Enhanced Cyclability of Lithium–Sulfur Batteries by a Polymer Acid

Article pubs.acs.org/cm

Enhanced Cyclability of Lithium−Sulfur Batteries by a Polymer AcidDoped Polypyrrole Mixed Ionic−Electronic Conductor Yongzhu Fu and Arumugam Manthiram* Electrochemical Energy Laboratory & Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: A mixed ionic−electronic conductor (MIEC) of polypyrrole (PPy) synthesized with poly(2-acrylamido-2methyl-1-propanesulfonic acid) (PAAMPSA), which is waterdispersible and is in the form of nanoparticles intertwined by the PAAMPSA, is explored as an additive in sulfur cathodes for rechargeable lithium−sulfur (Li−S) batteries. A S-MIEC composite containing a sulfur content of 75 wt % was synthesized by an in situ deposition of sulfur with MIEC. The sulfur retains an orthorhombic phase randomly mixed with MIEC nanoparticles, exhibiting a lower thermal decomposition temperature than the pristine sulfur. Cathodes containing the S-MIEC composite were prepared and evaluated in half cells by cyclic voltammetry and galvanostatic cycling. The S-MIEC composite cathode shows excellent electrochemical stability as the MIEC facilitates ion and electron transfer and capture intermediate polysulfides within the electrodes. The MIEC in the composite electrodes forms a porous, 3D heterostructure providing good electrochemical contact upon cycling as indicated by scanning electron microscopy and electrochemical impedance spectroscopy. The sulfur in the S-MIEC composite retains a capacity of >600 mA h g−1 at low rates and 500 mA h g−1 at 1C after 50 cycles. KEYWORDS: rechargeable lithium−sulfur battery, mixed ionic-electronic conductor, polymer acid-doped polypyrrole, electrochemical performance, cyclability

1. INTRODUCTION The development of electrochemical energy storage systems with high energy density for hybrid electric vehicles and electric vehicles has drawn much interest in low-cost rechargeable lithium−sulfur (Li−S) batteries. Sulfur, one of the most abundant materials in nature, can store up to two electrons per sulfur atom with a high theoretical capacity of 1675 mA h g−1, making it a potential high capacity cathode for lithium batteries.1 An order of magnitude higher capacity than that of the conventional insertion compound cathodes can enable packaged cells with an energy density of 400−600 Wh kg−1, which is two or three times higher than that of current Li-ion batteries. The low operating voltage of ∼2.1 V in contrast to 3.5 − 4.0 V of the transition-metal oxide cathode materials can offer better safety, which is desirable for transportation applications. The low-cost, high energy density Li−S batteries are also a promising candidate for grid energy storage for renewable energies such as solar and wind.2 However, a few challenges remain to be overcome to transform the Li−S battery technology to reality. Sulfur undergoes a series of structural and morphological changes during the charge−discharge process involving the formation of lithium polysulfides Li2Sx (x = 8, 6, 5, and 4) and sulfides Li2S2/ Li2S.3 In addition, the high resistance of sulfur and its intermediate compounds make it difficult to maintain stable electrochemical contact between the conductive additives and active materials within the sulfur electrodes, resulting in poor © 2012 American Chemical Society

utilization of active material and low rate capability ( x ≥ 4) and low-order polysulfides to Li2S2/Li2S, respectively. There is only one broad oxidation peak instead of two, showing the continuous conversion of Li2S2/Li2S to high-order polysulfides. For the pristine sulfur, the shift of the diminishing peaks with cycling clearly shows the electrochemical instability of the active materials within the sulfur electrode. In contrast, the SMIEC composite exhibits narrower peaks with slight shift with cycling, indicating its excellent electrochemical stability. Moreover, the two cathodic peaks increase with cycling, indicating its improved electrochemical contact upon cycling within the electrode and increased utilization of active materials. The Coulombic efficiency of the cell with the S-MIEC composite electrode, which is defined as the percentage of the area under the two reduction peaks to that under the oxidation peak, is 78.6% compared to 70.5% of the cell with the pristine sulfur electrode during the third cycle. The higher efficiency of the SMIEC composite cell indicates that the MIEC nanoparticles may help retain the active material (i.e., trap polysulfides) within the composite electrode. Figure 6 shows the representative charge and discharge voltage profiles and Table 2 shows the discharge capacities and Coulombic efficiencies of the 1st, 2nd, 5th, and 10th cycles of the pristine sulfur and S-MIEC composite at C/10 rate. The pristine sulfur (Figure 6a) shows a first discharge capacity of 747 mA h g−1 and low discharge capacities of 600−630 mA h g−1 during the subsequent cycles. The high charge capacities result in very low Coulombic efficiencies of 60−75% during the first ten cycles. In addition, the overpotential increases with cycling. These results indicate the loss of active materials and increasing electrochemical resistance within the sulfur electrode during the cycling process. In contrast, the S-MIEC composite (Figure 6b) shows a first discharge capacity of 968 mAh g−1 and discharge capacities of >710 mA h g−1 during the subsequent cycles, as shown in Table 2, and much lower charge capacities than the pristine sulfur. The Coulombic efficiencies start at 83.5%, followed by a slight drop during the second cycle and then an increase to 80.9% at the 10th cycle. The high discharge capacities and high Coulombic efficiencies prove the MIEC can improve the utilization of active materials within the sulfur electrodes. Moreover, the overpotential remains constant with cycling, which further confirms that the MIEC can maintain an electrochemically stable S-MIEC electrode. Extended cycling of the S-MIEC composite at various rates were also evaluated, as shown in Figure 7. Figure 7a shows the discharge capacities with cycling. A capacity drop during the first cycle was observed at all the rates evaluated, which is due to the large sulfur particles, resulting in poor electrochemical contact within the fresh electrodes. Afterward, very stable cycle life was obtained for the rates of C/10, C/5, and C/2. At 1C rate, a recovering process was observed, showing low discharge capacities during the first four cycles followed by steadily increasing capacities. Figure 7b presents the Coulombic efficiency of the S-MIEC composite at various rates. At C/10, C/5, and C/2 rates, the Coulombic efficiencies steadily increase after an initial decrease during the first couple of cycles. At 1C rate, the Coulombic efficiency follows an increasing-decreasing curve at the beginning and then a steadily increasing curve in

Figure 6. Voltage vs specific capacity profiles of the (a) pristine sulfur and (b) S-MIEC composite at 2.8−1.5 V and C/10 rate; the capacity values are in terms of the percentage of the sulfur active mass.

the following cycles. As the rate increases, the Coulombic efficiency increases. Coulombic efficiencies of 88 and 94% after 50 cycles were achieved at C/10 and 1C rates, respectively. The high discharge capacities, excellent cyclability, and steady increase in the Coulombic efficiency indicate that the MIEC nanoparticles within the S-MIEC composite facilitate electron and ion transport, retain active materials, and improve the electrode structure upon cycling. Figure 8a uncovers the structure/morphology of the crosssection of the cycled S-MIEC composite electrode. The inner space was filled with porous, 3D heterostructured, and well connected nanoparticles (i.e., MIEC). Such structure containing conductive PPy and ion-conducting PAAMPSA is ideal for capturing the active material upon cycling leading to excellent cyclability as shown in Figure 7a. To gain further understanding, EIS was employed to characterize the cell before and after cycling at C/10 rate, as shown in Figure 8b. The intercepts in the high-frequency regions are attributed to the bulk resistance of the liquid electrolyte. The cycled cell shows slightly increased bulk resistance compared to that before cycling due to the dissolved polysulfides increasing the viscosity of the electrolyte, which can retard lithium-ion transport.6j,17a There is only one semicircle in the high-medium frequency region before cycling, which is understood to be due to the sum of the resistances of the surface layers (i.e., electrode/electrolyte 3085

dx.doi.org/10.1021/cm301661y | Chem. Mater. 2012, 24, 3081−3087

Chemistry of Materials

Article

Table 2. Comparison of the Galvanostatic Cycling Performance of the Pristine Sulfur and S-MIEC Composite discharge capacity (mA h g−1)

Coulombic efficiency (%)

sample

1st

2nd

5th

10th

1st

2nd

5th

10th

sulfur S-MIEC

747.0 968.1

628.8 783.1

623.0 735.4

600.0 719.1

60.6 83.5

57.1 75.9

72.6 79.6

75.2 80.9

Figure 7. (a) Cyclability and (b) Coulombic efficiency of the S-MIEC composite at various C rates; the capacity values are in terms of the percentage of the sulfur active mass.

interfacial layers) on lithium and S-MIEC composite electrodes.17a The linear segment in the low-frequency region corresponds to the diffusion limitation within the electrodes.17b After the extended cycling, the resistance of the surface layers is reduced tremendously and another minor semicircle appears in the medium frequency region, which is assigned to charge transfer resistance.6j The reduction of the interfacial resistance is partially due to the loss of active materials upon cycling, leading to a high surface area of the S-MIEC composite electrode. The major contribution is attributed to the bifunctional MIEC nanoparticles, which electrochemically improve the contact between the electrolyte and the electrode. The ultralow charge transfer resistance indicates that the MIEC nanoparticles facilitate charge and ion transport within the electrodes at the same time. This result is consistent with the improving Coulombic efficiencies upon cycling as shown in Figure 7b. The favorable properties and performances MIEC exhibits prove that it is a promising additive material in sulfur cathodes for lithium−sulfur batteries.

Figure 8. (a) SEM image of the cross-section of the S-MIEC electrode after 50 cycles and (b) EIS analysis of the cell before and after 50 cycles, with the inset showing a magnified plot.

4. CONCLUSIONS A composite cathode material containing sulfur and an MIEC polymer has been prepared. The sulfur within the composite preserves the orthorhombic phase, but exhibits a lower decomposition temperature than pristine sulfur. The MIEC is in the form of nanoparticles intertwined by the PAAMPSA, which significantly improves the electrochemical performance of the S-MIEC composite electrode by reducing the overpotential and electrochemical impedance, improving cyclability and maintaining a robust but porous electrode structure. The SMIEC composite electrode shows high discharge capacities at various C rates and excellent cyclability. The study demon3086

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Chemistry of Materials

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(16) Beach, M. W.; Rondan, N. G.; Froese, R. D.; Gerhart, B. B.; Green, J. G.; Stobby, B. G.; Shmakov, A. G.; Shvartsberg, V. M.; Korobeinichev, O. P. Polym. Degrad. Stab. 2008, 93, 1664−1673. (17) (a) Kolosnitsyn, V. S.; Kuz’mina, E. V.; Karaseva, E. V.; Mochalov, S. E. Russ. J. Electrochem. 2011, 47, 793−798. (b) Choi, Y.J.; Chung, Y.-D.; Baek, C.-Y.; Kim, K.-W.; Ahn, H.-J.; Ahn, J.-H. J. Power Sources 2008, 184, 548−552.

strates that such MIEC material is promising to be used in sulfur cathodes for high performance lithium−sulfur batteries.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +1-512-471-1791. Fax: +1-512-471-7681. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Seven One Limited. The authors thank Thomas Cochell for his assistance with the XPS measurements.

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REFERENCES

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