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

Electrolytes with excellent ionic conductivity are needed for the development of safe and long-lasting all-solid-state batteries. Perfect candidates are Argyrodites, like Li 6 PS 5 X (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 31 P spin-lattice relaxation nuclear magnetic resonance (NMR) to find out whether rapid Li + ion dynamics in Li 6 PS 5 X is affected by rotational jumps of the PS 4 3− units. NMR revealed that in Li 6 PS 5 I, having an ordered anion sublattice, PS 4 3− 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 10 9 events per second. It clearly decreases when anion disorder is introduced. At the same time, anion substitution contracts the lattice when we go from Li 6 PS 5 I to Li 6 PS 5 Br and further to Li 6 PS 5 Cl as the ionic radii of X decrease along this series. While for Li 6 PS 5 I, with its soft lattice, the rotational jumps of the PS 4 3− units are decoupled, that is, independent from Li + translation, in Li 6 PS 5 Cl, 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 Li 6 PS 5 Br, 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.

Sulfide and Oxide Inorganic Solid Electrolytes for All-Solid-State Li Batteries: A Review

“…Owing to their high Li + ion conductivity at room temperature, sulfide-based materials are more promising electrolytes than oxide-based ones [ 159 , 160 , 161 , 162 , 163 , 164 , 165 , 166 , 167 , 168 , 169 , 170 , 171 , 172 , 173 , 174 , 175 , 176 , 177 , 178 , 179 , 180 , 181 , 182 , 183 , 184 , 185 , 186 , 187 , 188 , 189 , 190 , 191 , 192 , 193 , 194 , 195 , 196 , 197 , 198 , 199 , 200 , 201 , 202 , 203 , 204 , 205 , 206 , 207 , 208 ,…”

Li10GeP2S12‐Type Superionic Conductors: Synthesis, Structure, and Ionic Transportation

“…The role of rotation or vibration of polyhedra in ionic transport was discussed for several ionic conductors (e.g., hydride complex system [ 116 ] and argyrodite [ 117 ] ). The interaction of PS 4 tetrahedra with mobile lithium in LGPS was also discussed both experimentally and theoretically.…”

Changing the Static and Dynamic Lattice Effects for the Improvement of the Ionic Transport Properties Within the Argyrodite Li6PS5-xSexI

“…38,[49][50][51] Further, it was recently shown that additional polyhedral anion dynamics exist that are coupled to the Li + translation. 55 However, in the Li6PS5I series reported here, the Li + ion jumps and rotational dynamics are independent of each other, which beneficially prevents further complication of the analysis. In this work, the resulting lattice dynamics are assessed using the optical and acoustic modes of the dispersion relation.…”