Structural and Na-ion conduction characteristics of Na₃PS_xSe_4−x

Abstract

Abstract:The recent discovery of the isostructrual cubic Na 3 PS 4 and Na 3 PSe 4 as fast Na-ion conductors provided a general structural framework for the exploration of new sodium superionic conductors. In this work, we systematically investigated the structures and ionic conduction characteristics of a series of compounds with the general chemical formula of Na 3 PS x Se 4-x . Synthesis of Na 3 PS 4 at different conditions (e.g., temperature, reaction vessel, mass of the precursors) reveals the reactivity of the precursors with the reaction tubes, producing different polymorphs. X-ray diffraction studies on the solid solution phases Na 3 PS x Se 4-x identified a tetragonal-to-cubic phase transition with increasing Se concentration. This observation is consistent with the computed stability of the tetragonal and cubic polymorphs, where the energy difference between the two polymorphs becomes very close to zero in Se-rich compositions.Furthermore, ab initio molecular dynamic simulations suggest that the fast Na-ion conduction in Na 3 PS x Se 4-x may not be causally related with the symmetry or the composition of these phases.The formation of defects, instead, enables fast Na-ion conduction in this class of materials.2

Na3SbSe4−xS x as Sodium Superionic Conductors

“…With the increased S concentration in the solid solution phases, a gradual increase of activation energy can be observed for Na + migration. This trend is similar to what was observed in other Se-S solid solution ionic conductors 30,31 , resulted from a softer lattice with more polarizable Se 2− . The values of activation energies are listed in the Table S1 in supplementary information, together with the ionic conductivities of the compounds at room temperature.…”

Room-Temperature All-solid-state Rechargeable Sodium-ion Batteries with a Cl-doped Na3PS4 Superionic Conductor

“…In fact, we find no evidence of any significant difference in the predicted bulk Na + conductivities between the cubic and tetragonal Na 3 PS 4 , which is not surprising given the very small differences in lattice parameters between the two polymorphs38. We speculate that the nature of the defects present would promote the formation of one polymorph over another.…”

Ionic Conductivity of the NASICON‐Related Thiophosphate Na1+xTi2−xGax(PS4)3

“…[6,7,[16][17][18][8][9][10][11][12][13][14][15] Typical approaches to attain even higher ionic conductivities within the respective classes involve either the use of aliovalent substitution to increase the number of mobile chargeso rt he introduction of softer,more polarizable anions and aconcurrent widening of the diffusion pathways. [19][20][21] Indeed,t he softer anion lattice with more polarizable anions has been corroborated to lower the activationb arrier, [22][23][24][25] thereby explaining the high ionic conductivity in many of the Li + and Na + conductingt hiophosphates.H owever,i th as re-cently been shown that as ofter lattice not only lowers the migration barrier,but also affects the entropy of migration, which can also lead to an overall lower ionic conductivity. [23,24] It seems that the paradigm of "the softer,t he better" does not hold up within individual classes of materials, and new material classes need to be explored to obtain faster ionic conductors.…”

“…In particular, the thiophosphates, for example, Li 10 GeP 2 S 12 , Li 6 PS 5 X and Na 3 PS 4 , among others, have shown to be promising materials due to the soft mechanical nature and the intrinsically high ionic conductivity . Typical approaches to attain even higher ionic conductivities within the respective classes involve either the use of aliovalent substitution to increase the number of mobile charges or the introduction of softer, more polarizable anions and a concurrent widening of the diffusion pathways …”