relaxation, i.e., ionic conductivity measurements. Via both methods we were able to track the same transport mechanism in LiAlO 2 . Moreover, this enabled us to directly probe extremely slow Li exchange rates at temperatures slightly above 430 K. The results were compared with recent insights from nuclear magnetic resonance spectroscopy. Altogether, an Arrheniustype Li diffusion process with an activation energy of ca. 1.12 eV was revealed over a large dynamic range covering 10 orders of magnitude, i.e., spanning a dynamic range from the nano-second time scale down to the second time scale.

Nanostructured Ceramics: Ionic Transport and Electrochemical Activity

“…The value of 0.7 eV agrees with that derived from neutron diffraction data that indicated a curved pathway connecting two adjacent Li sites in LiAlO 2 [88]. Long-range Li ion transport, on the other hand, has to be described with a higher activation energy [86,89]. Both secondary ion mass spectrometry (SIMS) and conductivity spectroscopy revealed essentially the same activation energies of ca.…”

“…[86] the solid-state diffusion coefficient D σ was estimated via the Nernst-Einstein equation by assuming that the number density of charge carriers is simply given by the lithium concentration of the oxide. Mechanical loss spectroscopy confirmed these diffusion coefficients and activation energies [89]. Dynamic mechanical analysis, which is sensitive to ultraslow Li ion displacements, points to a Debye-like motional correlation that controls ion dynamics at low temperatures.…”

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

“…Materials with a large number of vacancies and many available Li voids, which can easily be reached by the charge carriers, also include oxides, such as Al-stabilized [ 48 , 49 ], and thiophosphates such as [ 50 ]. In contrast to compounds with strong site preferences such as [ 51 ] and [ 52 ], the Li sites of the partially occupied sublattices in these compounds are connected by small activation energies facilitating rapid local and also effective through-going ion transport. Whereas for the tetragonal form of LLZO the Li-ions are distributed over the Li voids in a regular manner, in cubic LLZO, which is stabilized by replacing some of the Li-ions by Al ions, the sites 24 d and the split-site 96 h are only partially occupied.…”

Extended Condensed Ultraphosphate Frameworks with Monovalent Ions Combine Lithium Mobility with High Computed Electrochemical Stability

“…These connectivity patterns allow silicates and aluminates to adopt more condensed anionic networks than those in ultraphosphates, such as pure 3-connected and 4-connected anionic networks. , Most tectosilicates and some alkali polyaluminates adopt 4-connected nets. − There are a large number of 4-connected porous tectosilicates with large cavities or tunnels, such as clathrasils and zeolites. , However, in general, compared with 2,3-connected nets, the pure 3-connected and 4-connected nets prefer the most stable dense networks with narrow interstices, which are unfavorable for forming 3D ion migration pathways. A paradigmatic example is γ-LiAlO 2 , which is one of the materials used as a coating for lithium-conducting electrodes and adopts a dense 4-connected tetrahedral network with small channels and void spaces (Figure f) that are associated with the extremely low Li-ion conductivity (below 10 –15 S cm –1 at room temperature). − In contrast, the more open 2,3-connected anionic networks of ultraphosphates are of interest because they could create large void spaces that form favorable channels or interstitial positions for cations to diffuse through the structure. For example, the ultraphosphate layer could provide large PO 4 3– based rings, such as 16-membered rings in the P 4 O 11 2– layer (Figure g), 20-membered rings in the P 5 O 14 3– layer, and 14-membered rings in the P 6 O 17 4– layer. ,, …”