Stabilized Lithium–Metal Surface in a Polysulfide-Rich Environment of

Letter pubs.acs.org/JPCL

Stabilized Lithium−Metal Surface in a Polysulfide-Rich Environment of Lithium−Sulfur Batteries Chenxi Zu 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: Lithium−metal anode degradation is one of the major challenges of lithium− sulfur (Li−S) batteries, hindering their practical utility as next-generation rechargeable battery chemistry. The polysulfide migration and shuttling associated with Li−S batteries can induce heterogeneities of the lithium−metal surface because it causes passivation by bulk insulating Li2S particles/electrolyte decomposition products on a lithium−metal surface. This promotes lithium dendrite formation and leads to poor lithium cycling efficiency with complicated lithium surface chemistry. Here, we show copper acetate as a surface stabilizer for lithium metal in a polysulfide-rich environment of Li−S batteries. The lithium surface is protected from parasitic reactions with the organic electrolyte and the migrating polysulfides by an in situ chemical formation of a passivation film consisting of mainly Li2S/Li2S2/CuS/Cu2S and electrolyte decomposition products. This passivation film also suppresses lithium dendrite formation by controlling the lithium deposition sites, leading to a stabilized lithium surface characterized by a dendrite-free morphology and improved surface chemistry. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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efficiency. In addition, Li−S batteries involve polysulfide shuttling and parasitic side reactions between polysulfides and lithium metal, causing passivation by bulk Li2S particles and electrolyte decomposition products on the lithium−metal surface; therefore, it is difficult to conserve a stable passivation film, and degradation of the lithium−metal anode is more severe in Li−S batteries. Excess lithium is usually required in Li−S batteries, decreasing their practical energy density. Consequently, the ultimate success of Li−S batteries depends on both improvements on polysulfide dissolution control and lithium−metal anode protection. Many efforts have been made to modify the passivation film and enhance the stability of the lithium−metal anode, including changing the electrolyte solvents, introducing additives, and designing novel electrode configurations.23−25 However, robust passivation films and a stable lithium−metal surface have yet to be realized by a facile method. Polysulfide dissolution is thermodynamically favorable in organic electrolytes.26 In addition, leaching of polysulfides from the cathode and polysulfide shuttling become more severe with higher sulfur content in Li−S batteries and prolonged cycling, directly impairing the stability of the lithium−metal anode and the safety of Li−S batteries. To suppress polysulfide shuttling and lithium degradation, LiNO3 was identified as an electrolyte additive in Li−S batteries because of its direct reduction by lithium to LixNOy species and its oxidation of sulfur species to LixSOy moieties, alleviating parasitic reactions between lithium

ithium−sulfur (Li−S) batteries, with a high theoretical energy density of ∼2500 W h kg−1, are considered as a promising alternative to conventional lithium ion batteries as next-generation secondary batteries.1−8 However, there are challenges that hinder the practical application of Li−S batteries, principally their poor cycling stability. The poor cycling stability is mainly attributed to the insulating nature of sulfur and its reduction products (i.e., Li2S/Li2S2), shuttling of the intermediate lithium polysulfides, and degradation of the lithium−metal anode. In order to address these problems for a better utility of Li−S batteries, tremendous efforts have been made to understand the physical and chemical properties of this battery system.9−21 For example, in operando characterization has been used to understand how the sulfur fraction and sulfide precipitation impact the utilization of sulfur.18 Furthermore, theoretical calculations were coupled with experiments to differentiate polysulfide species for a more comprehensive understanding of the sulfur redox reaction and polysulfide shuttling.21 Nevertheless, the problem of the lithium−metal anode degradation remains unsolved. Although metallic lithium has a high theoretical capacity of ∼3860 mA h g−1 as an anode, it suffers from dendrite formation and poor cycling efficiency with complicated surface chemistry, which were believed to result from the instability of the passivation film on lithium− metal surfaces.22 The unstable passivation film fails to accommodate the cycling-induced shape and volume changes of the lithium−metal electrode and causes nonuniform lithium deposition/dissolution and dendrite formation. Moreover, the passivation film breaks with cycling and results in an exposure of fresh lithium surface to the electrolyte and parasitic reactions to form a new passivation film, decreasing the lithium cycling © 2014 American Chemical Society

Received: June 30, 2014 Accepted: July 10, 2014 Published: July 10, 2014 2522

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Figure 1. (a) Schematic showing the structure of a CNF paper/dissolved polysulfide cell. (b) Discharge/charge voltage profiles of the CNF paper/ dissolved polysulfide cell with copper acetate in the electrolyte at a C/5 rate (1 C = 1672 mA g−1). (c) Cycling performance at the C/5 rate of the control cell without copper acetate and the experimental cell with copper acetate in the polysulfide-rich environment.

metal and polysulfide species.24 However, LiNO3 is consumed progressively with the breakdown of the formed passivation film, development of new dendrites, and the growth of a new passivation film during battery operation,27 limiting its ability to stabilize the lithium−metal surface for long-term cycling or in a polysulfide-rich environment. In this context, further investigation of the physical and chemical properties (e.g., electrochemical, morphological, structural, and chemical variations) of the lithium−metal anode in the Li−S battery system is critical. Because polysulfide dissolution is thermodynamically favorable, a reliable lithium−metal anode should be stable under a certain degree of attack by polysulfide species; otherwise, leaking of polysulfides would easily induce anode instability. We show here that copper acetate can act as a surface stabilizer for lithium metal in a polysulfide-rich environment of Li−S batteries. Carbon nanofiber (CNF) paper/dissolved polysulfide cells (Figure 1a) were used to explore this strategy as the open macrosize interspaces of CNF can readily allow the corrosive polysulfides to be in direct contact with the lithium−metal anode and help assess the effectiveness of the additive in protecting lithium metal when there is limited inhibition of polysulfide diffusion; accordingly, the sulfur content in the present study is increased further to ∼50 wt % and 2 mg cm−2 compared to that in our previous work, translating to 3 M sulfur in the polysulfide catholyte.28 Lithium trifluoromethanesulfonate (LiCF3SO3, 1 M) in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1 ratio, by volume) was used as the electrolyte. In a full cell, the polysulfide catholyte consisting of an active material of Li2S6 was added into a CNF paper electrode and coupled with the metallic lithium anode. In order to suppress polysulfide shuttling before a stable passivation film was formed, LiNO3 was added to the electrolyte that was introduced into the anode side for both the control cell and the experimental cell. Copper acetate (0.03 M) was then added only to the experimental cell, and the resulting discharge/ charge voltage profiles of the experimental cell are shown in Figure 1b. In contrast to that of the control cell without copper

acetate (Figure S1, Supporting Information), the lower-voltage plateau (∼1.9 V) of the experimental cell is transformed into a sloping region due to the formation of a passivation film in the first few cycles by parasitic reactions among copper acetate, lithium metal, and polysulfide species. Resistance to lithium ion conduction through the passivation film decreases with cycling, which is typical in the conditioning process.29 The conditioning process was completed after ∼25 cycles, as indicated by the recovery of the upper plateau. After 100 cycles, the classical discharge behavior of Li−S batteries was observed, in which the upper plateau at ∼2.25 V together with the following sloping region corresponds to the reduction of S/ Li2S8 to Li2S4, and the lower-voltage plateau at ∼2.0 V is attributed to the reduction of Li2S4 to Li2S2/Li2S.2 The corresponding oxidation plateaus at ∼2.35 and ∼2.5 V were observed during charge. Figure 1c presents the cycling performance of the CNF paper/dissolved polysulfide cells. The discharge capacity of the experimental cell is lower than that of the control cell in the first few cycles, possibly due to the chemical formation of the lithium surface passivation film in the polysulfide-rich electrolyte environment. The experimental cell containing copper acetate outperformed the control cell without copper acetate in cycling stability. The control cell without copper acetate experienced a sudden capacity and a Coulombic efficiency drop when approaching 100 cycles, possibly due to lithium anode degradation.22 In contrast, the experimental cell was stable in the highly corrosive polysulfide-rich environment, with a capacity retention of 75% after 300 cycles and a Coulombic efficiency of ∼100% for most of the cycles. In addition, as shown by the electrochemical impedance spectroscopy (EIS) results in Figure S2 (Supporting Information), while the control cell indicates an increase in electrolyte resistance and an emerged linear segment in the low-frequency region related to Li2S evolution and polysulfide migration after 100 cycles,30 the experimental cell maintains the conserved shape of the impedance spectrum with a lower resistance compared to that of the control cell, confirming the enhanced cycling 2523

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stability. The lower resistance of the experimental cell with cycling further ensures its better performance at C/2 and 1 C rates (Figure S3, Supporting Information). It is worth mentioning that the lithium protection strategy reported herein is also applicable for lower-cost Li−S cells consisting of commercial S/C cathode materials with an Al foil current collector, as seen in Figure S4 (Supporting Information). The results demonstrate the advantages of the lithium protection strategy in effectively enabling the use of low-cost cathodes without requiring intricate nanoengineering. In addition, sulfur utilization can be significantly improved by innovations in the cathode configurations. For example, while conserving the same sulfur content, we are able to assemble experimental cells with a reversible discharge capacity of ∼1300 mA h g−1 by simply switching to a cathode with a sandwich configuration (Figure S5, Supporting Information). The combination of an advanced cathode structure and a protected lithium−metal anode demonstrates the prospects of prototypes of Li−S batteries. In order to investigate the variation of the lithium surface morphology in the polysulfide-rich environment of Li−S batteries, scanning electron microscopy (SEM) characterization was conducted, and the results are shown in Figure 2. The

alone was not able to conserve the lithium morphology in the polysulfide-rich environment. In contrast, the experimental cell with copper acetate (Figure 2b) had a smoother lithium surface covered by a sulfur- and copper-containing passivation film. After the 100th charge, the lithium surface in the control cell (Figure 2c) exhibited dendritic morphology and buildup of sulfur-containing byproducts (see the EDS sulfur mapping in Figure S6, Supporting Information) in contrast to the experimental cell, where lithium deposition was uniform (Figure 2d) because of the reduced surface roughness of the lithium metal in the first cycle. The sulfur- and coppercontaining clusters could fill in the irregular pits left from lithium extraction, reducing the lithium−surface roughness (see the morphology of the lithium−metal surface after the first discharge in Figure S7, Supporting Information) and protecting the lithium surface from the bulk Li2S/Li2S2 deposition; therefore, more uniform lithium deposition and dissolution can be expected.22 Furthermore, the EDS line scans on the cross sections of lithium metal after the 100th charge in the control cell and in the experimental cell are shown, respectively, in Figure 2e and f, with the corresponding SEM images shown in Figure S8 (Supporting Information). The thickness of the passivation film is ∼20 μm in the experimental cell, as seen in Figure S8 (Supporting Information). As shown in Figure 2e and f, the sulfur signals are pierced deeply into the bulk of lithium metal in the control cell, while the sulfur signals are concentrated in a layer (∼20 μm) coated directly at the lithium surface in the experimental cell, indicating that the passivation film in the experimental cell is effective in preventing the migrating polysulfides from penetrating. The effective prevention of polysulfide penetration, which reduces contaminants in the lithium electrode, may help inhibit the formation of dendritic structures from the interior of lithium metal.31 Time-of-flight secondary ion mass spectrometer (TOFSIMS) was used to study lithium deposition through the sulfur- and copper-containing passivation film in the experimental cell. Figure 3a and b presents the TOF-SIMS chemical mapping images, showing, respectively, the overlay of sulfur and copper and the overlay of sulfur and lithium signals at the surface of the passivation film after the first charge. Lithium is identified to have good overlapping with copper and to be rich in the interspaces of the sulfur-containing clusters. Additionally, we speculate that the presence of copper on the lithium surface may suppress the formation of bulk Li2S particles due to the interaction between copper and sulfur species, interrupting the formation of crystalline Li2S with long-range order. The absence of crystalline Li2S on the lithium surface in the experimental cell is further confirmed by X-ray diffraction (XRD) patterns of the cycled lithium anode after the first discharge (Figure 3c) and after the first charge (Figure 3d) without obvious Li2S signals. In Figure 3c, two identical peaks at 2θ = 36.0 and 52.1° can be ascribed to the (011) and (002) peaks of crystalline lithium.32 The relative intensities of the (011) and (002) peaks change in the XRD pattern of the deposited lithium (Figure 3d), suggesting lithium deposition with a preferential orientation in contrast to the bulk lithium metal due to the presence of the passivation film.33 The varied surface condition of the passivation film could result in lithium deposition as particles instead of dendrites or whiskers (Figure 2b and d).34 It is evident from the results of TOF-SIMS and XRD characterizations that lithium deposition is influenced by the passivation film and that lithium ions deposit in the interspaces of the sulfur-containing clusters, where they are

Figure 2. SEM characterizations of the lithium−metal surface after the first charge in the (a) control cell and (b) experimental cell and after the 100th charge in the (c) control cell and (d) experimental cell. Cross-sectional EDS line scans (for sulfur) of the lithium metal after the 100th charge in the (e) control cell and (f) experimental cell. The scale bars in a−d represent 100 μm. The scales in e and f represent the position in lithium metal. The cells were cycled at C/5.

corresponding energy-dispersive X-ray spectroscopy (EDS) results are given in Figure S6 (Supporting Information). After the first charge, the lithium surface of the control cell (Figure 2a) was characterized by bulk Li2S/Li2S2 precipitates (see the EDS sulfur mapping in Figure S6, Supporting Information) and nonuniformly deposited mossy lithium, confirming that LiNO3 2524

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Figure 3. TOF-SIMS chemical mapping images showing the overlay of sulfur and copper ((a) the sulfur signal is in red, and the copper signal is in blue) and the overlay of sulfur and lithium ((b) the sulfur signal is in red, and the lithium signal is in blue) at the surface of the passivation film after the first charge in the experimental cell. XRD patterns of the lithium−metal anode after the first discharge (c) and after the first charge (d). (e) A simplified schematic showing the passivation film and lithium deposition. The cells were cycled at C/5.

closer to copper species and the bulk lithium metal, reducing the surface roughness. The corresponding simplified schematic is shown in Figure 3e. Detailed mechanistic investigation on the lithium deposition strategy will be conducted in the future. X-ray photoelectron spectroscopy (XPS) analysis was conducted to examine the effect of copper acetate on the lithium surface chemistry. High-resolution S 2p XPS spectra of the lithium surface after the first and 100th charge are shown in Figure 4 with the corresponding compositions shown in Tables S1 and S2 (Supporting Information). Lithium surface in the experimental cell had much less polysulfide adhesion after the first charge compared to that in the control cell, as indicated by the disappearance of the S 2p3/2 peak centered at 164.1 eV (Figure 4a).24 The passivation film on the lithium surface in the experimental cell is comprised of mainly Li2S, Li2S2/CuS/Cu2S species, lithium salts, and electrolyte decomposition products (denoted as LixSOy), with the characteristic S 2p3/2 peaks identified, respectively, at 159.8,24 162.1,35 169.1, and ∼167.0 eV.36 In contrast to that in the control cell, the concentration of electrolyte decomposition products, for example, sulfone or sulfite species,36 did not increase after 100 cycles in the cell containing copper acetate, indicating that fewer parasitic reactions have occurred on the lithium surface in the experimental cell (Figure 4b). The electronic conductivity of copper sulfide is ∼10−3 S cm−1, which is higher than that of bulk Li2S. Moreover, the concentration of copper acetate is only 0.03 M; therefore, copper sulfides will not dominate the electronic properties of lithium−metal surface.37,38 The electrolyte decomposition is more likely to be suppressed due to the smoother lithium surface and better mechanical strength of the sulfide matrix, leading to passivation films less likely to break and no continuous chemical reactions to form a new passivation film. To verify the chemical compositions in the cycled CNF electrode, UV−visible absorption spectroscopy analysis was performed on the cathode analyte after the 100th

Figure 4. High-resolution S 2p XPS spectra of the lithium surface after the first charge (a) and after the 100th charge (b). Each spectrum was fitted with peaks for differently bonded S species, and the sum of the fitting curves (black solid line) is consistent with the raw data (red dotted line). (c) UV−visible absorption spectra of the cathode analyte after the 100th charge. The cells were cycled at C/5.

charge (Figure 4c). Relative to the cathode analyte spectrum of the control cell that only exhibits a weak absorption peak at ∼310 nm attributed to S62− species, the cathode analyte spectrum of the experimental cell exhibits peaks of S62−, S8, and S42− species with higher intensities,39 confirming enhanced conservation of the active material in the experimental cell. It is worth mentioning that several factors are considered in selecting an appropriate additive salt, that is, the chemical 2525

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authors thank Dr. Yongzhu Fu, Dr. Wanli Yang, and Dr. Andrei Dolocan for their valuable suggestions. We also acknowledge the National Science Foundation Grant DMR-0923096 for the acquisition of the TOF-SIMS instrument as part of the Texas Materials Institute at the University of Texas at Austin.

properties of the transition-metal ions themselves, including expected resistivity and the predicted stability of the interface between the lithium−metal anode and the transition-metal ion. Furthermore, highly corrosive anions may impair the formation and diminish the stability of the passivation film. We also believe that the selected anion should control the release of the transition-metal ion in the specific solvent. Finally, the cost of the additive salts is critical to the practical utility of this technology. Examples showing cell performance with an additive salt consisting of varied cations (nickel acetate) and anions (copper nitrate) are demonstrated, respectively, in Figures S9 and S10 (Supporting Information) for a comparison. In summary, for the first time, we have demonstrated that the lithium surface in the corrosive polysulfide-rich environment of Li−S batteries can be stabilized by the addition of copper acetate, which is promising for effective lithium protection and a safe, long-term operation of high energy density Li−S batteries, especially with high sulfur content for practical use. The copper acetate additive improves the physical/chemical characteristics of the lithium−metal anode and the performance of Li−S cells. The improved surface morphology and chemistry could be attributed to the in situ chemical formation of a stable passivation film consisting of a good combination of a sulfide matrix and electrolyte decomposition products. The passivation film suppresses lithium dendrite development by controlling the lithium deposition sites. In addition, the robust passivation film with a sulfide matrix is less likely to break during battery operation, protecting the bulk lithium metal from parasitic reactions with the organic electrolyte and polysulfides. This is critical for maintaining the high energy density of Li−S batteries. Superior cycle stability and lower cell resistance upon cycling are obtained due to the stabilized lithium surface characterized by a dendrite-free morphology and less insulating passivation by large Li2S particles and electrolyte decomposition products/polysulfides. Considering the low cost and environmentally friendly nature of copper acetate, we believe that this lithium-protection strategy is benign for large-scale application of Li−S batteries. Furthermore, protection of lithium metal by the in situ surface chemical formation of a passivation film with a 3-D structure and a robust matrix could be extended for using a lithium−metal anode in lithium ion batteries or lithium−air batteries or for enabling the protection of other metallic anodes (e.g., Na, Mg, Al), significantly impacting the energy storage area for a sustainable and mobile society.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental methods, supplemental electrochemical performance, SEM images, and XPS results. 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]. Notes

The authors declare no competing financial interest.

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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. The 2526

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