Abstract
Lithium-thiophosphates have attracted great attention as they offer a rich playground to develop tailor-made solid electrolytes for clean energy storage systems. Here, we used poorly conducting Li6PS5I, which can be converted into a fast ion conductor by high-energy ball-milling to understand the fundamental guidelines that enable the Li+ ions to quickly diffuse through a polarizable but distorted matrix. In stark contrast to well-crystalline Li6PS5I (10–6 S cm–1), the ionic conductivity of its defect-rich nanostructured analog touches almost the mS cm–1 regime. Most likely, this immense enhancement originates from site disorder and polyhedral distortions introduced during mechanical treatment. We used the spin probes 7Li and 31P to monitor nuclear spin relaxation that is directly induced by Li+ translational and/or PS43– rotational motions. Compared to the ordered form, 7Li spin–lattice relaxation (SLR) in nano-Li6PS5I reveals an additional ultrafast process that is governed by activation energy as low as 160 meV. Presumably, this new relaxation peak, appearing at Tmax = 281 K, reflects extremely rapid Li hopping processes with a jump rate in the order of 109 s–1 at Tmax. Thus, the thiophosphate transforms from a poor electrolyte with island-like local diffusivity to a fast ion conductor with 3D cross-linked diffusion routes enabling long-range transport. On the other hand, the original 31P nuclear magnetic resonance (NMR) SLR rate peak, pointing to an effective 31P-31P spin relaxation source in ordered Li6PS5I, is either absent for the distorted form or shifts toward much higher temperatures. Assuming the 31P NMR peak as being a result of PS43– rotational jump processes, NMR unveils that disorder significantly slows down anion dynamics. The latter finding might also have broader implications and sheds light on the vital question how rotational dynamics are to be manipulated to effectively enhance Li+ cation transport.
Highlights
IntroductionThe enigmatic interplay between cation translational processes and rotational dynamics of complex anions[1−3] propelled the (re-)investigation of a range of Li-containing and Na-bearing thiophosphates.[4−7] The interest in fast ionic conductors[8] is spurred by the demand to develop high-performance energy storage systems relying on ceramic electrolytes.[9]
The enigmatic interplay between cation translational processes and rotational dynamics of complex anions[1−3] propelled theinvestigation of a range of Li-containing and Na-bearing thiophosphates.[4−7] The interest in fast ionic conductors[8] is spurred by the demand to develop high-performance energy storage systems relying on ceramic electrolytes.[9]
The paddlewheel mechanism[1,8,36] explains rapid cation translational dynamics by opening low-energy passageways through rotational jumps of the polyanions that form the polarizable matrix of the electrolyte.[1]
Summary
The enigmatic interplay between cation translational processes and rotational dynamics of complex anions[1−3] propelled the (re-)investigation of a range of Li-containing and Na-bearing thiophosphates.[4−7] The interest in fast ionic conductors[8] is spurred by the demand to develop high-performance energy storage systems relying on ceramic electrolytes.[9]. The validity of such mechanisms has been questioned in the past as the larger free transport volume for the rotational phases might play a role too;[3,10] an increase in lattice volume, e.g., by the introduction of larger lattice units, will facilitate anion rotation
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