Research Article www.acsami.org
Electrostatic Polysulfides Confinement to Inhibit Redox Shuttle Process in the Lithium Sulfur Batteries Min Ling,†,‡ Wenjun Yan,§,‡ Ayako Kawase,† Hui Zhao,† Yanbao Fu,† Vincent S. Battaglia,† and Gao Liu*,† †
Applied Energy Materials Group, Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Cationic polymer can capture polysulfide ions and inhibit polysulfide shuttle effect in lithium sulfur (Li−S) rechargeable batteries, enhancing the Li−S battery cycling performance. The cationic poly[bis(2-chloroethyl) ether-alt-1,3-bis[3(dimethylamino) propyl]urea] quaternized (PQ) with a high density quaternary ammonium cations can trap the lithium polysulfide through the electrostatic attraction between positively charged quaternary ammonium (R4N+) and negatively charged polysulfide (Sx2−). PQ binder based sulfur electrodes deliver much higher capacity and provide better stability than traditional polyvinylidene fluoride (PVDF) binder based electrodes in Li−S cells. A high sulfur loading of 7.5 mg/cm2 is achieved, which delivers a high initial areal capacity of 9.0 mAh/cm2 and stable cycling capacity at around 7.0 mAh/cm2 in the following cycles. KEYWORDS: cationic polymer binder, quaternary ammonium, electrostatic binding, lithium sulfur battery, high areal capacity
T
polyamidoamine (PAMAM) dendrimers,20 polyacrylonitrile (PAN),22 polyfluorene derived polymers,23 and gum arabic.16 The polar amino groups were evaluated and reported to effectively reduce the polysulfide dissolution.24 Polyquaternium (PQ) is one of the typical cationic polymers that are commercially available and widely used in the personal care products. The most significant feature of PQ is the presence of quaternary ammonium cations in the polymer. Because quaternary ammoniums are positively charged, they attract and immobilize the negative charges, such as Sx2−, through electrostatic force. Since the PQ polymer is fixed with carbon back in the electrode, the attracted polysulfides (Sx2−) are immobilized and confined with the polymer. The confinement mechanism is illustrated in Figure 1. The micron sulfur particles will form soluble polysulfide during initial lithiation process. The negative charged polysufide will be electrostatically
he increasing demands of grid-scale electric energy storage and electric vehicles calls for high energy density and low cost electrical energy storage solution. Lithium sulfur (Li−S) rechargeable batteries, which have a theoretical energy density of 2500 Wh/kg, could potentially satisfy the high energy density requirement. The main component, sulfur, is also abundant and can be extracted at very low cost.1,2 Despite these advantages, the commercialization of sulfur batteries has been hindered by major technical hurdles. The main obstacle is the formation of soluble polysulfides during the cell operation. The polysulfides can dissolve in the electrolyte and migrate to the anode called shuttle effect.3,4 Serious reversible and irreversible side reactions occur between the shuttling polysulfide and lithium metal, which lead to low Coulombic efficiency and fast capacity fade.5 To address the polysulfide shuttle effect, the efforts have focused on carbon matrix confinement,6−9 surfacefunctionalized carbon,10 metal oxide/sulfide materials11−14 and lately, functional polymer binder optimization.15−19 The functional groups in binders were investigated to immobilize the soluble lithium polysulfide through C−S, N− Li, or O−Li bonds,20 that is, polyvinylpyrrolidone (PVP),21 © 2017 American Chemical Society
Received: May 9, 2017 Accepted: August 15, 2017 Published: August 15, 2017 31741
DOI: 10.1021/acsami.7b06485 ACS Appl. Mater. Interfaces 2017, 9, 31741−31745
Research Article
ACS Applied Materials & Interfaces
Figure 1. Polysulfides confinement through cationic polymer. The electrostatic attraction between the PQ quaternary ammonium cations and polysulfide anions.
absorbed by the quaternary ammonium cations on the PQ polymer binder. The electrostatic absorption alleviates the polysulfide dissolution into electrolyte and reduces the shuttle constant. The electrostatic interaction with different ion species is simulated with density functional theory (DFT) prior the electrochemical test. Li2S6 is used as a predominant polysulfide species in the simulation because of its high solubility in the electrolyte.25 All the anions, including bis-trifluoromethanesulfonimide anion (TFSI), in the system are quantified through the simulation. The adsorption energy for 1,3-dioxolane (DOL)-Li2S6 and 1,2dimethoxyethane (DME)-Li 2 S 6 are 0.79 and 0.77 eV, respectively. The binding energy of PQ-TFSI anchoring system is 0.94 eV, which is higher than that of DOL-Li2S6 and DMELi2S6. For PQ-Li2S6, the binding energy is 1.89 eV, which is the highest among the interactions calculated in this study. Apparently, the adsorption strength between binders and lithium polysulfides prevails to the dissolution into the electrolyte solvents. The adsorption energy of PQ-Li2S6 is the highest among all other absorption-based interactions calculated in the system, but lower than the covalent bonding energy. The absorption binding energy between the polysulfides and various binders is depended on the functional groups in the polymer backbone. The values of absorption binding energy for polysulfide and various functional groups of ester, ketone, amide, imine, ether, nitrile, fluoroalkane, chloroalkane, bromoalkane, alkane is 1.10, 0.96, 0.95, 0.88, 0.71, 0.60, 0.40, 0.26, 0.23, and 0.23 eV, respectively. This theoretical calculation result of binding energy of ammonium cations with polysulfide is much higher than those of the literature reported absorption values based on other functional groups.15 Besides the theoretical calculation, the binding capability is evaluated through in situ Ultraviolet−visible spectroscopy (UV−vis).26 The characterization is focused on PQ with quaternary ammonium groups, which is different from the chemically inert binders (i.e., PVDF). UV−vis is conducted to confirm the electrostatic confinement as shown in Figure 2. The 0.1 g of PQ polymer is soaked in 1 mL of electrolyte sample solution consisting of 1 M Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) in DOL/DME (1/1 vol) with existence of 3 mM lithium polysulfide (average formula Li2S6) for 22 h to track the polysulfide concentration evolution. In the in situ UV−vis spectra, a significant absorption peak is observed at 427 nm. This absorption feature is attributed to polysulfide adsorption.27 A significant decrease of the absorbance at 427 nm is recorded for the solution with PQ polymer as shown in Figure 2b. The decrease of absorption is due to the removal of polysulfide in the solution, caused by the electrostatic attraction
Figure 2. In situ UV−vis full spectra for (a) PVDF and (b) PQ in 3 mmol/L lithium polysulfide in DOL/DME. (c) Absorption peak intensity change at 427 nm. (d) Calculated reaction kinetics based on the UV−vis spectra.
of polysulfide by PQ into solid polymer phase. In contrast, the absorption signals in Figure 2a for polysulfide solution with PVDF stay at the original level during the exposure time, indicating no adsorption of polysulfide to PVDF. To further illustrate the adsorption intensity evolution, the intensity of the absorbance peak are plotted versus time as shown in Figure 2c, which is further differentiated and plotted in Figure 2d. PQ shows significant polysulfide capturing ability as shown in Figure 2c and d in the current experimental condition. The actual rate of polysulfide capturing can be further modified through electrode and polymer morphology manipulation. On account of the calculations and UV−vis observation, improved electrochemical performances are expected due to the electrostatic attraction. The electrochemical performances are all based on micron size sulfur particles and in high sulfur loading electrodes conditions. The electrodes are composed of 60 wt % micron size sulfur particles. The sulfur particle size varies from several microns to tens of microns as shown in Figure S1. The thermostability of PQ is compared with that of the conventional PVDF binder as shown in Figure S2. From the isothermal test at 80 °C, the water content in PQ binder is around 3 wt %. The PQ polymer starts to decompose at around 150 °C during the temperature scan. The thermal stability window of the PQ binder is well beyond the battery operational window. Galvanostatic lithiation/delithiation studies are performed at a constant current rate of 0.05C (based on sulfur 31742
DOI: 10.1021/acsami.7b06485 ACS Appl. Mater. Interfaces 2017, 9, 31741−31745
Research Article
ACS Applied Materials & Interfaces
voltage increase for PQ-based sulfur electrode. Hence, a low shuttle constant is induced because of the low delithiation current, that is, 0.05C. The PVDF based sulfur electrodes show only the higher voltage in the charge plateau, which indicate only long-chain polysulfides are involved in the charge process. For the lithiation profile, the experimental high plateau lithiation capacity QHL is not always equal to the accumulated capacity. When the lithiation process starts, two parallel paths exist for high polysulfide reduction: the first, simple electrochemical reduction due to the lithiation current, and the second, reduction of polysulfides on the Li anode that depends on the shuttle constant.
mass in the electrode). As shown in Figure 3a, a high initial areal capacity of 9.0 mAh/cm2 is reached for PQ-based
Q LH = Q Hacc
electrode, corresponding to a specific capacity of 1201.2 mAh/ g. The sulfur utilization for the first cycle is 70.0% at the sulfur loading of 7.5 mg/cm2. The capacity is stabilized at 885.1 mAh/ g after 50 cycles. The higher rate performance (C/3) at this loading (7.5 mg/cm2) is plotted in Figure S3. The first two cycles are cycled at C/50. The initial specific and areal capacity is 1167.9 mAh/g and 8.8 mAh/cm2 respectively. Good cycling stability is achieved for the thick electrodes at C/3, with 40% capacity retention after more than 100 cycles. For comparison, the PVDF-based high loading sulfur (7.5 mg/cm2) electrode performance is also plotted. Because of the high loading and usage of micron size sulfur particles, the sulfur utilization is only 13% at the first cycle, and drops down to 5% at the 30th cycle. The PVDF-binder-based sulfur electrode gives very poor cycling performance compared to the PQ-based binder for the high-loading electrode. From Figure 3c, it is observed that PQ-based electrodes exhibited well-defined lithiation/delithiation plateaus corresponding to the formation of soluble long chain polysulfides and their further dissociation into short chain polysulfides. In contrast to PQ-based electrode, PVDF based electrode exhibited indistinguishable plateaus, which indicates disordered lithiation/delithiation process. To further analysis of the voltage profile, the delithiation-shuttle factor is described in the equation4 Id
fL
Analysis of the above equation shows that the high plateau lithiation capacity can reach its maximal value, which is equal to the lithiation accumulated capacity, only when the lithiationshuttle factor is close to zero. Experimentally, such conditions can be reached at a high enough lithiation current or a low shuttle constant. In this study, the PQ-based sulfur electrodes are lithiated at a very low current (0.05C) as shown in Figure 3c. Hence, a very well controlled shuttle constant is achieved for the PQ-based sulfur cell, which is in stark contrast with the lithiation profile of PVDF based cell in Figure 3d. The absence of the lower plateau of lithiaiton/delithiation profile for PVDFbased electrode is due to the failure to immobilize high order lithium polysulfides on the sulfur electrode. Accordingly, the further reduction to lower order polysulfides could not be realized. It is also worth noticing that the voltage spike in Figure 3c is caused by the growth of lithium dendrites on the surface of lithium foil due to the large amount of lithium being cycled.28 The Coulombic efficiency for both polymers based electrodes are shown in Figure S4. In the first three cycles of PVDF based electrode, the lithiation causes the dissolution of long chain polysulfide. Upon charging the dissolved polysulfides delithiate in the subsequent delithiaion process, leading to erratic Coulombic efficiency higher than 100%. In contrast, the initial Coulombic efficiency is 97.5% for the PQ based electrode, and 98% efficiency after 30 cycles. The consistent Coulombic efficiency of the PQ based sulfur electrode also support the confinement of long-chain polysulfide by the cationoic polymer.29,30 To investigate the morphology evolution after lithiation/ delithiation, scanning electron microscopy (SEM) of sulfur electrodes based on PQ at different state are shown in Figure 4. Bulk micron sulfur particles can be found with carbon black covering on the surface. In fully lithiated state, the porous surface is covered by flake-like lithiated product. Meanwhile, the bulk sulfur particles is converted into sulfide species after lithiation. Fully delithiated cathode shows the similar porous surface structure as fresh prepared cathodes, which indicate the successful oxidization of the lithiated product. The good reversibility is in accordance with the electrochemical performance of the cathode. The energy dispersive spectra (EDS) mapping of lithiated and delithiated PQ based sulfur cathode are collected in Figure 5. Overall, sulfur species are distributed homogeneously after lithiation/delithiation instead of the bulk micron sulfur particles in the fresh prepared electrode. The micron size sulfur particles are reduced and immobilized on the sulfur electrode side after cycling.31 The bulk sulfur particles can also be detected in the new PVDF polymer based
Figure 3. (a, b) Cycling behavior of Li−S batteries of PQ polymerbased in contrast to conventional PVDF-based sulfur electrode. The electrostatic effect of PQ polymer efficient confinement of the shuttle process. (c, d) Lithiation/delithiation voltage versus areal capacity profiles for PQ- and PVDF-based sulfur electrode, respectively.
ksqH[Stotal]
ln(1 + fL )
= fd
where ks is the shuttle constant, qH is the high plateau sulfur specific capacity, [Stotal] is the total sulfur concentration, Id is the delithiation current, fd is the delithiation-shuttle factor. At ksqH[Stotal]/Id < 1, when the shuttle constant is low or the charge current is high enough, the cell can be fully delithiated, showing a sharp voltage increase. Rather, at ksqH[Stotal]/Id > 1, the cell never reaches complete delithiation and shows a voltage leveling. The larger the value of fd, the lower the leveled voltage. The voltage profile in Figure 3c indicates an obvious sharp 31743
DOI: 10.1021/acsami.7b06485 ACS Appl. Mater. Interfaces 2017, 9, 31741−31745
ACS Applied Materials & Interfaces
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Research Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06485. Full experimental data, thermogravimetric profile, SEM images, high rate cycling performance, and Coulombic efficiency (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Min Ling: 0000-0001-6727-9585 Gao Liu: 0000-0001-6886-0507 Author Contributions ‡
M.L. and W.Y. contributed equally to the article. L.M. and G.L. conceived the idea and designed experiments. L.M., W.Y., A.K., H.Z., and Y.F. performed the experiments. V.S.B. provided guidance on cell testing. All authors provide input to the manuscript. L.M. and G.L. wrote the manuscript.
Figure 4. SEM images of PQ polymer-based sulfur electrodes. (a, b, c) fresh, (d, e, f) lithiated, and (g, h, i) delithiated images at different magnifications.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is funded by the Assistant Secretary for Energy Efficiency, Vehicle Technologies Office of the U.S. Department of Energy, under the Advanced Battery Materials Research (BMR) Program. UV−vis experiments are conducted at the Molecular Foundry of U.S. DOE user facility and are supported by the Director, Office of Science, Office of Basic Energy Sciences of the US Department of Energy under contract no. DE-AC02-05CH11231.
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