Influence of Lattice Dynamics on Na⁺Transport in the Solid Electrolyte Na₃PS_4–xSe_x

Abstract

Li+- and Na+-conducting thiophosphates have attracted much interest because of their intrinsically high ionic conductivities and the possibility to be employed in solid-state batteries. Inspired by the recent finding of the influence of changing lattice vibrations and induced lattice softening on the ionic transport of Li+-conducting electrolytes, here we explore this effect in the Na+ conductor Na3PS4–x Se x . Ultrasonic speed of sound measurements are used to monitor a changing lattice stiffness and Debye frequencies. The changes in the lattice dynamics are complemented by X-ray diffraction and electrochemical impedance spectroscopy. With systematic alteration of the polarizability of the anion framework, a softening of the lattice can be observed that leads to a reduction of the activation barrier for migration as well as a decreased Arrhenius prefactor. This work shows that, similar to Li+ transport, the softening of the average vibrational frequencies of the lattice has a tremendous effect on Na+-ionic transport and that ion–phonon interactions need to be considered in solid electrolytes.

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.…”

“…This conductivity is also comparable to those of recently reported sulfides such as Na 3 SbS 4 23 and Na 3 P 0.62 As 0.38 S 4 25 . This high ionic conductivity can be explained by the large unit cell of Na 3 SbSe 4 and the high polarizability of Se 2− , which lowers the electrostatic binding energy and weakens the attraction between Na + and the tetrahedron of [SbSe 4 3− ] 29,30 .…”

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.…”

“…Indeed, the softer anion lattice with more polarizable anions has been corroborated to lower the activation barrier, thereby explaining the high ionic conductivity in many of the Li + and Na + conducting thiophosphates. However, it has recently been shown that a softer lattice not only lowers the migration barrier, but also affects the entropy of migration, which can also lead to an overall lower ionic conductivity . It seems that the paradigm of “the softer, the better” does not hold up within individual classes of materials, and new material classes need to be explored to obtain faster ionic conductors …”

“…For instance, Na 3 PS 4 exhibits an ionic conductivity of up to 4.6×10 −4 S cm −1 in the cubic phase at room temperature . Consequently, in order to broaden the diffusion pathways and potentially increase the lattice polarizability, substitutions of S with Se were performed, which form stable solid solutions and influence the ionic conductivity . In addition, substitution of P 5+ with As 5+ increases the conductivity due to a widening of the diffusion pathways and possibly altered Na−S interactions .…”