Insights into the Transport of Alkali Metal Ions Doped into a Plastic Crystal Electrolyte

Abstract

The application of organic ionic plastic crystals (OIPCs) as a new class of solid electrolyte for energy storage devices such as lithium batteries and, more recently, sodium batteries is attracting increasing attention. Key to this is achieving sufficient target ion transport through the material. This requires fundamental understanding of the structure and dynamics of OIPCs that have been doped with the necessary lithium or sodium salts. Here we report, for the first time, the atomic level structure and transport of both lithium and sodium ions in the plastic crystalline phases of an OIPC diethyl­(methyl)­(isobutyl)­phosphonium hexafluorophosphate. These molecular dynamics simulations reveal two types of coordination geometries of the alkali metal ion first solvation shells, which cooperate closely with the metal ion hopping motion. The significantly different ion migration rates between two metal ion doped systems could also be related to the differences in solvation structures.

New insights into ordering and dynamics in organic ionic plastic crystal electrolytes

“…This would then account in part for the higher conductivity usually observed in doped OIPCs and also for the good performance of such electrolytes in solid-state devices [11,39]. More detailed analysis and a comparison between Li + and Na + is provided in our previous work [30].…”

“…Our recent MD simulations have also shown the details of alkali ion hopping in OIPCs containing either Li + or Na + ions, even if the MSD indicates overall lower diffusivity of these ions. [30] The van Hove analysis for the 8.3 mol% sodium ion containing OIPC is shown in Fig. 5 at various temperatures.…”

“…The details of the molecular dynamics simulations can be found in our previous publications on P 1224 PF 6 and vacancy model simulations [29] and also for the Li-doped P 1224 PF 6 simulations [30]. The vacancy model consists of 0.46 mol% vacancy (one cation-anion pair was removed from a box consisting of 216 ion pairs).…”

Toward High‐Energy‐Density Lithium Metal Batteries: Opportunities and Challenges for Solid Organic Electrolytes

“…The transport mechanism of the alkali metal ion dopants, including both Li + and Na + , and their impacts on ionic conductivity were also investigated through MD simulations in the [P 122i4 ][PF 6 ] and tetramethylammonium dicyanamide ([TMA][DCA]) OIPCs . In the [TMA][DCA] system, Li dopants were found to strongly coordinate with the DCA anions, resulting in the shift of the DCA anions from their lattice sites, which increases the free volume in the OIPC, thereby enhancing the diffusion of the remaining “free” DCA anions.…”

Symmetry breaking and unconventional charge ordering in single crystal Na2.7Ru4O9

“…7. This feature makes it attractive in terms of sodium batteries [37] and ion motion related applications [38][39][40][41]. Interestingly, formation of the tunnel structure in Na 2.7 Ru 4 O 9 is purely of edge-sharing MO 6 octahedra chains (single (1×1), double (1×2), and triple (1×3)), where M is Ru 3+ /Ru 4+ and three cations Na + can reside in those tunnels/channels as shown in Fig.…”