Fast Rotational Dynamics in Argyrodite-Type Li6PS5X (X: Cl, Br, I) as Seen by 31P Nuclear Magnetic Relaxation—On Cation–Anion Coupled Transport in Thiophosphates

Abstract

Abstract Electrolytes with excellent ionic conductivity are needed for the development of safe and long-lasting all-solid-state batteries. Perfect candidates are Argyrodites, like Li6PS5X (X: Cl, Br, I), that show an ionic conductivity in the mS/cm range. Translational Li+ diffusion in ceramic electrolytes is influenced by many factors such as crystal structure and defect chemistry. Considering thiophosphates, only little information is, however, available about the effects of rotational anion dynamics on Li+ jump processes. Here, we used 31P spin-lattice relaxation nuclear magnetic resonance (NMR) to find out whether rapid Li+ ion dynamics in Li6PS5X is affected by rotational jumps of the PS43– units. NMR revealed that in Li6PS5I, having an ordered anion sublattice, PS43– rotational dynamics are much faster than Li+ self-diffusion. At 223 K, the rotational correlation rate, which is obtained from NMR relaxation rate peaks, takes a very high value in the order of 109 events per second. It clearly decreases when anion disorder is introduced. At the same time, anion substitution contracts the lattice when we go from Li6PS5I to Li6PS5Br and further to Li6PS5Cl as the ionic radii of X decrease along this series. While for Li6PS5I, with its soft lattice, the rotational jumps of the PS43– units are decoupled, that is, independent from Li+ translation, in Li6PS5Cl, which has a smaller lattice constant, the much slower rotational dynamics seem to influence Li+ intercage hopping. The corresponding rotational jumps do occur almost on the same time scale as the Li+ exchange processes take place. For Li6PS5Br, on the other hand, an optimal balance between lattice properties, site disorder, and fast rotational jumps seems to be established, which leads to facile translational Li+ displacements. Our experiments show that rotational motions, if they are in resonance with cation exchange, should be considered when the origins of long-range Li+ transport in argyrodite-type thiophosphates need to be identified.