V2O5 Polysulfide Anion Barrier for Long-Lived Li–S Batteries

Article pubs.acs.org/cm

V2O5 Polysulfide Anion Barrier for Long-Lived Li−S Batteries Wen Li, Jocelyn Hicks-Garner, John Wang, Jun Liu,† Adam F. Gross, Elena Sherman, Jason Graetz, John J. Vajo,* and Ping Liu‡ HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, California 90265, United States S Supporting Information *

ABSTRACT: We describe a V2O5 polysulfide anion barrier for a Li−S battery containing a Li metal anode, an organic solvent-based electrolyte, and a nanostructured carbon−sulfur composite cathode that cycles without degradation from dissolution of soluble polysulfide anions. Degradation is prevented by the micrometer-scale V2O5 layer that physically isolates the anode and cathode electrolytes, thus blocking diffusion of dissolved polysulfide anions while permitting solid-state transport of Li+ cations. This divided cell architecture eliminates the interaction of soluble polysulfide with the Li anode. Mechanical integrity of the thin V2O5 layer is achieved by depositing the layer on a commercial polymeric battery separator. In addition, the isolated cathode electrolyte is optimized by the intentional addition of Li2S8, which suppresses redistribution of sulfur within the nanostructured carbon−sulfur composite cathode. A 5 mA h pouch cell of this design has been cycled >300 times over ∼1 year without noticeable degradation at capacities of 800 mA h g-sulfur−1.

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INTRODUCTION Concatenated molecular species of elemental sulfur (Sn, n = 2 to >8) and polyatomic sulfur anions (i.e., polysulfides, Sn2−) are common due to the relative stability of S−S single bonds.1−3 In addition, the relatively large size and distributed charge density in Sn2− anions reduces the lattice energy of polysulfide salts (e.g., Li2Sn), making salts of anions with n > ∼4 soluble in many organic solvents.4 This solubility has thus far impeded development of long cycle life rechargeable Li−S batteries despite intense interest motivated by material cost and abundance and a high theoretical specific energy density, ∼2550 W h kg−1 for the 2Li + S ↔ Li2S electrochemical reaction.5−10 The recharging and cycle life of Li−S batteries consisting of Li metal anodes, organic solvent-based electrolytes, and sulfur cathodes are compromised by the dissolution of soluble polysulfide anions formed at intermediate stages of the electrochemical oxidation and reduction reactions.11 Dissolved polysulfides diffuse throughout the battery electrolyte eventually reaching the Li metal anode. At the Li surface, chemical reduction of polysulfides may occur. Partial chemical reduction to lower soluble polysulfides (for example, dissolved Li2S8 being reduced by Li to dissolved Li2S6) enables chemical diffusion of these partially reduced polysulfides from the Li anode back to the sulfur cathode where they can be electrochemically reoxidized. During recharging, this diffusive anion transport opposes the electron transport in the external circuit leading to an endless charging current. Depending on the extent of dissolution and the rate of recharging, this current can limit the charging voltage preventing full recharging. The overall process, referred to as polysulfide crossover or the polysulfide shuttle,11 is essentially an internal chemical short. More complete chemical reduction to insoluble sulfides, such as Li2S2 or Li2S, can passivate the Li electrode with concomitant irreversible loss of sulfur from the cathode. Finally, even without reaction at the © 2014 American Chemical Society

Li anode, dissolution, disproportionation, and reprecipitation of polysulfides4 within the cathode can lead to significant sulfur redistribution that degrades the cathode kinetics and capacity. In addition to polysulfide solubility, commercially viable Li− S batteries have been difficult to realize due to the insulating nature of sulfur, which limits sulfur utilization. On the basis of the pioneering work of Ji et al.,12 this issue has been addressed using electrically conductive porous carbon scaffolds13 with nanometer scale pores that confine the sulfur and restrict electron transport distances.14−24 In this approach, sulfur is incorporated into the pore volume of nanoporous carbon scaffolds, often by melt-infusion, to form nanostructured carbon−sulfur composites. By adjusting the scaffold/sulfur composition, the pore volume can be appropriately partitioned to contain the sulfur while also accommodating volume changes and permitting electrolyte access. Thus, cathodes based on these carbon−sulfur composites can have high sulfur utilization and rate capabilities.21 The presence of electrolyte within the pore space also appears to sequester the partially reduced soluble polysulfides, inhibiting the crossover mechanism. Reduced diffusion of soluble polysulfides to the Li anode enables cells based on porous nanostructured carbon−sulfur composites to be recharged. However, a fraction of the recharging current still originates from the polysulfide shuttle. Although Coulomb efficiencies are often not reported, it appears that many nanostructured carbon−sulfur composites have efficiencies (discharge capacity/recharge capacity) of 75% to 85%.14−16 These efficiencies are high enough to permit significant recharging; however, cycling capacities usually still decline over ∼20 to 100 cycles. This degradation occurs because, eventually, (1) sulfur will be irreversibly lost at the Li Received: February 17, 2014 Revised: May 6, 2014 Published: May 15, 2014 3403

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a micrometer scale thickness V2O5 solid-state lithium ionconducting layer that physically separates the anode and cathode electrolytes. A schematic illustration of this divided cell architecture is shown in Figure 1.

anode and (2) the continual dissolution, disproportionation, and reprecipitation reactions, within the cathode, will lead to sulfur deposits that are electrochemically inactive. To further inhibit polysulfide diffusion, composite electrodes have been coated with polymers13,17 or mixed with porous silica adsorbents forming polysulfide reservoirs.25 Adsorbent layers of carbon have also been overlaid on sulfur cathodes.26,27 Sulfur has also been incorporated into an elastic polyaniline composite in nanotube form.28 These treatments further improve efficiencies but, nevertheless, some polysulfide diffusion and capacity degradation still occur. Alternative strategies have also been pursued. Lithium metal anodes can to some extent be protected from polysulfide anions by adding LiNO3 to the electrolyte.9 Remarkable cycling was reported by restricting the size of the polysulfide anions formed to only insoluble species using extreme confinement.29 However, the volume loading of 70% seems too large to accommodate the expected volume expansion (1.8×) even without considering electrolyte access. Nanoparticles of sulfur have also been coated with TiO2 to create yolk−shell structures.30 The TiO2 shell significantly reduced but did not eliminate polysulfide dissolution into the electrolyte. Recently, stable cycling over >500 cycles was attained by limiting the charging capacity.31 This prevented formation of soluble polysulfides but utilized at most only 75% of the theoretical capacity, ∼70% of the maximum Li−S energy. A hybrid anode was also used that contained a nonporous graphite layer acting as a solid electrolyte over the lithium metal.32 While this method would provide a formal barrier between the polysulfides and the anode, direct contact of a conductive barrier with a lithium metal anode could lead to lithium plating and dendrites (on top of the graphite), although this was not observed. The continued occurrence of polysulfide diffusion in nanostructured composites and other approaches involving polymer or gel electrolytes is not unexpected. Diffusion in the electrolyte contained within the pores of the composite may be slowed by the confined volume or adsorption on the pore walls, but it is not formally stopped. Similarly, polymers used as coatings or electrolytes, which must be permeable to solvated Li+ ions, will also necessarily be at least somewhat permeable to solvated polysulfide anions. In addition, adsorbent additives must be reversible and therefore function in equilibrium. This equilibrium will always permit some diffusion in the presence of a concentration gradient. Thus, these approaches will reduce but not eliminate polysulfide diffusion. To formally block polysulfide diffusion to the Li anode and thereby eliminate polysulfide solubility-based degradation, a barrier to polysulfide anions is required that transports Li+ cations without solvation. This requirement is satisfied by a solid-state Li+ ion conductor. While solid-state electrolyte Li−S batteries have been considered,33 an alternative approach is to include a solid-state Li+ conducting layer within an organic solvent electrolyte-based cell.34 This layer would completely isolate the electrolyte at the Li anode from the electrolyte at the sulfur cathode, thereby shutting down the polysulfide shuttle. Ideally, this configuration would allow separate optimization of the anode and cathodes electrolytes. The Li conducting ceramic LISICON has been considered for this approach, but its fragility (currently) limits the layer thickness to a minimum of 300 μm, which is too thick for practical cells.5,35 Here we describe a Li anode/organic solvent electrolyte/ nanostructured carbon−sulfur cathode Li−S battery containing

Figure 1. Schematic of divided cell architecture Li−S battery. A V2O5 layer (a) coated on one side of a commercial battery separator (b) divides the anode (c) and cathode (d) electrolytes, which are schematically shown greatly expanded. In actuality, the V2O5 layer contacts the sulfur cathode (e). A second separator (f) is used to ensure electrical isolation from the Li anode (g). The V2O5 layer transports Li+ cations but is an impermeable barrier to polysulfide anions and solvent molecules. This barrier, therefore, stops polysulfide crossover and enables separate optimization of the anode and cathode electrolytes. Other configurations with the layer in contact with the anode and isolated from both electrodes are shown in Supporting Information Figure S1.

As described above, the initial reduction of sulfur with Li produces intermediate polysulfide anions that are soluble in organic solvent-based electrolytes. These anions diffuse throughout the electrolyte and in conventional cells eventually reach the Li anode. In the divided cell, the dense and nonporous V2O5 layer presents a formal barrier to polysulfide anion diffusion. Dissolved polysulfide anions diffuse freely within the electrolyte contained in the cathode but are prevented from crossing into the anode electrolyte and reaching the anode. In contrast, V2O5 is a well-known Li battery cathode material and a good solid-state conductor of Li+ cations. Thus, by desolvating and resolvating at the V2O5/ electrolyte interfaces, Li+ cations can be transported between the anode and cathode through the V2O5 layer. Although V2O5 is a good solid-state lithium ion conductor, the diffusion coefficient for Li+ in V2O5 is much lower than in liquid electrolyte, ∼3 × 10−12 cm2 s−1 to 4 × 10−9 cm2 s−1 depending on Li content.36 Thus, to support the Li+ ion fluxes required for practical batteries (∼1 mA cm−2) without excessive resistance, i.e., IR losses, the V2O5 layer must have a thickness on the order of 1 μm. This restriction on thickness prevents use of freestanding layers. To use layers with micrometer scale thicknesses while maintaining the mechanical integrity necessary for handling and assembly in a practical battery, the V2O5 is deposited onto one side of a commercial polymeric battery separator. In addition to Li ion conductivity, V2O5 is also electronically conductive, a characteristic crucial in its performance as a cathode. Thus, to guard against an electrical short between the Li anode and sulfur cathode from any V2O5 that may have been deposited through the holes in the separator, a second 3404

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nomenclature in ref 37, the foam used in this work is designated 30%/100−1200 indicating synthesis using 30 wt % resorcinol− formaldehyde precursor solution with 100 cP viscosity silicone oil and a final firing temperature of 1200 °C. This composition produced a foam in which the spherical voids originating from the silicone oil droplets were partially collapsed (Supporting Information Figure S2). After firing, the foam was found to contain 43 wt % SiO2 from decomposition of residual silicone oil that was not fully washed out prior to firing. Trying to remove this SiO2 compromised the integrity of the foam, and so it was left in place. The pore size distribution, determined from N2 adsorption and Hg intrusion, was bimodal with peaks at ∼8 and 100 nm (Supporting Information Figure S3). The total pore volume was 1.23 cm3 g−1. The conductivity of the monolith measured using a 4-point probe method was 0.39 ± 0.07 S cm−1. This conductivity is lower than desired for optimal electrode performance,38 possibly due to the SiO2. Formation of the nanostructured carbon− sulfur composite was achieved by melt infiltration while heating the monolith with sulfur powder (purified by sublimation, −100 mesh, Aldrich), to 110 °C for 0.5 h in a sealed evacuated flask. After cooling, bulk sulfur on the exterior of the monolith was removed by scrapping. By weight, the composite was determined to be 61.8 wt % sulfur, corresponding to a volume loading of ∼60%. The composite monolith was subsequently hand-ground to a fine powder. Electrodes were formulated by mixing the nanostructured carbon foam/sulfur composite, SuperP carbon (MMM), and polyvinylidene fluoride (PVDF) binder (Kyner Flex 2801, Arkema) in a 80:10:10 weight ratio in N-methylpyrrolidinone (NMP) (99+%, Aldrich) until homogeneous slurries were obtained. Additional slurries containing bulk sulfur were prepared by mixing powder sulfur, TiS2 (99.9%, −200 mesh, Aldrich), to improve nucleation of Li2S, KS6 graphite (TIMCAL) to improve conductivity, SuperP carbon, and PVDF binder in a 60:10:10:10:10 weight ratio. Slurries of these mixtures were coated on Al foil (12 μm thick, Furukawa) and dried at 50 °C under vacuum (10−3 Torr) overnight to remove the solvent. These electrode tapes were then calendared. Areal sulfur densities were 1.25 mg-sulfur cm−2 for the nanostructured electrode and 3 mg-sulfur cm−2 for the bulk electrode. Electrochemical testing was performed using 2032 coin or 2 cm × 2 cm pouch half-cells. Cells were assembled in an argon filled glovebox using Li metal foil (Lectro Max 100, FMC Lithium), polypropylene separators (Celgard 3501), and 1 M lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) in 1:1 dioxolane (DOL)/ dimethoxyethane (DME) (Novolyte) electrolyte. Cells containing V2O5 layers supported on Celgard 3401 separators were assembled by first wetting the barrier layer side only with electrolyte and then placing the V2O5 layer facing the sulfur cathode. A second separator with no coating (Celgard 3501) was used to ensure electrical isolation from the Li anode. For cells without V2O5 layers, two separator layers were used to match the V2O5 layer cell design. Cells with added lithium polysulfide were assembled with barriers wetted with polysulfide by floating the separator containing the V2O5 layer (with the V2O5 layer facing down) on the surface of electrolyte containing 0.5 M Li2S8. The separator was carefully lifted off the polysulfide solution (excess electrolyte was allowed to drip off) and placed on the sulfur cathode with the V2O5 layer in contact with the sulfur electrode. From elemental analysis, the effective areal density of added sulfur was determined to be 0.4 mg/cm−2. Cells were assembled by stacking the component layers including the separator containing the V2O5 layer without attempting to seal the edges of the V2O5 layer. For coin cells, the separator containing the V2O5 layer was slightly larger than the cathode (1.5 cm vs 1.0 cm diameter). One or two drops of electrolyte were placed on the cathode (nanostructured or bulk) and one drop was placed on both the back (uncoated) anode side of the separator containing the barrier and on the second separator. Pouch cells were constructed using 2 cm × 2 cm sulfur cathodes with Al current collector tabs. Electrolyte was added dropwise (∼four or five drops) to the cathode just to wetness, with the edge still slightly dry. This was done to try to prevent electrolyte and thereby polysulfide from being pressed around the edges of the barrier layer. Separators coated with V2O5 layers were 3 cm × 3 cm. The second separator was 4 cm × 4 cm. Approximately three drops of

(uncoated) separator is placed between the Li anode and the coated separator. In general, the V2O5 layer can be utilized in three configurations (Supporting Information Figure S1): (1) in contact with the sulfur cathode; (2) fully isolated by using two additional separators, one on each side of the coated separator; or (3) in contact with the Li anode. Choosing to place the layer in contact with the sulfur cathode, as shown in Figure 1, allows the V2O5 to at least partially charge and discharge along with the sulfur cathode. Assembling a divided cell requires separately supplying electrolyte to the anode and cathode. This enables the electrolyte for each electrode to be optimized separately. For example, for the sulfur cathode, dissolution of soluble polysulfide will still occur within the electrode structure. Dissolved polysulfide anions can precipitate elsewhere in the cathode through disproportionation reactions or through oxidation during battery charging. Dissolution and precipitation redistributes sulfur within the cathode. This redistribution can lead to loss of electrochemically active sulfur through the formation of sulfur deposits that are electrically isolated or agglomerates that are too large for sufficient electron conduction. Redistribution of sulfur within the cathode may be suppressed by intentionally adding polysulfide anions (eg, as Li2S8) to the cathode electrolyte. Addition of Li2S8 does not change the rate constants for dissolution or diffusion but rather reduces the free energy for net dissolution. Following this design we demonstrate micrometer thick V2O5 films deposited on commercial polypropylene separators with the adhesion and mechanical robustness required for use as solid-state lithium ion conducting layers. We show the effectiveness of these layers at preventing soluble polysulfide crossover and the advantage of optimizing the isolated cathode electrolyte using added polysulfide anions. These features are utilized together in 5 mA h pouch cells that have been cycled >300 times over ∼1 year without noticeable degradation at capacities of 800 mA h g-sulfur−1.

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MATERIALS AND METHODS

Solid-state Li ion conducting layers were fabricated by coating commercial polymeric battery separators with a V2O5 sol−gel. Sol− gels of V2O5 were prepared by adding 1.62 g of V2O5 powder (99.6%, Aldrich) to 100 mL of 30 wt % H2O2 in water (Aldrich). This mixture was magnetically stirred for ∼80 min in a room temperature water bath. Hydrolysis and gelation began after ∼20 min. The sol−gel was then stored in a Nalgene bottle at room temperature for at least one week prior to use. Sols diluted from 1:2 up to 1:6 with deionized water by volume were prepared as needed and used immediately. Polypropylene separators containing surfactant coatings (Celgard, 3401 and 3501) were used as substrates for the V2O5 coatings. Substrates were prepared by taping pieces of separator around 4 in. × 4 in. glass slides, so that the separator was under tension. The separator was then rinsed with deionized water from a wash bottle until its appearance became translucent, ∼3 to 5 min. The slide was secured on a spin-coater (Chemat Technology, KW-4A) using double sided tape. Next, 3 mL of the V2O5 sol was applied to the center of the separator and spun for 15 s at 2000 rpm. This process was repeated to achieve the desired layer thickness. Layer thicknesses between 500 nm and 6 μm were used. The sol−gel layers were dried overnight in air and then heated for 4 h in air at 100 °C. Coated separators were stored at room temperature in a vacuum desiccator. Nanostructured carbon−sulfur composites were fabricated by infiltrating sulfur into electrically conductive carbon foam scaffolds. Scaffolds were synthesized in the form of monoliths using condensation of resorcinol-formaldehyde gels templated by silicone oil-based emulsions, as described previously.37 Following the 3405

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electrolyte were added to the anode side of the separator supporting the barrier and the second separator. The lithium anode was 3.5 cm × 3.5 cm and was pressed into a Ni grid custom etched from a Ni foil (99.8%, AMETEK 899L) with a Ni foil current collector tab. The stacked assembly was placed in a polyethylene bag and heat-sealed around the current collector tabs slightly over the edge of the second separator and up to the edge of the V2O5. This sealing and the much larger electrode area of the pouch cells likely reduced any electrolyte contact between the anode and cathode around the edge of the V2O5 although formally the edge was still not sealed. The sealed polyethylene bag was further sealed in a metallized bag. A schematic of the assembly is shown in Supporting Information Figure S4. Cycling was performed on a BT-2000 battery cycler (Arbin Instruments) between 3.0 and 1.0 V. Some cells were charged to 3.0 V with a subsequent voltage hold at 3.0 V until the current decreased by a factor of 10× or for a maximum of 5 h. A summary of the cells tested is given in Supporting Information Table S1.

Figure 2. Secondary electron image of V2O5 layer on polymeric battery separator. The sample was prepared by peeling a portion of the layer showing the V2O5 surface, V2O5 cross section, and the separator surface. The layer thickness is ∼1 μm. The arrow designates a portion of the layer that fractured internally and remained adhered to the separator.

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RESULTS AND DISCUSSION Achieving well-adhered crack-free V2O5 coatings is essential for preventing polysulfide anion crossover. Using surfactant coated polypropylene separators designed for aqueous electrolytes and then, crucially, rinsing the separator with deionized water enabled coatings with excellent adhesion. We speculate that the rinse removed excess surfactant, enabling the sol to interact with a monolayer of surfactant bound to the polypropylene. Surfactant removal by the water was indicated visually by the formation of stable bubbles in the rinsing solution and chemically by FTIR spectroscopy following drying a sample of the rinsed water. Complete wetting of the separator by the water indicated retention of at least a monolayer of surfactant because uncoated polypropylene separators are highly hydrophobic. The required duration of the rinse was determined by a change in appearance of the separator from opaque to translucent when sufficient surfactant was removed to permit water to enter the pores. Without rinsing, the sol still wet the separator well. However, the films cracked easily and exhibited poor adhesion as expected for a mechanically weak thick surfactant layer between the coating and the substrate. The porosity of the separator was also important for adhesion. The V2O5 coating must bridge the separator’s open pores. A porosity of 41% (Celgrad 3401) gave well adhered coatings while much poorer adhesion was found with a porosity of 55% (Celgard 3501) likely because of cracks initiated over the larger open pores. In addition to the substrate properties, the dilution of the sol also affected adhesion. Coatings without diluting the stock sol gave thick highly stressed layers, ∼5 μm per application that adhered poorly. Dilutions of 1:2 or 1:3, sol to deionized water, gave ∼0.5 μm thick layers per application with good adhesion. Dilutions > ∼1:4 gave incomplete layers even after multiple applications. Figure 2 shows a secondary electron image of a V2O5 layer in cross section after two applications of a 1:2 diluted sol on a water rinsed Celgard 3401 separator. A portion of the layer was intentionally cracked and peeled away from the separator to expose the cross section. Although polycrystalline and rough, the layer is dense, nonporous, and pinhole free. The thickness is uniformly ∼1 μm. Qualitatively, the adhesion is good with no delamination at the fractured interface. As shown, a shard of V2O5 remained attached to the substrate where the layer fractured internally, not at the interface. Additional images of the cross section and surface of the film are shown in Supporting Information Figures S5 and S6. The roughness originates from the holes and roughness of

the separator because layers deposited on glass are smoother (Supporting Information Figure S6d). After deposition, the coated separators, up to 10 cm × 10 cm, could be handled and even moderately bent without peeling or noticeable cracking. In addition, after wetting the barrier layer with the highly colored polysulfide solution, no color was observed on the back, uncoated, side of the separator. Layers thinner than ∼0.5 μm did not sufficiently cover the roughness and holes of the separator. However, only commercial separators were tried as substrates. We expect that layers with thicknesses of ∼0.1 μm should be possible on separators with surfaces optimized for barrier layer deposition. First cycle charge/discharge profiles of coin cells assembled using bulk sulfur electrodes, with and without V2O5 lithium ion conducting polysulfide anion barrier layers, are shown in Figure 3. Even for an extremely slow discharge rate of C/120 (1C = 1675 mA g−1), the discharge capacities for both cells are low (1 yr and undergoing >300 cycles, no clear indication of sulfur crossover was seen. The effectiveness of V2O5 (or LixV2O5) as a polysulfide anion barrier suggests the possibility that other diffusion barriers could be formed from dense (pinhole-free) films composed of fast Li+ conductors, such as conventional electrodes (e.g., lithiated carbon) or solid electrolytes (e.g., LiPON). Importantly, this concept of dividing the cell and restricting electrolyte crossover to Li+ ions may be used to improve capacity retention in more conventional lithium cells. For example, when paired with graphite, a number of high-energy lithium batteries suffer from severe capacity fade due to the dissolution and crossover of transition metal ions from the cathode (e.g., Mn2+, Ni2+, Cu2+).39−41 This problem is particularly acute with the Mn-based cathodes, which have a tendency to form soluble divalent Mn cations during cycling. Although there is some debate over the mechanisms involved,42 the interaction of Mn2+ with the graphite surface increases the loss of active Li+ (attributed to side reactions with the electrolyte), forms a thicker, more resistive solid electrolyte interphase, and results in significant capacity fade.43 Implementing a barrier layer by, for example, coating the separator with a thin lithium ion conductor would be a simple and effective method of eliminating the unwanted cation crossover and thereby improving capacity retention. In addition to preventing unwanted ion crossover, the use of a solid-state Li+ conducting barrier also enables divided cell architectures, providing a potentially useful extension of traditional battery designs. For example, in addition to separately optimizing the anode and cathode electrolytes, as described in this work, different organic solvents could be used for each electrolyte. This could enable higher voltage Li/metaloxide cells with different electrolyte solvents compatible with Li metal anodes and high voltage cathodes, respectively. Another

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SUMMARY In summary, we have described a micrometer-scale V2O5 barrier layer to separate the anode and cathode electrolytes of an organic electrolyte Li−S battery. This layer stops diffusion of dissolved polysulfide anions between the sulfur cathode and the Li anode while permitting solid-state transport of Li+ cations. Blocking diffusion of polysulfide anions to the Li anode eliminates polysulfide anion crossover, which is one of the major degradation mechanisms that plague Li−S batteries. Separation of the anode and cathode electrolytes also enables each electrolyte to be optimized individually. We optimized the cathode electrolyte with the addition of Li2S8 to reduce redistribution of sulfur within the cathode. The required mechanical integrity of such a thin V2O5 layer was achieved by coating the layer on one side of a commercial polymeric battery separator. We implemented this design with a high capacity nanostructured carbon−sulfur composite cathode based on a nanoporous carbon foam. A 2 cm × 2 cm pouch cell was demonstrated that cycled with a capacity of 5 mA h (800 mA h g-sulfur−1) without noticeable degradation for >300 cycles over ∼1 year.

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ASSOCIATED CONTENT

S Supporting Information *

Alternate cell configurations, SEM images of the carbon foam and pore size distribution plots, SEM images of the V2O5 layer, and additional cell discharge profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.J.V.). Present Addresses †

MK Tech. Ltd., Suzhou, Jiangsu Province, 215125 China (J.L.). ‡ Advanced Research Projects Agency - Energy (ARPA-E), 1000 Independence Avenue Southwest, Washington, DC 20585, United States (P.L.). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) In contrast, elemental and anionic species of oxygen are predominately diatomic due to the relative stability of OO double bonds (ref 2). (2) Purcell, K. F.; Kotz, J. C. Inorganic Chemistry; W. B. Saunders Co.: Philadelphia, PA, USA, 1977; pp 340−342. (3) Holleman, A. F.; Wiberg, E. Inorganic Chemistry, 34th ed.; Academic Press: San Diego, CA, USA, 2001; pp 505−518, English translation. (4) Rauh, R. D.; Shuker, F. S.; Marston, J. M.; Brummer, S. B. J. Inorg. Nucl. Chem. 1977, 39, 1761−1766. 3409

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dx.doi.org/10.1021/cm500575q | Chem. Mater. 2014, 26, 3403−3410