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Driving cyanide dynamics and ion transport through point disorder in the plastic crystal Li₆PS₅CN
Ray, Connor E.
Ray, Connor E.
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2023
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Abstract
Lithium argyrodites have demonstrated ionic conductivities approaching those of liquid electrolytes and provide a promising route to the next generation of solid state battery materials. The structure and ionic conductivity of the cyanide argyrodite, Li6PS5CN, has been shown to exhibit a lower energy barrier to ion diffusion compared to the bromide analogue, making it a promising candidate for further study. It has been hypothesized that this reduction in activation energy could be attributed to dynamic rotations through a "paddlewheel'' mechanism. Additionally, the polar nature of the CN- may elicit dipole frustration, which could provide additional low-energy ion migration pathways. Through a suite of experimental and computational probes, including 7Li SSNMR, temperature dependent structural characterization, and molecular dynamics simulations, we find that Li+ ions exhibit fast local hopping consistent with superionic conductivity, and that the Li+ ions preferentially orient along the negatively polarized end of CN-, suggesting lithium-cyanide coupling. However, we do not observe the onset of paddlewheel motion or temperature-dependent ordering of the cyanide moieties in this material. We demonstrate that site mixing disorder plays a key role in the presence of dynamic rotations in Li6PS5CN, and both avenues to enhanced conductivity are coupled.
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