Poly(ethylene oxide) Polymer

Letter pubs.acs.org/JPCL

Lithium Bis(fluorosulfonyl)imide/Poly(ethylene oxide) Polymer Electrolyte for All Solid-State Li−S Cell Xabier Judez,† Heng Zhang,*,† Chunmei Li,*,† José A. González-Marcos,‡ Zhibin Zhou,§ Michel Armand,† and Lide M. Rodriguez-Martinez† CIC Energigune, Parque Tecnológico de Á lava, Albert Einstein 48, 01510 Miñano, Á lava, Spain Department of Chemical Engineering, Faculty of Science and Technology, University of the Basque Country UPV-EHU, P.P. Box 644, 48080 Bilbao, Spain § Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China † ‡

S Supporting Information *

ABSTRACT: Solid polymer electrolytes (SPEs) comprising lithium bis(fluorosulfonyl)imide (Li[N(SO2F)2], LiFSI) and poly(ethylene oxide) (PEO) have been studied as electrolyte material and binder for the Li−S polymer cell. The LiFSI-based Li−S all solid polymer cell can deliver high specific discharge capacity of 800 mAh gsulfur−1 (i.e., 320 mAh gcathode−1), high areal capacity of 0.5 mAh cm−2, and relatively good rate capability. The cycling performances of Li−S polymer cell with LiFSI are significantly improved compared with those with conventional LiTFSI (Li[N(SO2CF3)2]) salt in the polymer membrane due to the improved stability of the Li anode/electrolyte interphases formed in the LiFSI-based SPEs. These results suggest that the LiFSI-based SPEs are attractive electrolyte materials for solid-state Li−S batteries.

triflate (CF3SO3−),16 bis(trifluoromethanesulfonyl)imide ([N(SO2CF3)2]−, TFSI−),17,18 and so on. For a decade now, lithium bis(fluorosulfonyl)imide (Li[N(SO2F)2], LiFSI) has been intensively investigated as next-generation conducting salt for Li batteries due to its high ionic conductivity, good chemical stability, and capability of forming stable SEI films on various electrodes.19−24 Very recently, LiFSI was introduced as a conducting salt for SPEs by using PEO,20 poly(ethylene carbonate) (PEC), 25,26 and polymeric ionic liquids (PILs)22,27,28 as polymer matrices. It was demonstrated that LiFSI-based SPEs showed much better compatibility with Li metal electrode than the LiTFSI-based ones.20 Inspired by the advantageous properties of LiFSI/PEO electrolyte mentioned above, in this communication, we report the electrochemical performances of LiFSI/PEO as SPE in all solid-state Li−S cells. The molar ratio of [EO]/[Li+] of 20 was used because the concentration of lithium salt around this ratio of [EO] to [Li+] generally delivers relatively high ionic conductivities.19 The effects of the important parameters (e.g., the active material loading and the thickness of S cathode) on the cycling performances are presented. Figure 1a shows a self-standing and transparent LiFSI/PEO electrolyte membrane. The ion-transport behavior of the LiX/

S

ulfur has been recognized as one of the most promising cathode materials to improve the gravimetric energy density of state-of-art battery because of its high theoretical capacity (1675 mAh gsulfur−1), low cost, and less toxicity.1−3 However, the commercial success of Li−S batteries is limited by several serious problems, including insoluble and electronically insulating nature of sulfur at the cathodes, dendrite formation at Li metal anode, side reactions of soluble lithium polysulfide (shuttle effect), and so on. These drawbacks lead to irreversible loss of active materials in the cathode, low coulombic efficiency (CE), and fast capacity fading.4−8 Solid polymer electrolytes (SPEs) have been suggested as safe electrolytes for application in all solid-state rechargeable Li batteries. This is motivated by their advantages over conventional liquid electrolytes, including nonvolatility, no leakage, and suppression of Li dendrites.9−12 In addition, SPEs have a low density compared with inorganic electrolytes (e.g., garnets) and good processability, and thus they offer advantages versus other solid conductors and even liquid cells in terms of gravimetric energy density.13 Among various kinds of SPEs, high-molecular-weight poly(ethylene oxide) (PEO) is the most commonly used polymer matrix due to its good mechanical properties and the high solvation power of ethylene oxide (−CH2CH2O−, [EO]) unit.14 For lithium salts applied in all solid-state Li−S polymer cells, previous work was mainly focused on those with weakly coordinating anions, including tetrafluoroborate (BF4−),15 © 2017 American Chemical Society

Received: March 10, 2017 Accepted: April 13, 2017 Published: April 13, 2017 1956

DOI: 10.1021/acs.jpclett.7b00593 J. Phys. Chem. Lett. 2017, 8, 1956−1960

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Physical appearance of the as-prepared membrane of the LiFSI/PEO electrolyte. (b) Total and Li+ conductivity of the LiFSI/PEO and LiTFSI/PEO electrolytes at 70 °C. (c) SEM images of the sulfur cathode (40 wt % content of S) using the LiFSI/PEO electrolyte as a binder.

Figure 2. (a) Discharge/charge profiles of the Li−S cell using the LiFSI/PEO and LiTFSI/PEO electrolytes in the first cycle at a charge/discharge rate of 0.1/0.1C at 70 °C. (b) SEM images of the Al current collector, pristine and after cycling in LiTFSI-based cell. (c) Optical and (d) crosssection images of the Li−S cells opened after the first cycle using the LiFSI/PEO and LiTFSI/PEO electrolytes.

LiTFSI/PEO, indicating that the difference of ion-transport behavior in both electrolytes has a negligible effect on distinguishing cycling performance of Li−S cells. Figure 1c shows the typical scanning electron microscope (SEM) images of the electrode with 40 wt % S loading, in which the LiFSI/ PEO electrolyte is used as binder and also ion-transport media

PEO electrolytes (X = FSI and TFSI) is shown in Figure 1b. It is noteworthy that the Li+ conductivity (σLi+ = σtotal × TLi+) (see Figure S1 and Table S1 in the Supporting Information (SI) for lithium-ion transference number (TLi+) measurements) for both kinds of electrolytes is quite comparable at 70 °C, 9.0 × 10−5 S cm−1 for LiFSI/PEO versus 1.0 × 10−4 S cm−1 for 1957


Letter


Figure 3. Discharge/charge profiles of the Li−S cells using the LiFSI/PEO electrolyte in the first cycle at a discharge/charge rate of 0.05/0.05C at 70 °C. (a) S cathodes with the thicknesses around 30 μm but different S contents. (b) S cathodes with the same S content of 50 wt % but different electrode thicknesses. Discharge capacity and areal capacity (c) and Coulombic efficiency (d) versus cycle number for the LiFSI-based Li−S cells at 70 °C.

in the electrode.29,30 The S/carbon mixtures are well-dispersed in the presence of high-molecular-weight PEO, confirming the homogeneity and the good binding properties of PEO.31 Figure 2a presents discharge/charge profiles of a Li−S polymer cell using the LiFSI/PEO and LiTFSI/PEO electrolytes at the first cycle, respectively. The LiFSI-based cell with 40 wt % S shows a high discharge capacity of 800 mAh gsulfur−1 and CE of 95% at 0.1C. On the contrary, overcharging occurred in the LiTFSI-based cell after the first discharging process (i.e., the cutoff voltage of 2.8 V for constant current charging could not be reached). It is worth mentioning that this overcharging was found in all TFSI-based cells using various S weight contents (Figure S2). One possible reason could be Al corrosion in contact with the LiTFSI/PEO electrolyte on charging, as it could occur in liquid electrolyte with the presence of sulfur species at 2.5 V.32 Another reason could be the side reaction of the polysulfides on Li metal anode due to the poorly formed SEI film in LiTFSI/PEO electrolyte.19 To address this issue, post-mortem analysis of the cycled Li−S cell was performed. Figure 2b shows the surface morphologies of pristine Al current collector and the one recovered from LiTFSI-based cell after 250 h of charging. No significant change can be observed in both SEM images, suggesting that Al corrosion is unlikely to be responsible to the overcharging in the LiTFSI-based cell. Furthermore, the good electrochemical stabilities of Al current collector with both SPEs were confirmed by the experiments of cyclic voltammograms (Figure S3) and electrochemical impedance spectroscopy (Figure S4) of Al electrode. Moreover, in both the LiFSI- and LiTFSI-based cells, a good contact between SPEs and electrode was obtained, and no dendritic lithium was formed (Figure 2c,d). However, the dissolution and diffusion of polysulfide species in both SPEs occurred, after the first cycle, suggested by the brown color of the membrane (Figure 2c). These results indicate that the poor SEI film between LiTFSI/PEO and Li metal, especially in the presence

of polysulfides, could be the main reason for the deteriorating of cycling performance of the Li−S cell. Thus on the basis of aforementioned results and the better chemical and electrochemical compatibility with Li metal electrode in the LiFSI/PEO electrolyte when compared with the LiTFSI/PEO one, we may anticipate that a stable and polysulfide-resistant SEI film has been formed on the interphase of Li|SPEs in the LiFSI/PEO electrolyte. In this sense, FSI− might play a role similar to that of NO3− used in liquid Li−S cells.5,6 This would be presumably due to the inorganic F-SO2− group in FSI− instead of organic CF3−SO2− group in TFSI− that can be beneficial for forming a robust (e.g, LiF rich) SEI film at Li metal electrode, as evidenced by the improved cycling stability of Li metal in the LiFSI/1,2dimethoxyethane (DME, which can be viewed as the monomer of PEO) liquid electrolyte than that in the LiTFSI-based one.23,24 The exact effect of FSI− on the interphase between Li metal and PEO-based electrolyte in Li−S batteries needs to be further investigated; however, it is not within the scope of this communication. High sulfur loading in the cathode is critical for achieving a high areal capacity and energy density for Li−S batteries; however, previous work on Li−S batteries with polymer electrolyte seldom presented real S loading in the composite cathode.5 In this communication, the effect of important parameters (i.e., the S content and thickness of S cathode) on the cycling performance of Li−S polymer cells is studied and presented in Figure 3. In the first cycle (Figure 3a), the cells with 30 wt % S show fairly similar discharge capacity to that with 40 wt % S, that is, 900 mAh gsulfur−1 for 30 wt % S versus 800 mAh gsulfur−1 for 40 wt % S, while the cell with 50 wt % S loading presents significantly reduced discharge capacity of 600 mAh gsulfur−1. This would be attributed to the pronounced effect of the electronically insulating nature of S when decreasing the relative amount of conductive carbon in the cathode.5 There is 1958



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a significant difference of discharge capacity (600 to 250 mAh gsulfur−1) and polarization between the cells using electrodes with various thicknesses (30 and 65 μm), shown in Figure 3b. This would be explained by (1) longer and more tortuous ion pathway with increasing the thickness of electrode, leading to salt depletion as the transference number of Li+ is less than unity; (2) the reductions of polysulfide on the surface of the electrode due to their difficulty of back diffusion into the electrode, thus causing insulating layers and further inhibiting utilization of the deeper-residing sulfur;5 and (3) the intrinsic insulating nature of S whose effect is amplified in thicker electrodes. The discharge capacity and CEs decrease with increasing the S content in all C-rate tests, as seen in Figure 3c,d. A highest areal capacity of 0.5 mAh cm−2 at 0.1C with 40 wt % S content was obtained, which can be explained as a trade-off between sulfur utilization and S loading. It is important to note that the specific discharge capacity of all LiFSI-based cells maintains 100−400 mAh gsulfur−1 at 0.5C and the initial capacity is regained when returning to 0.1C (Figure 3c), suggesting an improved rate capability compared with those of other SPEsbased Li−S cells.17,18 Moreover, capacity fading was not observed after 50 cycles, regardless of the S content (30, 40, and 50 wt %, though the latter shows evidence of soft dendrites) and rate, displaying superior performances compared with those results with solid-state Li−S polymer cells reported in literature (e.g., a capacity decay of 70% after 10 cycles were observed for Li−S cell using LiCF3SO3/PEO electrolyte with ceramic fillers;17 see Table S2). See Figure S5 for the cycling performance of the LiFSI-based cells in terms of mAh per gram of total electrode weight. Although the exact mechanism underlying the excellent cyclability of the cell with the LiFSI/PEO electrolyte is not clear at present, it seems that stable interphases would play a decisive role in sustaining the stable cycling performances of the Li−S polymer cell. This is also supported by the results that the electrochemical reduction of LiFSI-based liquid electrolytes at a potential close to that of sulfur cathode could form in situ protective coating on both cathode and anode surfaces.23 In summary, LiFSI/PEO polymer electrolyte has been studied as electrolyte for the all solid-state Li−S cell. The LiFSI-based cell delivers high specific discharge capacity of 800 mAh gsulfur−1 and high areal capacity of 0.5 mAh cm−2 with good rate capability and cyclability. In contrast, the LiTFSIbased cell shows poor cyclic performance, which most likely suffers from the Sn2− shuttle effect of polysulfide due to the inferior quality of the interphases between Li metal and the LiTFSI/PEO electrolyte. These results suggest that the LiFSIbased SPEs could be promising candidates as electrolyte materials for all-solid Li−S batteries.

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Letter

AUTHOR INFORMATION

Corresponding Authors

*H.Z.: E-mail: [email protected]. *C.L.: E-mail: [email protected]. ORCID

Heng Zhang: 0000-0002-8811-6336 Chunmei Li: 0000-0003-4438-0458 José A. González-Marcos: 0000-0002-5962-7938 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by GV-ELKARTEK-2016 from the Basque Government and MINECO RETOS (ref: ENE201564907-C2-1-R) from Spanish Government. X.J. thanks the Government of the Basque Country for funding through a Ph.D. Fellowship. We thank 3M for providing the Al current collector.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00593. Experimental details; discharge/charge profiles the Li−S cells using the LiTFSI/PEO electrolyte; cyclic voltammograms and electrochemical impedance spectroscopy (EIS) profiles of Al electrode; and discharge/charge profiles and discharge capacity (mAh per gram of total electrode weight) for Li−S cells using the LiFSI/PEO electrolyte. (PDF) 1959


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Poly(ethylene oxide) Polymer

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