Long-Chain Sodium

Letter pubs.acs.org/JPCL

Highly Reversible Room-Temperature Sulfur/Long-Chain Sodium Polysulfide Batteries Xingwen Yu and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: In a room-temperature sodium−sulfur (RT Na−S) battery, the complicated reduction reaction of the sulfur cathode generally involves two main steps: (i) transformation of elemental sulfur into long-chain soluble sodium polysulfides (Na2Sn 4 ≤ n ≤ 8) and (ii) conversion of the long-chain sodium polysulfides into solid-state short-chain polysulfide Na2S2 or disulfide Na2S. It is found that the slow kinetics of the second step limits the efficiency of discharge and induces irreversible capacity loss during cycling. Accordingly, we present here a RT Na−S cell operated with the sulfur/long-chain sodium polysulfide redox couple to avoid the capacity fade. An advanced cathode structure has been developed by inserting a carbon nanofoam interlayer between the sulfur cathode and the separator to localize the soluble polysulfide species and prevent its migration to the anode. The highly reversible sulfur/long-chain sodium polysulfide cell presented here can provide a stable output energy density of 450 Wh kg−1 at an extremely low energy cost of ∼$10 kWh−1 (based on the active material of anode and cathode). SECTION: Energy Conversion and Storage; Energy and Charge Transport to a cathode capacity of 1675 mAh g−1, which is an order of magnitude higher than that of conventional insertion-cathode materials based on transition-metal oxides. Therefore, use of less expensive sodium metal as an anode to couple with the high-capacity sulfur cathode is promising to develop Na−S batteries with high energy density at low cost. Actually, since 2006, there have been a few reports on low-temperature Na−S batteries, employing sodium metal as the anode, composite sulfur cathode, and electrolytes based on organic solvents, polymers, and solid Na+-ion conductors.15−20 However, the operation of a Na−S battery at room temperature faces the critical challenge of extremely fast capacity fade during cycling due to a series of known (e.g., dissolution of polysulfide into electrolyte19) and unknown issues. In light of the discharge mechanism and cycling characteristics, we present here an approach to achieve high-power room-temperature Na−S (RT Na−S) batteries with low capacity fade through the operation of the cells with sulfur/long-chain sodium polysulfide redox couple. In order to avoid the shuttling of soluble sodium polysulfide, an advanced cathode structure is developed by inserting a nanostructured, carbon-based interlayer between the sulfur cathode and the separator. The configuration of the RT Na−S battery in this study is displayed as Figure 1a, in which a carbon nanofoam (CNF) interlayer is placed in between the separator and the sulfur

A

mong the various available energy storage technologies, the Li-ion batteries have attracted almost all the attention not only in the portable electronic market but also as the prime candidate to power the next generation electric vehicles and to modulate various renewable energy sources on a large scale.1 However, the limited abundance of lithium resources poses serious concerns for their adoptability for large-scale storage of renewable energies. In this regard, there is a need to explore low cost, safe, long-life rechargeable energy storage systems based on abundant resources. Compared with Li, Na has huge availability in natural resources. Therefore, development of room-temperature Na-ion batteries with a similar working principle as Li-ion batteries for large-scale energy storage is an attractive alternative.2,3 To date, many efforts have been made and a large number of compounds have been proposed as electrode materials for the development of the “rocking chair” Na-ion batteries.4−7 However, it remains a great challenge for developing high-energy and high-power density materials with low volume change during cycling. One successful Na-ion energy storage technology is the high-temperature molten electrode Na−S battery, which has been used to support stationary energy storage systems for several decades.8−11 However, the molten Na−S batteries require high operating temperatures of >300 °C, which limits their propagation into a wide range of applications. Recently, there has been great progress in the lowtemperature sulfur-cathode chemistry in organic electrolytes toward the development of high-energy Li−S batteries.12−14 Sulfur can theoretically accept two electrons per atom leading © 2014 American Chemical Society

Received: April 30, 2014 Accepted: May 16, 2014 Published: May 16, 2014 1943

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with the carbon nanotube and mesoporous carbon have been discussed in our previous studies of Li−S batteries.21,22 The carbon nanofoam used here is similar to the carbon nanotube and mesoporous carbon used by us before, but with better mechanical properties and robustness. Such a conductive interlayer with three-dimensional nanoframework and tortuous interspace not only prevents or inhibits the migration of soluble sodium polysulfides (Na2Sn) from the cathode to the anode but also acts as a secondary current collector to capture and retain the polysulfide species by electrochemical deposition at the end of discharge−charge. In addition, the interlayer is also able to accommodate some of the stress and volume expansion during discharge of sulfur and endure the volume change of the trapped active material during cycling.21,22 Figure 2a shows the discharge capacities as a function of cycling number for the RT Na−S batteries with and without a carbon nanofoam interlayer and operated with a discharge cutoff voltage of 1.2 V. The capacity of the cell with a CNF interlayer is significantly higher than that without an interlayer throughout the 20 cycles. Both cells either with or without an interlayer experience a sharp decline in capacity in the first cycle or first few cycles, and then the fade rate diminishes. Figure 2b shows the representative first and the fifth discharge curves of the cell with a CNF interlayer. According to the discharge mechanism of the RT Na−S batteries,19 the upper plateau region in Figure 2b is contributed from the transformation of elemental sulfur into long-chain soluble sodium polysulfides (Na2Sn 4 < n ≤ 8), whereas the lower plateau region corresponds to the conversion of the long-chain sodium polysulfides into solid-state short-chain polysulfide Na2S2 or disulfide Na2S. Of particular interest in Figure 2b is that the decrease in capacity within the two potential plateau regions is not synchronous. It is evident that the capacity decrease is less severe in the upper-voltage-plateau region relative to that in the

Figure 1. (a) Schematic representation of the room-temperature sodium−sulfur battery with an interlayer. The anode, separator, interlayer, and cathode are firmly packed with mechnical contact. The spaces between the cell components are filled with liquid electrolyte; the space between the sulfur cathode and the interlayer also consists of polysufide. (b) SEM image showing the structure of the carbon nanofoam. (c) Properties of the carbon nanofoam.

cathode. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD) characterizations of the sulfur electrode are provided in the Supporting Information as Figures S1 and S2, respectively. The carbon nanofoam is a unique class of porous material with good electrical conductivity and is available in the form of a paper. The lightweight foam used here has nanometer-scale dimensions of the solid frame and pore spaces, as shown in Figure 1b, and presents ideal properties for electrode applications (Figure 1c). Such structure and properties are able to provide subtle surface contact with the sulfur cathode to reduce the electrical resistance and allows electrolyte penetration during cycling. Multiple functions of the interlayer

Figure 2. Electrochemical characteristics of the room-temperature Na−S batteries: (a) Discharge capacities as a function of cycle number for the Na−S batteries with or without an interlayer and operated with a discharge cutoff voltage of 1.2 V. (b) Representative discharge curves of the cell with a CNF interlayer at the first and fifth cycles. (c) Cyclic voltammogram profiles of the Na−S cell at the first and fifth cycles at a scan rate of 0.1 mV s−1. (d) Representative upper plateau discharge profiles of the Na−S battery with a CNF interlayer. (e) Representative cyclic voltammetry diagrams of the RT Na−S battery between 2.8 and 1.8 V at a scan rate of 0.1 mV s−1. (f) Discharge capacity of the room-temperature sulfur/longchain sodium polysulfide batteries as a function of cycle number. 1944

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Figure 3. X-ray photoelectron spectroscopy (XPS) and ultraviolet−visible spectroscopy (UV−vis) analysis of the discharge product of the RT Na−S batteries. Left: High-resolution S 2p XPS spectra of the sulfur cathode being discharged to 2.2 V (a) and 1.8 V (b) at a scan rate of 0.1 mV s−1. Right: UV−vis absorption spectra of the discharge product of the sulfur cathode being discharged to 1.8 V (c) and in-house synthesized 0.25 mM Na2S6 in tetraglyme (d).

lower-voltage-plateau region. A similar phenomenon is also observed in the cyclic voltammogram profiles of the same RT Na−S cell (Figure 2c), where there are only small current or potential changes in the first (high-voltage) discharge peaks (2.2 V) and the corresponding charge peaks (2.4 V) with repeated scans, attesting to the superior reversibility and cycling stability between elemental sulfur and long-chain soluble sodium polysulfides (Na2Sn, 4 ≤ n ≤ 8). On the other hand, the second (low-voltage) discharge peaks (1.65 V) and the corresponding charge peaks (1.85 V) show apparent unstable and diminished CV patterns in the successive cycles, indicating the relatively poor reversibility between the Na2S4 and insoluble short-chain sodium polysulfide or disulfide (Na2Sn, 1 ≤ n < 4). According to these cycling characteristics, utilizing only the upper plateau by adjusting the discharge cutoff voltage is expected to be a promising means to achieve low capacity fade RT Na−S batteries. Figure 2d displays the representative upper-plateau discharge profiles (operated with a discharge cutoff voltage of 1.8 V) of the Na−S battery with a CNF interlayer at the 1st, 10th, 25th, and 50th cycles. Relatively lower voltage in the first discharge cycle is due to the poor wetablility of the interlayer material by the electrolyte. After the CNF interlayer is soaked with the electrolyte upon a complete cycle, the discharge voltage becomes normal and there is almost no change in the shape of the discharge curves in the following cycles. Figure 2e shows the representative cyclic voltammetry diagrams of the RT Na− S battery between 2.8 and 1.8 V at a scan rate of 0.1 mV s−1. The reduction and oxidation peaks are located at ∼2.2 and ∼2.4 V, respectively, which is consistent with the discharge plateau shown in Figure 2d and corresponds to the transition of elemental sulfur to the long-chain sodium polysulfides (Na2Sn, 4 ≤ n ≤ 8). The reversibility in a single scan is nearly perfect, demonstrating the excellent reversibility of the sulfur/longchain sodium polysulfides redox couple. Notably, there are no apparent current or potential changes in the overlapped peaks with repeated scans in Figure 2e (except the first cycle), attesting to the superior cycling stability of the sulfur/longchain sodium polysulfide batteries. Figure 2f shows the discharge capacity of the room-temperature sulfur/long-chain sodium polysulfide batteries as a function of cycling number. Extremely stable discharge capacity is achieved throughout the tested cycle life (50 cycles).

Due to the extremely air sensitive nature of the sodium polysulfide compounds and the lack of well-established in situ analytical platforms, the discharge products of the RT Na−S battery are difficult to be captured and effectively characterized. Here, we employed X-ray photoelectron spectroscopy (XPS) and ultraviolet−visible spectroscopy (UV−vis) to analyze the discharge products of the sulfur/long-chain sodium polysulfides batteries. A wide survey-scan XPS spectrum of the discharged (to a voltage of 1.8 V) sulfur electrode is provided in Supporting Information Figure S3, and it displays the characteristic peaks of sodium, sulfur, carbon, oxygen, and fluorine. The sodium peak comes from either the discharge product or the precipitation of sodium salt from the electrolyte. Sulfur signals belong to active sulfur compounds or the longchain polysulfide discharged product. The carbon peaks can be attributed to the carbon particles (super P) and binder in the sulfur cathode. Fluorine comes mainly from the binder. Figure 3a and b show the high-resolution S 2p spectra obtained from the sulfur cathodes discharged to various voltages. The linear-scan voltammogram profiles of these electrodes as in the Na−S batteries are presented in Supporting Information Figure S4a and b, respectively. The 1.8 V is the cutoff discharge voltage employed for the sulfur/long-chain sodium polysulfide batteries in this study. Another experiment in which the sulfur electrode was discharged to 2.2 V is also given here for a comparison. The elemental sulfur usually has two split peaks in the S 2p region, corresponding to 2p3/2 (164.0 eV) and 2p1/2 (165.2 eV).23 The dominant S 2p3/2 peak for sodium sulfide is located at 161.6 eV.23 After discharging the cell, the oxidation state of sulfur in the cathode changes dramatically. As expected from the battery chemistry, the majority of sulfur was reduced to lower oxidation states as indicated by the lower binding energies shown in Figure 3. A new peak located at 162.1 eV also appears, which is neither from elemental sulfur nor from sodium sulfide, implying that long-chain sodium polysulfide compounds have been formed. There have been overviews of binding energy ranges for the sulfur-rich compounds.24−28 Of particular interest is the assignment for polysulfide components. In general, the average binding energy of polysulfides existing as Sn2− (n = 1−8) is 163.7 eV. Particularly the binding energy for S52− is 161.9−163.2 eV and S42− is 162.0−163.0 eV.24−28 The peak in Figure 3a located at high binding energy (165.2 eV) is contributed from the S 2p1/2 of the unreduced sulfur. The dominant peak at 164.0 eV is supposed to be from both the 1945

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The Journal of Physical Chemistry Letters elemental sulfur (S 2p3/2) and the polysulfides. The above features indicate that the elemental sulfur is not completely reduced to polysulfide in this case (being discharged to 2.2 V). It is obvious that the S 2p1/2 peak shrinks (almost disappears) in Figure 3b, indicating that the elemental sulfur has been mostly or completely reduced to a lower-valent state. According to the above discussion, the shadow area in Figure 3 at the binding energy of 161.6−164.1 eV is most reasonably assigned to the long-chain sodium polysulfides Na2Sn (4 ≤ n ≤ 8). On the other hand, the peak located at 162.1 eV becomes more prominent with the increase in the depth of discharge, suggesting an increase in sodium polysulfide products. Figure 3c shows the ultraviolet−visible (UV−vis) absorption spectra of the discharge product from a sulfur cathode that is discharged to 1.8 V; the sample was collected by rinsing the discharged cathode and the interlayer. This spectrum shows almost the same characteristics of the in-house synthesized Na2S6 in tetraglyme solvent as shown in Figure 3d. The spectra for both the samples have a band at ca. 620 nm, characteristic of S3•−, which is from the dissociation of S62− (S62− → 2S3•−).29,30 The broad peaks in the 400−500 nm region is in agreement with the S42− and S62− species, indicating the possible coexistence of multiple forms of polysulfides in equilibrium (2S62− → S42− + S82−).29−36 According to the discussion above, the anode, cathode, and overall reactions during charge− discharge of the sulfur/long-chain sodium polysulfide cell can be expressed as Anode: Na ↔ Na + + e−

Cathode: (4 ≤ n ≤ 8)

(2)

Overall: nS + 2Na ↔ Na 2Sn

(4 ≤ n ≤ 8)

EXPERIMENTAL METHODS

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ASSOCIATED CONTENT

Pristine sulfur with a small particle size was synthesized in aqueous solution in accordance with the reaction Na2S2O3 + 2HCl → 2NaCl + SO2 + H2O + S↓. Typically, 5.0 g of Na2S2O3·5H2O (Fisher Scientific) was dissolved in 750 mL of deionized water by magnetic stirring, followed by the addition of 2.0 mL of 10 M HCl. The above mixture was stirred for 24 h at room temperature. Then the sulfur product was filtered and thoroughly rinsed with deionized water, ethanol, and acetone successively. Finally, the sulfur powder was dried in an air oven at 50 °C for 24 h. The synthesized sulfur powder (60 wt %), super P carbon powder (30 wt %), and polyvinylidenefuoride (PVDF, Kureha) binder (10 wt %) were homogeneously mixed in a NMP (Sigma-Aldrich) solvent with magnetic stirring. The slurry was then coated uniformly on an aluminum foil and dried under vacuum at 50 °C for 24 h to form the carbon−sulfur composite electrode. The charge/discharge and electrochemical performances of the batteries were tested with a coin cell configuration in which a sodium−metal foil was used as the anode. The electrolyte was 1.5 M NaClO4 and 0.3 M NaNO3 in a tetraglyme (TEGDME) solvent. Glass fiber (Merk Millipore Ltd.) was used as a separator. In particular cases, an interlayer with CNF thin film was placed between the cathode and the separator. Cell assembly was carried out in an argon-filled glovebox. The coin cells were cycled between the cutoff voltages of 2.8 and 1.2 V on a battery test instrument (Arbin Instruments) at room temperature. Cyclic voltammetry (CV) measurements were performed with a VoltaLab potentiostat at a scan rate of 0.1 mV s−1. The morphology of the carbon nanofoam interlayer was examined by scanning transmission electron microscopy (STEM, Hitachi S-5500). Surface analysis of the discharged sulfur electrodes were carried out with X-ray photoelectron spectroscopy (XPS, Kratos Analytical Company) analysis. UV− visible absorption spectroscopy (Varian Cary 5000 UV−vis− NIR Spectrophotometer) analysis was conducted to identify the polysulfide species. The samples for UV−vis experiment from a discharged cell were collected by rinsing the discharge cathode and interlayer with tetraglyme (TEGDME) solvent inside the glovebox.

(1)

nS + 2Na + + 2e− ↔ Na 2Sn

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(3)

According to the data shown in Figure 2d and f, the highly reversible sulfur/long-chain sodium polysulfide battery can provide an energy density of ∼450 Wh kg−1 at an extremely low energy cost of ∼ $10.0 kWh−1 (based on the active materials of anode and cathode), offering a competitive option for low-cost, large-scale energy storage applications. The battery chemistry presented here also sheds light on the fundamental understanding of alkali metal-ion batteries and sulfur chemistries in organic media. In summary, it is found that the slow transition kinetics of the short-chain sodium polysulfides contributes to the irreversible capacity fade of the RT Na−S batteries during cycling. Operation of the cell with the sulfur/long-chain sodium polysulfide redox couple can avoid the capacity loss from the transition of short-chain polysulfides and gives a highly reversible RT Na−S battery system with stable capacity output. Incorporation of a nanostructured, carbon-based interlayer between the sulfur cathode and the separator localizes the soluble polysulfide species and prevents its migration to the anode. The highly reversible sulfur/long-chain sodium polysulfide cell presented here can provide a stable output energy density of 450 Wh kg−1 at an extremely low energy cost of ∼$10 kWh−1 (based on the active material of anode and cathode), offering a practically superior cost−performance ratio to the traditional Li-ion batteries and Li−S batteries.

S Supporting Information *

SEM images and EDS elemental mapping (sulfur and carbon) analyses of the sulfur−carbon composite cathode; X-ray diffraction (XRD) patterns of the pristine sulfur−carbon composite cathode; XPS survey spectrum of the discharged (to 1.8 V) sulfur electrode; voltammogram profiles of the sulfur electrodes prepared for XPS analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest. 1946

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0005397.

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