Mechanical detection of ultraslow, Debye-like Li-ion motions in LiAlO2 single crystals

Single crystalline LiAlO2 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. LiAlO2 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 LiAlO2 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 LiAlO2. Moreover, this enabled us to directly probe extremely slow Li exchange rates at temperatures slightly above 430K. The results were compared with recent insights from nuclear magnetic resonance spectroscopy. Altogether, an Arrhenius-type Li diffusion process with an activation energy of ca. 1.12eV 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.