Na₃PSe₄: A Novel Chalcogenide Solid Electrolyte with High Ionic Conductivity

Abstract

a high conductivity of 0.46 mS cm −1 at room temperature because the three-dimensional pathways in the open framework benefi t the diffusion of Na ions. [ 5 ] Further improvement (60%, 0.74 mS cm −1 ) has been obtained by substitution of Si on P sites in 94Na 3 PS 4 -6Na 4 SiS 4 . [ 6 ] However, the ionic conductivity is still low in comparison to liquid electrolytes, and therefore SEs with higher ionic conductivity need to be sought.Tatsumisago and co-workers found that an appropriate diffusion channel size is critical for fast ion diffusion and anion substitutions have a greater effect on ionic diffusivity than cation substitutions. [ 7 ] Moreover, Se-substituted lithium sulfi des demonstrate an enhanced ionic conductivity in comparison with their pristine compounds. [ 8 ] The advantages of Se-doping lie in two aspects. On one hand, the atomic radius of Se is bigger than that of S, so Se substitution on S sites may expand the lattice. On the other hand, the higher polarizability of Se 2− may weaken the binding energy between the moving ion and the anion framework. These modifi cations may be benefi cial for Na + diffusion because of the big ionic radius of sodium. It is therefore highly interesting to synthesize Na 3 PSe 4 and evaluate its electrochemical performance.In this study, cubic Na 3 PSe 4 was synthesized for the fi rst time and its crystal structure, spectra, and electrochemical performance were investigated. A ionic conductivity of 1.16 mS cm −1 was observed; to the best of our knowledge, this is one of the best values among sodium ion conductors and is the highest value reported for sulfi des to date. Figure 1 a shows the X-Ray Diffraction (XRD) pattern of Na 3 PSe 4 . The halo patterns in both cases refl ect the polyimide fi lm. The crystal structure of Na 3 PSe 4 has not been reported yet. Here, the integrated intensities from powder XRD data were extracted by the Le Bail method using the FullProf program. The crystal structure was solved by using the direct space method and was then refi ned by the Rietveld method. The crystal structure was determined to be cubic with the space group I -43 m (No. 217) and Z = 2. The plots of the observed, calculated, and difference patterns from the Rietveld refi nement (Figure 1 a) evidence the formation of single-phase Na 3 PSe 4 . The refi ned crystallographic data are listed in Table 1 . The cell has a lattice parameter a = 7.3094(2) Å, which is much larger than that of Si-doped Na 3 PS 4 ( a = 6.9978 Å). [ 6 ] A negative isotropic atomic displacement parameter ( U iso ) for P atoms and large U iso values for Na and Se atoms are obtained, indicating large disorders in the crystal structure. Comparison of the XRD patterns of Na 3 PSe 4 before and after ball milling ( Figure S1, Supporting Information) shows that only peak broadening is observed. This observation is in accordance with Differential scanning calorimetry (DSC) results ( Figure S2,The development of large-scale energy-storage system attracts worldwide attention because of the rapidly increasing de...

Structural and Na-ion conduction characteristics of Na3PSxSe4−x

“…The close similarity between Na 3 PS 4 and Na 3 PSe 4 implies that a solid solution between these two end members may form, which could provide an effective means to adjust the physical/chemical properties of the Na 3 PS x Se 4-x compounds. Indeed, in a very recent paper, the syntheses of Na 3 PS 0.5 Se 3.5 and Na 3 PS 2 Se 2 were reported, although neither diffraction patterns nor structural parameters were reported in that work 14 .…”

“…Cubic Na 3 PSe 4 is isostructral to cubic Na 3 PS 4 [12][13][14][15] . Both phases crystallize in the I-43m (#217) space group.…”

“…Indeed, previous impedance measurements showed that cubic Na 3 PS 4 and Na 3 PSe 4 exhibit high conductivity at room temperature (0.1-1.0 mS/cm), and low activation energies for Na + diffusion (20-30 kJ/mol) [12][13][14] . However, our previous computational investigation on cubic Na 3 PSe 4 indicates that the defect-free structure does not permit rapid Naion diffusion 12 .…”

Development of Sulfide Solid Electrolytes and Interface Formation Processes for Bulk-Type All-Solid-State Li and Na Batteries

“…Increased Na + ion concentration is presumed as the reason for the enhancement of conductivity. Very recently, high conductivity of more than 10 −3 S cm −1 has been reported in crystalline Na3PSe4 (Zhang et al, 2015). However, studies of new sulfide Na + ion conductors are few.…”

Sodium thiophosphate electrolyte thin films prepared by pulsed laser deposition for bulk-type all-solid-state sodium rechargeable batteries

“…13) Substituting the anion S by Se (Na 3 PSe 4 ) 14) and the cation P by Sb (Na 3 SbS 4 ) 15) further increased the conductivity of Na 3 PS 4 to >10…”