Superion Conductor Na11.1Sn2.1P0.9Se12: Lowering the Activation

Article Cite This: Chem. Mater. 2018, 30, 4134−4139

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Superion Conductor Na11.1Sn2.1P0.9Se12: Lowering the Activation Barrier of Na+ Conduction in Quaternary 1−4−5−6 Electrolytes Marc Duchardt,† Sven Neuberger,# Uwe Ruschewitz,‡ Thorben Krauskopf,§ Wolfgang G. Zeier,§,∥ Jörn Schmedt auf der Günne,# Stefan Adams,⊥ Bernhard Roling,† and Stefanie Dehnen*,†

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Fachbereich Chemie and Wissenschaftliches Zentrum für Materialwissenschaften (WZMW), Philipps-Universität Marburg, Hans-Meerwein-Straße 4, D-35043 Marburg, Germany ‡ Department of Chemistry, University of Cologne, Greinstraße 6, D-50939 Köln, Germany § Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany ∥ Center for Materials Research (LaMa), Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany # Inorganic Materials Chemistry, Department of Chemistry and Biology, University of Siegen, 57068 Siegen, Germany ⊥ Department of Materials Science and Engineering, National University of Singapore, Singapore 117576, Singapore S Supporting Information *

ABSTRACT: We report on the first quaternary selenide-based Na+ superionic solid electrolyte, Na11.1Sn2.1P0.9Se12 (further denoted as NaSnPSe), which shows virtually the same room temperature Na+ ion conductivity (3.0 mS/cm) as the current record holder for sulfide-based systems, Na11Sn2PS12 (denoted as NaSnPS), but with a considerably lower activation energy of 0.30 eV. Both electrolytes belong to the currently highly topical class of solids comprising group I, IV, V, and VI atoms, which we summarize as 1−4−5−6 electrolytes. Herein, they are compared to each other with regard to their structural characteristics and the resulting ion transport properties. The lower activation energy of Na+ ion transport in NaSnPSe is well in line with the results of speed of sound measurements, Raman spectroscopy, bond-valence site energy calculations, and molecular dynamics simulations, all of which point to a lower lattice rigidity and to weaker Na−chalcogen interactions as compared to NaSnPS.

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Nowadays, both commercial Li+ ion batteries and the still experimental room temperature Na+ ion batteries draw on organic carbonates as electrolytes.12−14 But, as frequently reported, the usage of these electrolytes poses safety risks due to potential leakages and their flammability.1 Solid electrolytes (SEs), on the other hand, represent a suitable alternative, especially regarding their high ionic conductivities. Among the SEs, non-oxide chalcogenides have attracted much attention due to their greater softness and hence easier processability compared to oxides. Fast-ion conducting sulfide-based Li+ ion conductors have been known for many years now.15 The ternary system Li−P− S has been intensively studied by many groups.16−21 In 2011, Kamaya et al. succeeded in the pioneering discovery of Li10GeP2S12, which adopts the newly found LGPS-type structure. In a comprehensive theoretical study in 2013, Ceder and coworkers investigated various compounds of the general formula type Li10TtP2E12 (Tt = Si, Ge, Sn; E = O, S,

INTRODUCTION To proceed to a sustainable economy, modern society has to turn its back on fossil fuels.1 But the most important alternatives, wind and solar energy, can be produced only on a fluctuating basis, giving rise to the need for safe, reliable, and affordable ways to store electrical energy.2,3 High-energy Li+ ion batteries can surely contribute to the mitigation of this problem. But already today they are widely used for a multitude of different devices like, for example, smartphones, laptops, or electric cars. Therefore, justifiable doubts arise whether the future lithium supply will meet the continuously increasing demand, and at what price.2−5 Another pressing issue for Li+ ion batteries resides in the already beginning shortage in the supply of cobalt, which represents a crucial component in Li+ ion cathode materials.6 For Na+ ion batteries, on the other hand, a multitude of cobalt-free cathode materials do exist.7−9 Even though a further replacement of cobalt inside Li+ ion cathode materials would surely reduce the demand for this raw material, the demand for Li would remain unaffected, thus raising the battery price considerably.6 In the context of these potential shortages, Na + ion batteries attract renewed interest.7,10,11 © 2018 American Chemical Society

Received: April 20, 2018 Revised: May 23, 2018 Published: May 24, 2018 4134

DOI: 10.1021/acs.chemmater.8b01656 Chem. Mater. 2018, 30, 4134−4139

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Chemistry of Materials Se), that were supposed to adopt an LGPS-type structure.22 From this study, important trends for Li+ superionic conductors were derived that suggested increasing conductivity in the order Sn → Ge → Si and O → S → Se. Later experimental studies succeeded in the realization of several of the proposed substitutions, e.g. the replacement of expensive Ge by Si or Sn. 23−27 The resulting compounds turned out to be isostructural to the mother compound LGPS, but while incorporation of Sn indeed leads to a lower conductivity due to a tightening along the diffusion channels in z-direction,25 the expected increase for the Si homologue was disproved experimentally.24 However, except for a minor substitution,28 the suggested complete replacement of sulfur with selenium was never accomplished until now (cf. Figure S1). Hence, none of the respective quaternary selenides Li10+xTt1+xP2‑xSe12 (Tt = Si, Ge, Sn) was reported, although the ternary Li4SnSe4, with an ionic conductivity of 2 × 10−5 S/cm, is known. Quaternary sodium-ion compounds of the type Na10+xSn1+xP2−xS12 have been reported only recently.29−31 The foundation was led by theoretical studies by the groups of Waghmare32 and Ceder,33 followed by the observation of a very high Na+ ion conductivity for Na11Sn2PS12 (3.7 mS/cm) in spite of a relatively large activation barrier (0.39 eV).30 Inspired by further theoretical studies22 and by the known fact that softening of the lattice may diminish the activation barrier,34 which would allow lower-temperature applications of according electrolytes, besides their general suitability for usage in the rather new field of sodium−selenium batteries,35−37 we currently expand our studies toward the related selenides. To explore whether sodium, with its greater ionic radius as compared to that of lithium, might stabilize a potential Na11Sn2PSe12, we tried to transfer the newly found Sn:P ratio of 2:1 in this quaternary sulfide (which generally deviates from the ratio found in lithium compounds, Tt:P = 1:2) to the respective selenides.29,30,38 As a result, we present in this study the new superionic sodium ion conductor Na11.1Sn2.1P0.9Se12, which represents the first quaternary selenium-based Na+ superionic conductor reported to date. We note that Na11.1Sn2.1P0.9Se12 together with Na 11 Sn 2 PS 12 29,30 and the very recently reported Na4‑xSn1−xSbxS438 (Figure 1) are the first examples of a new structural class of chalcogenide-based sodium ion conductors, for which we suggest the abbreviations NaSnPSe and NaSnPS, respectively, in analogy to LGPS (Li10GeP2S12) and LSnPS (Li10SnP2S12) used for the structurally related lithium ion conductors.23,26,27,39 These Li+ and Na+ ion conductors

together form the overarching class of 1−4−5−6 electrolytes, named after the main group affiliation of the constituting elements.

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RESULTS AND DISCUSSION The title compound NaSnPSe was synthesized in a high temperature reaction. Stoichiometric amounts of Na2Se, Sn, red phosphorus, and selenium powder were mixed, ground in an agate mortar, and vacuum-sealed in a quartz ampule. A general temperature profile comprised a heating rate of 40 °C/h up to 600 °C, dwelling for 12 h, and subsequent cooling at a rate of 20 °C/h. To obtain single crystals of NaSnPSe (and also for Na11Sn2PS12, see Supporting Information), a lower cooling rate of 2 °C/h was applied. As typical for chalcogenide-based alkali metal superionic conductors, both phases are hygroscopic and (especially in the case of Se as the chalcogenide) also sensitive to (humid) air and thus need to be handled under dry and nonoxidative conditions. According to both single crystal and powder X-ray diffraction experiments, NaSnPSe is isostructural to the previously reported sulfide NaSnPS and thus crystallizes in the space group I41/acd (Figure 2). The lattice parameters a and c (at

Figure 2. Section of the crystal structure of NaSnPSe, viewed along [001] (a) and [100] (b). Sn atoms and [SnSe4]4− tetrahedra are drawn in gray, P atoms and [PSe4]3− tetrahedra in orange, Se atoms in dark red, and Na+ cations in different shades of blue and green to differentiate between the six crystallographically distinct sites. For a comprehensive visualization of the [PS4]3− tetrahedra, both possible orientations are shown. Their superimposition leads to the appearance as a cube. (c) Rietveld refinement of a diffraction pattern collected at beamline BL9 of the DELTA synchrotron radiation facility, Dortmund/Germany (λ = 0.49594 Å; T = 295 K). Experimental data points (black crosses), calculated profile (red solid line), and difference curve (blue curve below) are shown. Purple vertical bars mark the positions of Bragg reflections. The refinement residuals are given in the inset.

100 K) increase from 13.5309(14) to 14.1562(10) Å (+4.4%) and from 27.125(4) Å to 28.322(2) Å (+4.6%) (Table S2), respectively. The associated unit cell volume increases from 4966.2 to 5675.7 Å (+14.3%), which matches very well the increase reported for the substitution of selenium for sulfur in the structurally related ternary compound Na3PS4 both in its cubic (+14.3%) and tetragonal modification (+13.9%).40 A detailed comparison of the sulfide versus selenide structure is provided in the Supporting Information (Table S3).

Figure 1. Overview of the hitherto known combinations of tetrel (Tt), pentel (Pn), and chalcogen (E) atoms in quaternary 1−4−5−6 electrolytes, forming the Li+ compounds with the generalized formula Li10TtPn2E12 (left) and the Na+ compounds with the generalized formula Na11Tt2PnE12 (right). The newly found NaSnPSe is denoted as a purple sphere. 4135


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Chemistry of Materials

of 10 nF/cm2, indicating that this semicircle reflects a grain boundary impedance. The spike observed in the low frequency regime is ascribed to electrode polarization. From a fit to the equivalent circuit, the grain resistance Rg and the grain boundary resistance Rgb were obtained, and the grain conductivity σg = dA−1Rg−1 and the total conductivity σtotal = dA−1(Rg + Rgb)−1 were calculated. In the respective Nyquist plot at 20 °C (Figure S2), the former two processes are not resolved anymore due to the high ionic conductivity of the compound. Figure 3b displays an Arrhenius plot of the grain conductivities σg of both NaSnPSe and NaSnPS.30 The grain conductivity of NaSnPSe could be obtained in the temperature range from −120 to 0 °C and exhibits an activation energy EA of 0.30 eV. This value is considerably lower than the one measured recently for Na11Sn2PS12.30 The grain conductivity extrapolated to 25°C of NaSnPSe, on the other hand, amounts to σg = 3.0 mS/cm (σtot = 1.7 mS/cm) and is thus slightly lower than the conductivities observed for the best sulfide-based superionic Na+ conductor NaSnPS with σg = 3.7 mS/cm (σtot = 3.0 mS/cm) and the best selenide-based superionic conductor Na3SbSe4.30,41 However, this value is higher than for other high-performance chalcogenide-based electrolytes such as Na3SbS442,43 and Na3PSe4.44 The high Na+ ion mobility is confirmed by the fast spin−lattice relaxation of 23Na, which yielded a relaxation time of T1 = 4.7(3) × 10−4 s. The static 23 Na NMR powder spectrum shows a single peak only (Figure 3c). Even with an excitation bandwidth in the MHz regime, no satellite transitions could be detected, which indicates that all Na+ ions are involved in fast ionic motion on the NMR time scale. In contrast to the theoretical prediction for the related LGPS-type and Na7P3E11-type electrolytes (E = O, S, Se),45 the replacement of sulfur by selenium apparently does not lead to a conductivity increase. Even though the new NaSnPS-type electrolytes differ in both their composition and structure from their lithium analogues, one would intuitively assume to equally raise the ionic conductivity by expanding the lattice parameters and the polarizability of the lattice, as it was, for instance, observed for the replacement of oxygen by sulfur in Li10GeP2S12−xOx46 and for the replacement of sulfur by selenium in Na3PS4−xSex.40 To investigate whether for the new NaSnPS and NaSnPSe families of Na+ ion conductors, the same correlations for the lattice dynamics and the ionic transport can be observed as for Na3PS4−xSex and Li6PS5X (X = Cl, Br, I),34,40 we implemented ultrasonic speed of sound measurements. Substituting sulfur by the more easily polarizable selenium leads to a lower speed of sound and thus a lower Debye frequency νD, which points to a lower lattice rigidity. The positive correlation between the Debye frequency and the activation energy of the conduction process, as shown in Figure S2, is in good accordance with the trends recently reported by Kraft et al.34 and Krauskopf et al.40 The assumptions regarding the enhanced structural freedom and the results found for the activation energy of the conductivity are supported by Raman spectra (Figure 4). A blue shift of the Raman peaks associated with the symmetric stretches of the Sn−S and P−S bonds (A1,Sn−S and A1,P−S) by 5 cm−1 is observed for NaSnPS when compared to the ternary sulfides NaSnS and NaPS. This blue shift implies stronger Sn− S and P−S bonds in the quaternary compound, which should go along with weaker Na···S interactions. Qualitatively, the same blue shift is also observed for the A1,Sn−Se and A1,P−Se mode in NaSnPSe when compared to those in Na4SnSe4 and

However, although NaSnPSe is isostructural to NaSnPS, thus comprising tetrahedral [PSe4]3− and [SnSe4]4− anions (with one crystallographically independent site for each type of anion) and six crystallographically independent Na+ sites, we also identified notable differences in their structural details. First, we found different site occupational factors for the Na1− Na6 sites in comparison with the respective sites in the sulfide (Table S3). Second, the atomic position of the P atom (Wyckoff site 8a) revealed to be partially occupied by ca. 10% of a second Sn atom (thus increasing the amount of sodium atoms from 11 to 11.1 per formula unit). Third, we found a rotational disorder of the tetrahedral anion at this position in a 9:1 ratio. The (P/Sn)−Se distances refine to the same (average) value in both orientations, indicating a rotational disorder of both [PSe4]3− and [SnSe4]4− tetrahedra. Despite extensive attempts and high-quality single crystal data, it was not possible to obtain a reasonable superstructure model, so we describe the structure in the mentioned disordered model. Whether the disorder of the tetrahedra is dynamic or static is not clear until now, but it clearly points toward a higher degree of positional freedom for the anionic positions as compared to the sulfide. The ionic conductivity of annealed pellets of polycrystalline Na11Sn2PSe12 was examined by means of impedance spectroscopy over broad temperature and frequency ranges, enabling us to differentiate between grain, grain boundary, and electrode contributions to the overall impedance. In Figure 3a, the

Figure 3. (a) Nyquist plot of NaSnPSe after annealing, together with the equivalent circuit utilized for fitting the spectra. The different contributions to the total impedance, obtained from the fits, are highlighted in different colors. (b) Arrhenius plot of the grain and total conductivity of NaSnPSe obtained from the mean values of measurements on three annealed NaSnPSe pellets. For comparison, the grain conductivity of NaSnPS is also shown.30 (c) Static 23Na NMR spectrum of NaSnPSe (90° pulse length of 0.5 μs).

Nyquist plot of a NaSnPSe pellet measured at −100 °C is depicted. The spectrum shows exemplarily the three detected processes. The high-frequency semicircle is characterized by a capacitance of 23 pF/cm2 and is thus assigned to ion transport in the crystalline grains. A smaller intermediate-frequency semicircle is observed, which is characterized by a capacitance 4136


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Figure 4. (a and b) Raman spectra of NaSnPS in comparison with those of Na3PS4 and Na4SnS4. (c and d) Raman spectra of NaSnPSe in comparison with those of Na3PSe4 and Na4SnSe4 (b and d are zoomed-in images of a and c, respectively).

Figure 5. (a) BVSE models of migration barriers for various relaxed local structure models of NaSnPSe, containing P(Se3)4 (violet squares), Sn2(Se3A)4 (dark gray diamonds), P(Se3A)4 (light blue triangles), or Sn2(Se3)4 (light gray circles). The lowest energy threedimensional (3D) pathway (migration barrier of 0.25 eV) is formed in the P(Se3)4 model by Na4−i1−Na1−i1−Na5 networks (including the split positions from the SCXRD structure). A separate 3D pathway is formed by interconnecting local low energy Na2−Na6−Na3 paths via the interstitial site i1. In the case of the Sn(Se3A)4 environment, these paths percolate with a migration barrier of 0.4 eV. At an only slightly higher migration barrier (0.41−0.44 eV), both networks merge to a common dense pathway network. (b) Arrhenius plot of results from NVT molecular dynamics (MD) simulations for Na11.1Sn2.1P0.9Se12 and Na11Sn2PS12. Lines are linear fits as a guide to the eye. The activation barriers mentioned are based on regressions over the linear regime up to 350 K.

Na3PSe4. Here, the A1,P−Se mode is blue-shifted by even 8 cm−1. Thus, one might suspect that the strengthening of the P−Se bond in the quaternary solid and the concomitant weakening of the Na···Se interaction results in a more notable widening of the whole structure, as compared to the quaternary sulfide. Hence, we conclude the following: (a) the Na···chalcogen interaction is weaker in the selenide, which parallels the lower activation barrier observed in this Na+ ion conductor, and (b) this observation is accompanied by new features like partial disorder of Sn versus P sites and the rotational disorder of the [PSe4]3− unit. Additional bond valence site energy (BVSE) calculations47,48 for Na+ in various local structure models were conducted to investigate the influence of the observed structural disorder on the ion transport model (Figure 5 and Figure S7). Besides the partially occupied Na sites, the BVSE calculations shown in Figure 5a consistently reveal one type of low energy interstitial site, the position of which depends on the orientation of the (P/Sn)(Se3/Se3A) tetrahedra. When comparing pathways in NaSnPSe with those in NaSnPS, both phases exhibit a dense 3D pathway network that may be thought of comprising two interpenetrating 3D pathway networks with similar migration barriers involving either Na1, Na4, Na5, and i1 or Na2, Na3, Na6, and i1. As discussed above, the larger unit cell volume and higher polarizability (cf. Figure S2) of the selenide will contribute to a reduction of the migration barriers in relaxed local structure models from 0.35 to 0.25 eV (cf Figure S8) but may not be the only factor. The seemingly counterintuitive combination of lower migration barrier with slightly lower room temperature conductivity may be caused by different relative contributions of the two subnetworks to the overall conductivity: Figure S8 shows that Na6 forms an extremely shallow minimum in NaSnPS, while the site energy of Na6 in NaSnPSe is considerably lowered, and the minimum adopts a more regular shape. Along with the drastically reduced (vanished) number of vacancies on the Na6 site, this reduced local mobility of Na6 limits the relative contribution of the (slightly higher energy) Na2−Na6−Na3−i1 pathway branches.

Moreover, Sn on the Sn2/P site will impede transport via adjacent i1 sites, while the local mobility of ions on the i1 sites around P will be higher in NaSnPSe. Overall, this reduces the density of the effective pathway network in local structure models of NaSnPSe, thus counterbalancing the lower migration barriers. The predictions of the static BVSE models were further tested by empirical molecular dynamics simulations of Na11Sn2PS12 (NaSnPS) and Na11.1Sn2.1P0.9Se12 (NaSnPSe), see Figure 5b. As commonly found for NVT simulations of sulfide-based solid electrolytes based on the softBV force field49 and such relatively small supercells, the experimental conductivity is somewhat underestimated, while the observed activation energy of 0.28 eV for the temperature range up to 350 K harmonizes to the experimental findings (cf. Figure 3b). In accordance with the experiments, we find similar roomtemperature conductivities despite the clearly lower activation energies for NaSnPSe compared to NaSnPS. The Na + trajectories for the supercell model harmonize with the predictions from the BVSE models (cf. Figures S9 and S10). It may be noticed that, as suggested from the static pathway models, the mobility in the low-energy local pathway cluster (Na2)4−Na6−(Na3)2 is somewhat less pronounced in the selenide. Furthermore, it is important to mention that the observed differences between NaSnPSe and NaSnPS are not caused by the higher Sn content of NaSnPSe because, in the 4137


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support from the NUS “Centre for Energy Research” grant as well as from National Research Foundation, Prime Minister’s Office, Singapore under the Competitive Research Programme (CRP Award NRF-CRP 10-2012-6).

MD simulations, only a small difference in the Na+ ion conductivity between a so far hypothetical stoichiometric compound Na11Sn2PSe12 and the Sn-enriched compound Na11.1Sn2.1P0.9Se12 (NaSnPSe) is observed (Figure S11).

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CONCLUSION The first example of a quaternary selenium-based alkali superionic conductor, Na11.1Sn2.1P0.9Se12 (NaSnPSe), was synthesized. With this new compound, we extended the spectrum of a new promising structural class of Na+ conductors, which may be summarized as 1−4−5−6 electrolytes, toward the so far unprecedented selenides. While the Na+ ion conductivity is nearly as high as that for the related sulfide (NaSnPS), the activation energy of the conduction process is significantly lower as a consequence of the softer crystal lattice of the selenide, which is advantageous for battery-based energy storage at temperatures considerably lower than 298 K. Further substitutions are likely to additionally foster the ionic conductivity in future works as it has been shown for the lithium counterparts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01656. Experimental data, more details on the computational studies, 31P, 119Sn, 77Se NMR data, and results of the structure refinements for both compounds (Tables S4 −S7) (PDF) Crystallographic data (single crystal) for Na11Sn2PSe12 (CIF) Crystallographic data for (single crystal) Na11Sn2PSe12 at 298 K (CIF) Crystallographic data (powder) for Na11Sn2PSe12, measured at the DELTA synchroton radiation source in Dortmund, Germany(CIF) Crystallographic data (single crystal) for Na11Sn2PS12 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Uwe Ruschewitz: 0000-0002-6511-6894 Wolfgang G. Zeier: 0000-0001-7749-5089 Jörn Schmedt auf der Günne: 0000-0003-2294-796X Stefan Adams: 0000-0003-0710-135X Stefanie Dehnen: 0000-0002-1325-9228 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.D. acknowledges a fellowship from the Marburg Research Academy (MARA). The authors thank Melanie Werker and Dr. Christian Sternemann for collecting synchrotron powder diffraction data and DELTA (Dortmund/Germany) for providing synchrotron radiation. B.R. thanks the German Federal Ministry for Economy and Technology for financial support in the framework of the SME Central Innovation Programme (ZIM Grant ZF4430001ZG7). S.A. acknowledges 4138


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