Fast Na-Ion Conduction in a Chalcogenide Glass–Ceramic in the

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Fast Na-Ion Conduction in a Chalcogenide Glass−Ceramic in the Ternary System Na2Se−Ga2Se3−GeSe2 Seong K. Kim, Alvin Mao, Sabyasachi Sen,* and Sangtae Kim* Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, United States ABSTRACT: All solid-state rechargeable sodium-ion batteries are of key importance for future energy storage applications and novel electrolytes with improved performance are continuously being sought after for such batteries. Here we report large sodium-ion conductivity (>10−5 S/cm) at ambient temperature in a layered chalcogenide compound of composition Na2(Ga0.1Ge0.9)2Se4.95, synthesized as a glass−ceramic composite in the ternary system Na2Se−Ga2Se3−GeSe2. Alkali-ion-containing crystalline chalcogenides are typically hygroscopic and consequently unstable in an ambient environment. However, in the present study the crystal phase is possibly stabilized against moisture via encapsulation in an inert glass matrix and forms a three dimensionally continuous percolation network for fast sodium-ion conduction. This result demonstrates that such layered chalcogenide compounds with planar pathways for alkali-ion diffusion can serve as viable alternatives to the conventional solid electrolytes.

1. INTRODUCTION Inorganic compounds that conduct alkali-metal ions are of particular interest because of their potential importance as solid electrolytes (SEs) used in all solid-state rechargeable batteries.1,2 These types of batteries eliminate the issues associated with flammable liquid electrolytes, leading to enhanced safety, extended cycle life, and reduced cost. Numerous studies on such cation conductors are available in the literature and the vast majority of them focus primarily on Li+ conductors since lithium batteries offer superior volumetric and gravimetric energy densities.1,3 However, rapidly growing demands for low-cost energy storage have renewed interest in the batteries based on Na+ conductors because of the natural abundance and low cost of sodium.4−8 Crystalline NASICON compounds that belong to the family Na1+xZr2P3−xSixO12 (0 ≤ x ≤ 3) are typical examples of such SEs that are characterized by relatively large electrical conductivities on the order of 10−4 S/cm at ambient temperature.9,10 Another prominent example is the Na+-conducting β″-Al2O3 that is used in the conventional sodium−sulfur (Na/S) batteries as an electrolyte.5,11 These batteries hold several advantages including high-energy density up to 760 W·h/kg, nearly 3 times higher than that of lead-acid batteries. However, they are operated at temperatures above 300 °C where both electrodes (i.e., Na and S) are in their molten states, and are thus much more reactive and corrosive compared to their solid counterparts. Hence, chemically stable Na+-conducting SEs suitable for low-temperature all solid-state batteries are continuously being sought after. Recently, Hayashi et al. have demonstrated that a sulfide glass−ceramic-containing crystalline Na3PS4 in its cubic form can have Na+ conductivity of ∼10−4 S/cm at room temperature, significantly higher (by a factor of ∼30) than that of the parent glass phase.12−14 This © 2014 American Chemical Society

conductivity is in fact by far the highest among those reported for Na+-conducting sulfides. It is interesting to note here that earlier studies reported substantially lower conductivity (∼10−6 S/cm) of the Na3PS4 crystal in its tetragonal form at the same temperature and showed little change in the conductivity after its phase transformation from tetragonal to cubic phase.15 It is worth noting that the chalcogenide crystal structure of this superionic conductor is characterized by channel-like pathways in an open framework for the diffusion of Na ions. On the other hand, little is known about Na+ conduction in chalcogenide compounds with layered structures where a twodimensional pathway may be available for ionic diffusion. Na2Ge2Se5, a selenogermanate compound better known for its highly nonlinear optical property,16,17 provides such a structure type that enables investigation into the effect of the structural topology on Na+ conduction. The crystal structure of this compound is comprised of two-dimensional anionic layers/ sheets of composition [Ge2Se5]2− that can be denoted as 2 2− ∞[Ge2Se5 ] and are formed by GeSe4 tetrahedra sharing corners with three neighboring tetrahedra. These anionic units in the crystal structure can be charge-balanced by Na+ in planes intercalated with the [Ge2Se5]2− layers (Figure 1). Na+ in such a two-dimensional structure is expected to be highly mobile and yet Na+ conductivity in this compound remains unexplored, primarily because compounds in the ternary Na−Ge−Se system often tend to be highly hygroscopic and consequently unstable in an ambient environment.18 However, it is possible to synthesize crystals as being encapsulated in an inert glass Received: July 12, 2014 Revised: September 5, 2014 Published: September 8, 2014 5695

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in Figure 2. Prior to the electrical measurement, powdered samples were formed into pellets through cold-isostatic pressing at 300 MPa.

Figure 2. Ternary composition diagram showing the glass and glass− ceramic compositions used in this study. Compositions within the shaded region form glass upon quenching in water while those within the unshaded region partially crystallize to form glass−ceramic with the Ga-doped Na2Ge2Se5 crystal phase. The glass and the glass− ceramic compositions used in this study are denoted with the filled circle and the cross, respectively.

Figure 1. Structure of Na2Ge2Se516 showing the alternating layers consisting of Na atoms (yellow spheres) and helical chains of GeSe4 tetrahedra (purple polygons). Se atoms are denoted as green spheres.

matrix via a glass−ceramic route where the parent melt transforms into a composite of glass and crystals. The crystals may precipitate during cooling due to the incongruent crystallization of the parent melt wherein the residual melt forms the glass matrix. Alternatively, the melt can be quenched into a homogeneous glass that is subsequently heated above its glass transition temperature to nucleate and grow the crystals. Here we report the Na+ conductivity of a Na−Ga−Ge−Se glass−ceramic containing Ga-doped Na2Ge2Se5 crystals that are formed during the cooling of the parent melt, dispersed, and encapsulated in an inert glassy matrix, thus forming a composite that can possibly be stable against moisture attack at ambient conditions.

Pressed pellets were sputtered with platinum on both sides to make electrodes. Impedance measurements were carried out under a dry nitrogen environment to protect the sample from any chemical reaction with oxygen or moisture. Impedance was measured at temperatures between −20 and 20 °C. Obtained ac-impedance spectra were fitted using the Z-View software (Scribner Associates Inc.) to extract electrical conductivities at different temperatures. The sample resistance was determined from the best fit of the semicircular arcs, using an equivalent circuit model consisting of two parallel RQ circuits (where R is a resistor and Q is a constant phase element, the relation of which with capacitance, C, is given as C = (QR)1/nR−1) in series. Frequency-dependent ac conductivity σ(ω) was calculated using real and imaginary impedance, Z′(ω) and Z″(ω) as

2. EXPERIMENTAL SECTION 2.1. Synthesis. A glass of nominal composition (in mol %) 15% Na2Se−25%Ga2Se3−60%GeSe2 and a glass−ceramic of nominal composition 25%Na2Se−15%Ga2Se3−60%GeSe2 were synthesized in 10 g batches from Na2Se (5 N purity, Materion Advanced Chemicals, Inc.) and Ga−Ge−Se precursor alloys, using the traditional meltquench method. Ga−Ge−Se precursor alloys were batched using the pure elements (5 N purity, Alfa Aesar) as starting materials in a dry nitrogen atmosphere and placed into fused silica ampules, which were then evacuated to ∼10−5 Torr and flame-sealed. Loaded ampules were placed in a rocking furnace, and the temperature was slowly ramped to 950 °C over several hours. The melts were rocked at this temperature for 24 h followed by quenching in water. To minimize any oxygen uptake in the melt from possible chemical reactions between Na2Se and the silica ampule, the Na2Se + Ga−Ge−Se alloy mixtures were loaded into ampules that were precoated with a thin layer of Si via vapor deposition. Furthermore, the final, Na-containing mixtures were heated to 950 °C for only about 12 h before quenching in water. Electron probe microanalyses of the resulting samples indicated a maximum possible level of oxygen incorporation of ≤2 wt %. Differential scanning calorimetry at a heating rate of 10 °C/min yields a glass transition temperature Tg of ∼370 °C for the glass sample. 2.2. X-ray Diffraction. Powder X-ray diffraction (XRD) patterns were obtained using a Scintag X-ray diffractometer with a Cu Kα X-ray source. Diffraction patterns were obtained at 2θ angles ranging between 10° and 60°, at an effective scanning rate of 2.4°/min (step size of 0.02° with a dwell time of 0.5 s). 2.3. Alternating Current (ac)-Impedance Spectroscopy. Electrical conductivity was measured using the ac-impedance technique with a frequency analyzer (Novocontrol Alph-AN). Composition of measured glass and glass−ceramic samples is shown

σ(ω) =

⎛L⎞ ⎜ ⎟ Z′(ω) + Z″(ω) ⎝ A ⎠ 1

2

2

(1)

where A and L are the electrode area and the distance between two electrodes, respectively.

3. RESULTS AND DISCUSSION Figure 3 shows a scanning electron micrograph of the fracture surface of the as-synthesized glass−ceramic. The composite nature of the sample with coexisting glassy and crystalline regions is immediately apparent in this image, as the two regions are characterized by distinctly different surface textures and reflectivity. The glass−ceramic appears to contain a substantial volume of the crystal phase forming a continuously percolating network. A representative powder XRD pattern of the glass−ceramic exhibits multiple sharp peaks (see the upper panel in Figure 4), indicating the presence of a significant crystalline fraction. The Bragg peaks in the XRD pattern at 2θ ≤ 40° are nicely consistent with those recently reported for crystalline Na2Ge2Se5 (the intensity of the peaks appearing at higher angles are too weak for reliable comparison), confirming that a phase similar to Na2Ge2Se5 is indeed crystallizing in the glass−ceramic as the dominant phase. In contrast, an XRD pattern of a glass of similar composition [15%(Na2Se)− 25%(Ga2Se3)−60%(GeSe2)] in this system (hereafter, the glass 5696

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crystal appears to be slightly (∼5%) Na-deficient compared to the concentration required to charge-balance the [GaSe4]−1 tetrahedra, such a deficiency is well within the experimental errors associated with the technique. Similar substitution of tetrahedral Ge sites by Ga has recently been reported in the literature for glasses in the binary system Ga2Se3−GeSe2.19 Substitution of Ga for Ge in the crystal structure is also consistent with the fact that the positions of the major Bragg peaks in the XRD pattern of the glass−ceramic are all shifted by ∼3°−4° toward higher 2θ compared to those characteristic of the pure Na2Ge2Se5 phase,16 indicating a shrinking of the unit cell upon Ga substitution. Figure 5 presents an ac-impedance spectrum (viz. a Nyquist plot) of the glass−ceramic (the main panel) measured under

Figure 3. Representative scanning electron micrograph of the assynthesized glass−ceramic exhibiting distinctly different surface textures of glassy (left bottom corner) and crystalline regions (upper right corner).

Figure 5. Nyquist plots of impedance measured for the glass−ceramic (main panel) and the glass (inset) samples under dry nitrogen at a given temperature. The solid lines indicate the best fit.

nitrogen at 20 °C in the frequency range of 10−1 to 107 Hz, consisting of two well-separated semicircular arcs. The spectrum also indicates the existence of an additional large arc that appears at frequencies below 10−1 Hz. We note that the presence of three separated arcs in series is a common characteristic of Nyquist plots of ion-conducting polycrystalline ceramics: the first and second arcs typically correspond to the impedance in the bulk (i.e., the crystal interior) and at the grain boundaries (i.e., the interfaces between crystallites in a ceramic), respectively, whereas the third one is attributed to the double-layer impedance at the contact between the sample and metal electrodes applied to it, often being very large for ionic conductors. Both the resistance and the capacitance in the bulk (Rb and Cb, respectively) and at the grain boundaries (Rgb and Cgb, respectively) of the glass−ceramic can be determined from the best fit to the Nyquist plots (e.g., solid line seen in Figure 4) with an appropriate equivalent circuit (see the Experimental Section above for details), and those at 20 °C are Rb = 1.2 × 106 Ω, Cb = 1.5 × 10−12 F and Rgb = 1.1 × 105 Ω, Cgb = 3.0 × 10−9 F. The value of Cb corresponds to a dielectric constant of ∼10, typical of these types of crystals. The Cgb/Cb in the range of 1000 confirms that the second arc resulted from the grain boundary, the width of which is normally less than 1 nm in such polycrystalline ceramics with a grain size of a few micrometers. The inset of Figure 5 shows a representative

Figure 4. Representative XRD patterns of the glass−ceramic sample (upper panel) and the glass sample (lower panel).

sample), shown in the lower panel in Figure 4, exhibits only weak and broad peaks characteristic of glassy materials. It is a challenging task to exactly determine the crystal:glass volume ratio in this glass−ceramic. Nevertheless, analyses of the scanning electron micrographs indicate that the volume fraction of glassy phase in the glass−ceramic is more than 50%, which is sufficient to encapsulate the crystal phase. This estimation is also consistent with the X-ray diffraction data when the integrated area under the Bragg peaks for the crystal phase is compared with that under the broad amorphous peaks corresponding to the glassy fraction. The results of energy-dispersive X-ray spectroscopy (EDS) analysis yield the composition of the crystalline regions in this glass−ceramic to be approximately Na2(Ga0.1Ge0.9)2Se4.95, corresponding to Na2Ge2Se5 with about 10% of the tetrahedral Ge sites in the structure being occupied by Ga atoms. Henceforth, this crystalline phase will be referred to as Gadoped Na2Ge2Se5. Although the chemical composition of this 5697

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Nyquist plot of the glass sample for comparison, indicating no sign of any grain-boundary arc as expected. The Arrhenius plots of both the bulk and the total Na+ conductivity (σb and σt, respectively) of the glass−ceramic at temperatures ranging between −20 and 20 °C are shown in

Figure 7. dc-polarization curves of the glass−ceramic sample with sputtered Pt electrodes measured at three different voltages under the conditions indicated. The inset demonstrates ohmic behavior of the cell in the voltage range from 10−30 mV.

Figure 7 confirms that the dc-polarization measurements were carried out in the ohmic regime (10−30 mV). (The inset also shows that the current measured at a voltage higher than 30 mV deviates from the ohmic behavior.) The initial current at time t = 0 measured at 30 mV was 3.7 nA while the final current at t = 360 s was 0.08 nA. Assuming that the latter is sufficiently close to the steady-state current (i.e., the current at t = ∞), the ratio of the current at t = 0 to the current at t = ∞ is ≥47, meaning that the transference number for Na+ conductivity of the glass−ceramic sample is 0.97 or higher. Therefore, it is evident that the majority charge carriers in the glass−ceramic sample are Na ions. Figure 8 exhibits the ac conductivity σ(ω) of both the glass− ceramic and the glass samples as a function of angular

Figure 6. Arrhenius plots of both the bulk and the total conductivities of the glass−ceramic and the glass samples.

Figure 6 and compared with that of the glass sample. The conductivity, σ, of the samples is calculated using the equation

σ = R−1(L /A)

(2)

where A and L are the electrode area and the distance between two electrodes, respectively. It should be noted that σb and σt were computed with R = Rb and R = Rb + Rgb, respectively. As can be seen, the σb reaches ∼10−5 S cm−1 at room temperature (25 °C) under nitrogen atmosphere with an activation energy Ea of ∼0.63 eV. The difference in σb between the glass−ceramic and the glass samples is remarkable, the conductivity of the former being higher than that of the latter by more than 4 orders of magnitude at room temperature! Clearly such a tremendous enhancement in σb observed for glass−ceramic cannot be attributed to the difference in the Na content that differs by only less than a factor of 2. We attribute the enhancement to Na+ conduction through the percolating network of the Ga-doped Na2Ge2Se5 crystal phase formed in the glass−ceramic as evidenced by the Nyquist plot shown in Figure 5. Na+ in this crystal structure is expected to migrate rapidly through the two-dimensional space between the anionic layers (Figure 1) as mentioned earlier and will be demonstrated below. The difference in the Ea of the σb between the glass− ceramic and the glass samples (0.63 vs 0.50 eV) also supports the hypothesis that the conduction mechanisms in these materials are different from one another. It is to be noted here that the electrical conductivity of the crystalline phase in the temperature range of interest is expected to be predominantly, if not entirely, ionic in view of the relatively wide band gap (2.38 eV) reported for Na2Ge2Se5.16 The substantial doublelayer impedance of the Pt electrodes used for the impedance measurements shown in Figure 5 also implies that the charge carriers in the glass−ceramic are ions, the direct current (dc) of which is blocked by the Pt electrodes. More importantly, the dc-polarization curves (Figure 7) of the glass−ceramic sample with ionically blocking electrodes (i.e., sputtered Pt) measured under dry N2 at 6 °C clearly demonstrates that the cell undergoes rapid polarization under a constant dc bias due to accumulation of Na+ at the negative electrode. The inset of

Figure 8. Bode plots showing the frequency-dependent ac conductivity of the glass−ceramic (red circles) and the glass (black circles) samples at 20 °C. The arrows indicate the hopping frequencies of Na+ in the two samples.

frequency (ω = 2πf with f being frequency in Hz) applied, given by the Almond-West expression: σ(ω) = σdc{1 + (ω/ω h)n }

(3)

where σdc and ωh denote the bulk dc conductivity and the average hopping frequency of the ions, respectively, and n is a fitting parameter. Equation 3 implies that σ(ω) becomes frequency-independent (i.e., σ(ω) = σdc) where ω is sufficiently lower than ωh, while σ(ω) increases with ω (i.e., σ(ω) = σdc(ω/ ωh)n) where ω is sufficiently higher than ωh as is clearly seen in 5698

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Figure 8 for both samples. The σ(ω) of the glass−ceramic measured at 20 °C shows an additional plateau at lower frequencies, representing the grain-boundary dc conductivity. The value of ωh of Na+ in the glass−ceramic is estimated to be ∼1.05 × 107 rad/s at 20 °C, approximately 4 orders of magnitude higher than that (4.71 × 102 rad/s) observed for the glass sample as indicated in Figure 8. This difference in ωh is consistent with the corresponding difference in σb between the two samples (5.1 × 10−6 vs 3.8 × 10−10 S cm−1), strongly suggesting that the enhancement in σb observed for the glass− ceramic is essentially driven by the higher ωh of Na+ in the Gadoped Na2Ge2Se5 crystal phase. Previous studies have shown that the power-law exponent n in eq 3 for dispersive conductivity σ(ω) is related to the effective dimensionality of ion diffusion in solids.20 For glassy materials ionic diffusion takes place in a three dimensionally disordered potential energy landscape and n is universally ∼0.6−0.7. On the other hand, for two-dimensional ionic diffusion in crystals such as Na+-conducting β″-Al2O3, n is reduced to ∼0.55 while for one-dimensional diffusion n < 0.4.20 The n values determined for both our glass−ceramic and glass samples are indicated in Figure 8. We note that the value of n for the glass−ceramic is ∼0.55, consistent with the values characteristic of ion conduction in crystals with layered structure characterized by two-dimensional pathways for ionic diffusion. This result thus confirms our hypothesis that Na+ conduction takes place through the percolating network of the Ga-doped Na2Ge2Se5 crystal phase formed in the glass− ceramic. The n value of ∼0.72 estimated for the glass sample agrees well with the values previously reported in the literature for a wide range of glassy materials.20,21 It is worth noting that the σb of ∼105 S cm−1 estimated for the glass−ceramic is an effective conductivity, that is, a summation over all conductivities, where each conductivity (σi) is weighted by the volume fraction of the corresponding phase (φi) in a composite, given as

⟨σ ⟩ =

∑ φi σ i i

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Bruce Aitken and Mr. Steve Currie at Corning Incorporated for their help with sample synthesis. S.K. and S.K.K. are grateful for the partial financial support from USIsrael Binational Science Foundation (BSF 2012237). S.S. and A.M. were supported by an NSF grant (DMR 1104869).

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REFERENCES

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(4)

where ⟨σ⟩ denotes the effective conductivity of a network of resistors in parallel. Accordingly, the actual σb of the Ga-doped Na2Ge2Se5 crystal must be substantially higher than what is shown in Figure 6 for the glass−ceramic since φ of the crystalline phase is much less than 1. It is also worth mentioning that the conductivity of this sample remained unchanged over more than 12 months when kept in a conventional desiccator.

4. CONCLUSION We have measured the effective conductivity of a glass−ceramic containing a Ga-doped Na2Ge2Se5 crystalline phase: a layered chalcogenide with a novel topology of a two-dimensional anionic framework of 2∞[Ge2Se52−] separated by layers of Na ions. The three dimensionally continuous percolation network of the crystal phase in the glass−ceramic allows for long-range transport of Na+ through this phase. The effective conductivity measured around room temperature under nitrogen is appreciably high (>10−5 S/cm). This result clearly demonstrates that such layered chalcogenide compounds with fast diffusing Na ions can be viable alternatives to the conventional SEs, and the synthetic approach taken here may lead to the development of novel glass−ceramics with potential applications in low-temperature sodium batteries. 5699

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