Novel Li[(CF3SO2)(n-C4F9SO2)N]-Based Polymer Electrolytes for

Oct 11, 2016 - ... Polymer Electrolytes for Solid-State Lithium Batteries with Superior ... but also afford good cycling performances for the Li|LiFeP...
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Novel Li[(CF3SO2)(n‑C4F9SO2)N]-Based Polymer Electrolytes for SolidState Lithium Batteries with Superior Electrochemical Performance Qiang Ma,†,‡ Xingguo Qi,† Bo Tong,‡ Yuheng Zheng,† Wenfang Feng,‡ Jin Nie,‡ Yong-Sheng Hu,*,† Hong Li,† Xuejie Huang,† Liquan Chen,† and Zhibin Zhou*,‡ †

Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ 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, Wuhan 430074, China S Supporting Information *

ABSTRACT: Solid polymer electrolytes (SPEs) would be promising candidates for application in high-energy rechargeable lithium (Li) batteries to replace the conventional organic liquid electrolytes, in terms of the enhanced safety and excellent design flexibility. Herein, we first report novel perfluorinated sulfonimide salt-based SPEs, composed of lithium (trifluoromethanesulfonyl)(nnonafluorobutanesulfonyl)imide (Li[(CF3SO2)(n-C4F9SO2)N], LiTNFSI) and poly(ethylene oxide) (PEO), which exhibit relatively efficient ionic conductivity (e.g., 1.04 × 10−4 S cm−1 at 60 °C and 3.69 × 10−4 S cm−1 at 90 °C) and enough thermal stability (>350 °C), for rechargeable Li batteries. More importantly, the LiTNFSI-based SPEs could not only deliver the excellent interfacial compatibility with electrodes (e.g., Li-metal anode, LiFePO4 and sulfur composite cathodes), but also afford good cycling performances for the Li|LiFePO4 (>300 cycles at 1C) and Li−S cells (>500 cycles at 0.5C), in comparison with the conventional LiTFSI (Li[(CF3SO2)2N])-based SPEs. The interfacial impedance and morphology of the cycled Li-metal electrodes are also comparatively analyzed by electrochemical impedance spectra and scanning electron microscopy, respectively. These indicate that the LiTNFSI-based SPEs would be potential alternatives for application in high-energy solid-state Li batteries. KEYWORDS: poly(ethylene oxide), LiTNFSI, polymer electrolytes, solid-state batteries, Li batteries

1. INTRODUCTION In the past 20 years, tremendous progress has been achieved in the field of lithium-ion batteries (LIBs) with nonaqueous organic liquid electrolytes. However, the traditional LIBs based on graphitized carbon as anode have a maximum theoretical gravimetric capacity limited to about 350 mA h g−1, which is too low to be employed in practical large-scale scenarios.1,2 Lithium (Li) metal as an anode would be a promising material for application in large-scale systems, because of the extremely high theoretical specific capacity (3.86 A h g−1), lightest equivalent weight (6.94 g mol−1), and the lowest negative electrochemical potential (−3.04 V vs SHE).3,4 Unfortunately, using metallic Li as an anode for the organic liquid electrolytesbased Li batteries would cause safety issues, because of the high reactivity and flammability of the organic liquid electrolytes in the presence of metallic Li. It finally hinders its practical applications in rechargeable Li batteries,5 especially in largescale energy storage systems, in which safety is considered as the primary concern. Clearly, it is extremely urgent to seek the electrolytes with superior safety for the rapid development of Li-ion and/or Li batteries in the next-generation large-scale applications.6−9 © 2016 American Chemical Society

One of the best strategies to deal with the aforementioned concerns is to develop solid polymer electrolytes (SPEs), which possess great advantages over the liquid or gel electrolytes,10−15 including the enhanced safety, nonflammability, nonleakage, and excellent design flexibility. Earlier works on the poly(ethylene oxide) (PEO)-based SPEs for Li batteries have concentrated on conventional lithium bis(trifluoromethanesulfonyl)imide (Li[(CF3SO2)2N], LiTFSI) as a conducting salt. This is mainly attributed to the application of LiTFSI/PEO SPEs in Li|LiFePO4 batteries for an electric car, Autolib.16 However, the LiTFSI/PEO SPEs may not be the best choice for Li batteries, because of the relatively unstable interphases between the Li-metal anode and SPEs (LiTFSI/ PEO) for the Li|Li cells in comparison with lithium bis(fluorosulfonyl)imide (Li[(FSO2)2N], LiFSI)/PEO SPEs, as have been demonstrated in the previous report.17 Recently, we reported a new liquid electrolyte based on lithium (fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide (LiReceived: August 23, 2016 Accepted: October 11, 2016 Published: October 11, 2016 29705

DOI: 10.1021/acsami.6b10597 ACS Appl. Mater. Interfaces 2016, 8, 29705−29712

Research Article

ACS Applied Materials & Interfaces

mixing the CMK-3/S composite, super P, SPEs (LiTNFSI/PEO, EO/ Li+ = 20), and PVDF at a mass ratio of 65:10:20:5 in appropriate acetonitrile under strong agitation. The resultant slurry was cast onto a carbon-coated aluminum foil with doctor blade, and dried at 60 °C for 12 h under high vacuum conditions. 2.5. Characterization of Polymer Electrolyte. The phase transition behaviors of the as-prepared LiTNFSI/PEO blended polymer electrolyte were characterized by differential scanning calorimeter (DSC, Netzsch 200 F3). The sample (ca. 5−10 mg) was hermetically sealed in an aluminum pan and placed at room temperature for 10 days prior to measurement in an argon-filled glovebox. The DSC instrument was programmed to heat the sample from −150 to 150 °C (or 300 °C) at a rate of 10 °C min−1 under a flow of nitrogen. Thermal gravimetric analysis (TGA) was performed on a NETZSCH STA 449C thermoanalyzer. The sample (ca. 5−10 mg) was put into an aluminum pan and heated at a ratio of 10 °C min−1 from 30 to 600 °C under a flow of argon. The crystalline phases of the sample were determined by X-ray diffraction (XRD, D8 X-ray diffractometer, Bruker) equipped with Cu Kα radiation (λ = 1.54 Å) in the 2θ range from 5 to 90° at 40 mA and 40 kV. The ionic conductivities of LiTNFSI/PEO blended polymer electrolyte were evaluated by AC impedance with an amplitude of 5 mV over the frequency range from 0.01 to 106 Hz (Autolab PGSTAT128N, Netherland). The sample was sandwiched between two stainless steel blocking electrodes and heated to 70 °C for 2 h to obtain good contact between the electrolytes and electrodes, and was then placed at room temperature for 10 days prior to the measurement in an argon-filled glovebox.11,17 Subsequently, the measurements were performed in the temperature range from 25 to 90 °C for the heating scan. The sample was thermally equilibrated at each temperature at least for 1 h prior to the measurement. The Li-ion transference number (tLi+) of LiTNFSI/PEO blended polymer electrolyte was measured by a combination measurement of AC impedance and DC polarization, which were proposed by Bruce and Abraham.24,25 The symmetric [Li|SPEs|Li] coin-type cell was first heated to 70 °C for 2 h to obtain good contact between the electrolytes and electrodes. Then, a DC voltage of 10 mV was applied to the cell to obtain the initial and steady currents. Meanwhile, the EIS spectra of the same cell before and after the DC polarization were also monitored using AC impedance with an amplitude of 5 mV over the frequency range from 0.05 to 106 Hz. The anodic electrochemical stability of LiTNFSI/PEO blended polymer electrolyte was investigated by linear sweep voltammetry (Shanghai Chenhua CHI627D, China). The [Li|SPEs|SS] coin-type cell was measured over a wide range of potentials from the open circuit potential (OCP) to 6.0 V vs Li+/Li at a scan rate of 1 mV s−1 at 70 °C. The morphology of the membrane for the LiTNFSI/PEO blended polymer electrolyte was observed by scanning electron microscopy (SEM) equipped with a vacuum transfer box (Hitachi S-4800). The morphology and elemental mapping of Li metal anodes were also observed by SEM and energy-dispersive X-ray spectroscopy (EDS) equipped with a vacuum transfer box (Hitachi S-4800). 2.6. Cell Fabrication and Cell Performance Evaluation. The coin-type (2032) cells were assembled by using Li-metal anode, SPEs (LiTNFSI/PEO, EO/Li+ = 20), and the CMK-3/S composite (or LiFePO4) cathode in an argon-filled glovebox. The area of electrodes (i.e., CMK-3/S composite and LiFePO4 cathodes) was 1.13 cm2 (diameter of 12 mm). The active material mass loading on the electrodes were ca. 1.0 and 1.5 mg cm−2 for the CMK-3/S composite and LiFePO4 cathodes, respectively. The applied currents and specific capacities of the cells were calculated on the basis of the weight and theoretical capacity of cathodes. The galvanostatic charge/discharge tests were conducted on a Land CT2001A battery test system (Wuhan, China) at various current rates at 60 °C. The symmetric [Li| SPEs|Li] coin-type cells were assembled by using Li metal as the working and counter electrodes. All the cells were charged and discharged at a current density of 0.2 mA cm−2 at 60 °C.

[(FSO2)(n-C4F9SO2)N], LiFNFSI), which exhibits good cycling performances at elevated temperature for LIBs, compared with the commercial lithium hexafluorophosphate (LiPF6)-based electrolyte.18 This would be ascribed to the relatively stable SEI films formed on the graphite anode, because of the reductive decompositions of FNFSI− anions. Besides, similar to the LiFNFSI with C4F9− group, a new perfluorinated sulfonimide salt, namely lithium (trifluoromethanesulfonyl)(n-nonafluorobutanesulfonyl)imide (Li[(CF 3SO 2)(n-C4 F9 SO 2)N], LiTNFSI) has also been reported. The obtained excellent cycling performances in liquid Li−S batteries and superior cycling stability with metallic Li have proved more stable SEI films formed on the Li-metal anode in the LiTNFSI-based electrolyte, compared with conventional LiTFSI-based one.19 All of the above results demonstrate that C4F9-group (in LiFNFSI and LiTNFSI) plays a key role in forming stable SEI films on the graphite and Limetal anodes.18−22 On the basis of the above understanding, we herein report a new type of SPEs, which were composed of the LiTNFSI and PEO with the different molar ratios of EO/Li+ = 8, 16, 20, and 30 for the first time. The physicochemical and electrochemical properties of the LiTNFSI/PEO blended polymer electrolytes were intensively investigated, in terms of their phase transition behavior, XRD characterization, thermal stability, ionic conductivity, Li-ion transference number, and anodic electrochemical stability. More importantly, the cycling stability for metallic Li and cycling performances for Li|LiFePO4 and Li−S cells with the LiTNFSI-based SPEs, have also been comparatively investigated with the LiTFSI-based electrolyte.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Poly(ethylene oxide) (PEO, Mw = 6 × 105 g mol−1, Acros) was dried at 50 °C for 12 h under high vacuum conditions. Lithium foil (China Energy Lithium Co., Ltd., 600 μm in thickness) was used as received. Sublimed sulfur (S) (99.5%, Alfa Aesar), polyvinylidene fluoride (PVDF, Alfa Aesar), conductive carbon black (super P, Timcal), and CMK-3 (Nanjing XF-nano, China) were used as received. Lithium iron phosphate (LiFePO4) powder (gifted by Suzhou Phylion battery, China) was dried at 120 °C for 12 h under high vacuum conditions before use. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (99%, Solvay, China) and lithium (trifluoromethanesulfonyl)(n-nonafluorobutanesulfonyl)imide (LiTNFSI) (99%, Suzhou Fluolyte Co., Ltd., China) were used as received without further treatment. Anhydrous acetonitrile (99.9%, Alpha Aesar) was dried by 4 Å molecular sieves for 48 h before use. All the procedures related to the handing moisture or oxygen sensitive materials were carried out in an argon-filled glovebox (MBraun, H2O and O2 < 1 ppm). 2.2. Fabrication of Polymer Electrolyte Membrane. The membrane of LiTNFSI/PEO blended polymer electrolytes was prepared by the solution casting technique. First, the complex of LiTNFSI/PEO was prepared by mixing and dissolving the LiTNFSI and PEO with a certain proportion in dried acetonitrile in an argonfilled glovebox. Second, the homogeneous viscous solution was spread onto a Teflon plate and let to evaporate the solvent slowly at least for 12 h at room temperature. Finally, the obtained film was formed and dried at 60 °C for 48 h under high vacuum conditions to remove any residual solvent. 2.3. Synthesis of Cathode Material. The CMK-3/S composite was prepared by a melt-diffusion method according to the previous report.23 First, the CMK-3 and S powders were ground together. Second, the mixture was hermetically sealed in a glass tube in an argon-filled glovebox, and heated at 155 °C for 24 h. 2.4. Fabrication of Cathode Sheet. The cathode sheets of CMK3/S composite (or LiFePO4 with same procedure) were fabricated by 29706

DOI: 10.1021/acsami.6b10597 ACS Appl. Mater. Interfaces 2016, 8, 29705−29712

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endothermic peak at about 60−70 °C is observed in these LiTNFSI/PEO SPEs, except for the LiTNFSI/PEO electrolyte with a molar ratio of EO/Li+ = 8, which corresponds to the classic melting of crystalline PEO.26,27 It is worth noting that a decrease in concentration of Li salts in the LiTNFSI/PEO (EO/Li+ = 16, 20, and 30) blended electrolytes leads to an increase in both melting point (Tm) and enthalpy of melting for these SPEs (Table S1). This would be attributed to the plasticizing effects of Li salt on these SPEs. This trend in the DSC traces is coincident with previous report.28 XRD patterns of the LiTNFSI/PEO blended polymer electrolytes with the different molar ratios (i.e., EO/Li+ = 8, 16, 20, and 30), as well as the neat PEO for comparison, are presented in Figure 2b. It can be seen that two characteristic diffraction peaks of the crystalline PEO at 2θ = 19.2 and 23.3° are observed.29,30 With the increase in Li salt content, the intensities of these two characteristic diffraction peaks for neat PEO decrease rapidly in the series of the LiTNFSI/PEO (EO/ Li+ = 8, 16, 20, and 30) blended electrolytes. It is interesting to note that these two peaks become very broad at the lowest molar ratio of EO/Li+ = 8, confirming that the presence of mainly amorphous phase.30 This trend in the XRD patterns is in good agreement with the DSC results in Figure 2a (Table S1). Therefore, the incorporation of Li salt into these LiTNFSI/PEO blended electrolytes will decrease crystallinity (χc) of PEO, favoring to the ionic conduction in these SPEs. The thermal stabilities of the LiTNFSI/PEO blended polymer electrolytes with the different molar ratios (i.e., EO/ Li+ = 8, 16, 20, and 30), as well as the neat PEO and LiTNFSI, are comparatively shown in Figure 2c. It can be inferred that the decomposition temperature of the neat LiTNFSI salt can reach 378 °C (Table S1), which is stable enough as a conducting salt for application in high-energy solid-state Li batteries. Meanwhile, the decomposition temperatures of the LiTNFSI-based SPEs can also reach 365 °C (Table S1), suggesting that the LiTNFSI-based SPEs can satisfy the application conditions of the above-mentioned solid-state Li batteries. Figure 3 compares the logarithmic temperature dependence of the ionic conductivities (σ) for the LiTNFSI/PEO blended

3. RESULTS AND DISCUSSION 3.1. Physicochemical and Electrochemical Properties of Polymer Electrolyte. Figure 1a shows the photograph for

Figure 1. (a) Photograph and (b) SEM image for the membrane of the LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte.

the membrane of the as-prepared LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte. It can be seen that the selfstanding membrane has been successfully fabricated. The thickness of the membrane is about 150 μm, which was measured by the micrometer. The SEM image for the membrane of the LiTNFSI/PEO (EO/Li+ = 20) electrolyte is displayed in Figure 1b. It is clear that the as-prepared membrane is relatively homogeneous and compact (inset in Figure 1b). Furthermore, the thickness of the membrane determined by the SEM image is consistent with the measurement of micrometer (Figure 1b). The phase transition behaviors of the prepared LiTNFSI/ PEO blended polymer electrolytes with the different molar ratios of EO/Li+ = 8, 16, 20, and 30, as well as the neat PEO and LiTNFSI, were characterized by DSC. All the measurement data are summarized in Table S1. Figure 2a comparatively displays the DSC traces of the LiTNFSI/PEO blended polymer electrolytes with the different molar ratios (i.e., EO/Li+ = 8, 16, 20, and 30), as well as the neat PEO. All these LiTNFSI/PEO SPEs show a low glass transition temperature (Tg), and the Tg increases with increasing the EO/Li+ ratio (Table S1). Moreover, a sharp

Figure 3. Temperature dependence of the ionic conductivities for the LiTNFSI/PEO blended polymer electrolytes with the different molar ratios of EO/Li+ = 8, 16, 20, and 30. Figure 2. (a) DSC traces of the LiTNFSI/PEO blended polymer electrolytes with the different molar ratios of EO/Li+ = 8, 16, 20, and 30. (b) XRD patterns of the LiTNFSI/PEO blended polymer electrolytes with the different molar ratios of EO/Li+ = 8, 16, 20, and 30, as well as the neat PEO. (c) TGA traces of the LiTNFSI/PEO blended polymer electrolytes with the different molar ratios of EO/Li+ = 8, 16, 20, and 30, as well as the neat PEO and LiTNFSI.

polymer electrolytes with the different molar ratios (i.e., EO/ Li+ = 8, 16, 20, and 30). As shown in Figure 3, for these LiTNFSI/PEO blended electrolytes, the ionic conductivities generally decrease in the order of EO/Li+ = 16 ≈ 20 > 8 > 30 above 60 °C. This would be attributed to the consequence of a trade-off between the crystallinity of polymer and concentration 29707

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ACS Applied Materials & Interfaces of charge carrier.1,11,13,30,31 It can be noted that the LiTNFSI/ PEO electrolyte with a molar ratio of EO/Li+ = 8 displays a linear relation between ionic conductivity and temperature, while a clear ionic conductivity jump between 60 and 70 °C is found in these LiTNFSI/PEO SPEs with higher EO/Li+ ratios, due to the transitions for PEO from crystalline/semicrystalline to amorphous phase.26,30 This result can be confirmed by the DSC results in Figure 2a (Table S1). It is interesting to note that the LiTNFSI/PEO electrolyte with a molar ratio of EO/ Li+ = 20 is chosen for deeper investigation because of the consequence of a trade-off between the mechanical property and ionic conductivity, even though the ionic conductivities for the LiTNFSI/PEO (EO/Li+ = 16) electrolyte are slightly high above 60 °C. Note that the LiTNFSI/PEO (EO/Li+ = 20) electrolyte shows a relatively low ionic conductivity (σ = 1.21 × 10−6 S cm−1) at 25 °C, which is not sufficient to be used as SPEs for application in solid-state Li batteries. This is a common feature for the PEO-based SPEs, owing to the crystallization of PEO below 60 °C. Fortunately, a relatively high ionic conductivity (σ = 1.04 × 10−4 S cm−1) for the LiTNFSI/PEO (EO/Li+ = 20) electrolyte can be obtained above 60 °C, which would be ascribed to the melting of crystalline PEO, as verified by the DSC traces in Figure 2a. Some of representative values for the ionic conductivities of the LiTNFSI/PEO (EO/Li+ = 8, 16, 20, and 30) blended electrolytes are summarized in Table S2. Figure 4 shows the EIS spectra and DC polarization curve of Li-ion transference number (tLi+) for the LiTNFSI/PEO (EO/

ascribed to the concurrent oxidative decomposition of both TNFSI− anions and PEO. This indicates that the LiTNFSIbased SPEs can be used as potential candidates for application in 4 V solid-state Li batteries. 3.2. Compatibility with Lithium Metal. It is widely known that the ionic conductivity and interfacial stability are two key issues in all kinds of solid-state Li batteries. Therefore, the interfacial compatibility of the LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte with Li metal was evaluated by the symmetric [Li|SPEs|Li] coin-type cell at a current density of 0.2 mA cm−2 at 60 °C, as well as the LiTFSI/PEO electrolyte with the same molar ratio for comparison. As seen from Figure 6a, the polarization potential decreases gradually in the first few

Figure 4. (a) Impedance spectra and (b) time-dependence response of DC polarization for the LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte on the symmetric [Li|SPEs|Li] coin-type cell at 70 °C, polarized with a potential of 10 mV.

Figure 6. Cycling performances (a) and electrochemical impedance spectra (b) for the symmetric [Li|SPEs|Li] coin-type cell with LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte at a current density of 0.2 mA cm−2 at 60 °C.

Li+ = 20) blended polymer electrolyte at 70 °C, and the corresponding calculated values are summarized in Table S3. It can be concluded that the value of tLi+ for the LiTNFSI/PEO (EO/Li+ = 20) blended electrolyte is 0.16, which is comparable to the results for the classic ambipolar LiFSI/PEO (EO/Li+ = 20) and LiTFSI/PEO (EO/Li+ = 20) SPEs,13 indicating that the LiTNFSI-based SPEs exhibit a dual-ion conducting behavior. The anodic electrochemical stability of the LiTNFSI/PEO blended polymer electrolytes is an important parameter to evaluate the feasibility of SPEs for application in solid-state Li batteries, which was investigated by linear sweep voltammetry. The polarization curve for the LiTNFSI/PEO (EO/Li+ = 20) blended electrolyte on [Li|SPEs|SS] coin-type cell at 70 °C is shown in Figure 5. It is interesting to note that a small anodic current (below 5 μA cm−2) at ca. 4.0 V vs Li+/Li is observed, which is related to the initializing oxidative decomposition of PEO. This result is in good agreement with our previous report.11 Moreover, a large anodic current (above 10 μA cm−2) is also observed only after ca. 5.0 V vs Li+/Li, which would be

cycles, and then maintains relatively stable at a low value of ∼70 mV for more than 400 h (100 cycles). Simultaneously, the negligible variable values of total impedance (i.e., diameter of semicircle) from the 10th to the 100th cycle can also be delivered from the EIS spectra in Figure 6b. The aforementioned results indicate that the relatively stable interphases between the LiTNFSI-based SPEs and Li metal can be formed, which is different to the LiTFSI-based SPEs, wherein the random fluctuations of polarization curve (inset in Figure S1) and diameter of semicircle from the EIS spectra (Figure S2) are observed.17 Clearly, such a difference in the interfacial stability with Li metal should arise from Li salt. This may be expected from the participation of C4F9-group (in LiTNFSI) in forming stable interphases with the Li metal, which can be proved by the LiTNFSI-based electrolyte in liquid Li−S batteries,19 in comparison with LiTFSI-based electrolyte. Detailed research would be desired to clarify the difference between LiTNFSI and LiTFSI-based SPEs in terms of the interfacial stability with Li metal.

Figure 5. Linear sweep voltammogram of the LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte on [Li|SPEs|SS] coin-type cell, measured over a wide range of potentials from the open circuit potential (OCP) to 6.0 V vs Li+/Li at 70 °C.

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ACS Applied Materials & Interfaces 3.3. Cell Performance. Li|LiFePO4 and Li−S cells were used in the present study to evaluate the electrochemical performances of the LiTNFSI-based SPEs, as well as the LiTFSI/PEO (EO/Li+ = 20) electrolyte for comparison. Figure 7 shows the charge/discharge profiles for the Li|LiFePO4 and

8d) for the LiTFSI-based blended electrolyte are presented. It is worthy to note that the random fluctuations of capacity retention are observed in the Li|LiFePO4 cell with LiTFSIbased SPEs after 35 cycles, which may be due to the unstable interfacial films formed on the aforementioned electrodes. This result can be further confirmed by the SEM images in Figure 9c. Comparing Figure 9a−d, a significant difference observed

Figure 7. Voltage profiles of Li|LiFePO4 cell (a) at 0.1C and Li−S cell (b) at 0.05C with LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte at 60 °C.

Li−S cells with LiTNFSI/PEO (EO/Li+ = 20) blended electrolyte at various current rates at 60 °C (Note that the photograph of membrane for the LiTNFSI/PEO (EO/Li+ = 20) electrolyte in the heating process at 80 °C is displayed in Figure S3). As shown in Figure 7a, b, there are no obvious polarizations in the Li|LiFePO4 and Li−S cells, which is the same with other advanced electrolytes-based Li batteries.32−36 This implies that the LiTNFSI-based SPEs can afford good electrochemical performances. It can be observed that the LiTNFSI/PEO (EO/Li+ = 20) blended electrolyte can deliver the relatively high initial discharge specific capacities (e.g., 146.0 mA h g−1 for Li|LiFePO4 cell, and 851.4 mA h g−1 for Li−S cell) and initial Coulombic efficiencies (CEs) (e.g., 97.5% for Li|LiFePO4 cell, and 85.3% for Li−S cell). As seen from Figure 8a, b, the LiTNFSI/PEO (EO/Li+ = 20) blended electrolyte

Figure 9. SEM images of Li-metal anodes for the (i) Li|LiFePO4 cells and (ii) Li−S cells at various rates at 60 °C. (a) Li|LiFePO4 cell with LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte after 300 cycles at 1C (after 5 cycles for activation at 0.1C); (b) Li−S cell with LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte after 200 cycles at 0.2C (after 5 cycles for activation at 0.05C); (c) Li|LiFePO4 cell with LiTFSI/PEO (EO/Li+ = 20) blended polymer electrolyte after 100 cycles at 1C (after 5 cycles for activation at 0.1C); (d) Li−S cell with LiTFSI/PEO (EO/Li+ = 20) blended polymer electrolyte after 200 cycles at 0.2C (after 5 cycles for activation at 0.05C).

from these images is that the surfaces of the cycled Li-metal anodes with LiTNFSI-based SPEs (Figure 9a, b) are more smooth and dense than those with LiTFSI-based SPEs (Figure 9c, d), suggesting that the interfacial films formed on the electrodes with LiTNFSI-based SPEs would be more compact and roust (Figure 10). It may be due to the participation of the C4F9− group (in LiTNFSI) in forming stable interphases with the electrodes, which can be further demonstrated by the nearly invariable values of total impedance from the 10th to the 300th cycle for the Li|LiFePO4 cell (Figure 11a), and from the initial to the 200 cycle for the Li−S cell (Figure 11b).19,37 Clearly,

Figure 8. Cycling performances and Coulombic efficiencies of (i) Li| LiFePO4 cells and (ii) Li−S cells at various current rates (after 5 cycles for activation at 0.1C) at 60 °C. (a, b) LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte; (c, d) LiTFSI/PEO (EO/Li+ = 20) blended polymer electrolyte.

can display the relatively good cycling performance (i.e., slow capacity decay) with capacity retention rate of 72% after the following 300 cycles at 1C in Li|LiFePO4 cell, and excellent cycling performance almost without capacity fading even after 500 cycles at 0.5C in Li−S cell. However, the obviously inferior cycling performances in the corresponding Li|LiFePO4 (only cycling for 35 cycles, Figure 8c) and Li−S cells (lower average discharge specific capacity of ∼200 mA h g−1 at 0.5C, Figure

Figure 10. Schematic illustration of the formed interfacial films on the electrodes. 29709

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Figure 11. Electrochemical impedance spectra of (a) the Li|LiFePO4 cell and (b) Li−S cell with LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte at 1C and 0.5C (after 5 cycles for activation at 0.1C), respectively, at 60 °C.

such a difference in the cycling performances (Figure 8), SEM images (Figure 9), and variations of impedance (Figure 11 and Figure S4) should be due to the different Li salt used as it is the only variable. Additionally, Figure 12 shows the current-rate (C-rate) capabilities of the Li|LiFePO4 cell with LiTNFSI/PEO (EO/Li+

Figure 13. Cycling performances of the Li−S cells at 0.2C (after 5 cycles for activation at 0.05C) at 60 °C. (a) LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte; (b) LiTFSI/PEO (EO/Li+ = 20) blended polymer electrolyte.

S5), which is commonly observed in the liquid Li−S cells without lithium nitrate (LiNO3) as additive.19,46−48 As seen from Figure 14, no obvious sulfur-containing species formed on

Figure 12. Rate capabilities of the Li|LiFePO4 cell with LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte at 60 °C.

= 20) blended polymer electrolyte at various C rates of 0.2, 0.5, 1, and 2C at 60 °C. The LiTNFSI-based SPEs can retain 66% of its discharge specific capacity at 2C (90.8 mA h g−1) with respect to the discharge specific capacity at 0.2C (137.1 mA h g−1). Moreover, when the C rate is switched back to 0.2C, the discharge specific capacity can still be reverted to 132.0 mA h g−1, indicating that the excellent C-rate capability of the Li| LiFePO4 cell with LiTNFSI-based SPEs. As shown in Figure 13a, the Li−S cell with LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte can afford the average discharge specific capacity of ∼450 mA h g−1 at 0.2C for more than 200 cycles, which is higher than that of ∼310 mA h g−1 at 0.5C at 60 °C (Figure 8b), though the discharge specific capacity of the Li−S cell with LiTNFSI-based SPEs still remains to be improved. This may be due to the relatively poor C-rate capability for the CMK-3/S composite cathode.38 Fortunately, the cycling stability is superior with negligible capacity loss for hundreds cycles. However, this trend in continuous capacity loss (i.e., poor cycling stability) during cycling (200 cycles) at 0.2C at 60 °C is observed in the LiTFSI-based electrolyte (Figure 13b). Thus, we believe that the practical performances for the Li−S cell with LiTNFSI-based SPEs can be further improved by the optimization of electrode structure design.39−45 It is worthy to note that the so-called shuttle effect has been effectively suppressed in the Li−S cell with LiTNFSI-based SPEs (Figure

Figure 14. EDS maps of Li-metal anodes for the Li−S cells after 200 cycles at 0.2C (after 5 cycles for activation at 0.05C) at 60 °C. (a) LiTNFSI/PEO (EO/Li+ = 20) blended polymer electrolyte; (b) LiTFSI/PEO (EO/Li+ = 20) blended polymer electrolyte.

the Li-metal anodes are observed in the LiTNFSI-SPEs, however, the conspicuous sulfur-containing species formed on the Li-metal anodes can be observed in the LiTFSI-SPEs, indicating that the so-called shuttle effect cannot be effectively suppressed in the Li−S cell with LiTFSI-based SPEs. This result can be further confirmed by the cycling performances of Li−S cells (Figure S5). Clearly, such a difference in the EDS maps should be due to the different Li salt used as it is the only 29710

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(5) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (6) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (7) Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652−657. (8) Suo, L. M.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X. L.; Luo, C.; Wang, C. S.; Xu, K. Water-in-Salt” Eelectrolyte Enables HighVoltage Aqueous Lithium-Ion Chemistries. Science 2015, 350, 938− 943. (9) Suo, L. M.; Borodin, O.; Sun, W.; Fan, X. L.; Yang, C. Y.; Wang, F.; Gao, T.; Ma, Z. H.; Schroeder, M.; von Cresce, A.; Russell, S. M.; Armand, M.; Angell, A.; Xu, K.; Wang, C. S. Advanced High-Voltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chem., Int. Ed. 2016, 55, 7136−7141. (10) Hallinan, D. T.; Balsara, N. P. Polymer Electrolytes. Annu. Rev. Mater. Res. 2013, 43, 503−525. (11) Ma, Q.; Zhang, H.; Zhou, C. W.; Zheng, L. P.; Cheng, P. F.; Nie, J.; Feng, W. F.; Hu, Y.-S.; Li, H.; Huang, X. J.; Chen, L. Q.; Armand, M.; Zhou, Z. B. Single Lithium-Ion Conducting Polymer Electrolytes Based on a Super-Delocalized Polyanion. Angew. Chem., Int. Ed. 2016, 55, 2521−2525. (12) Wang, Y.; Zhong, W.-H. Development of Electrolytes towards Achieving Safe and High-Performance Energy-Storage Devices: A Review. ChemElectroChem 2015, 2, 22−36. (13) Ma, Q.; Xia, Y.; Feng, W. F.; Nie, J.; Hu, Y.-S.; Li, H.; Huang, X. J.; Chen, L. Q.; Armand, M.; Zhou, Z. B. Impact of the Functional Group in the Polyanion of Single Lithium-Ion Conducting Polymer Electrolytes on the Stability of Lithium Metal Electrodes. RSC Adv. 2016, 6, 32454−32461. (14) Zhou, D.; Liu, R. L.; He, Y.-B.; Li, F. Y.; Liu, M.; Li, B. H.; Yang, Q.-H.; Cai, Q.; Kang, F. Y. SiO2 Hollow Nanosphere-Based Composite Solid Electrolyte for Lithium Metal Batteries to Suppress Lithium Dendrite Growth and Enhance Cycle Life. Adv. Energy Mater. 2016, 6, 1502214. (15) Lin, Y.; Li, J.; Lai, Y. Q.; Yuan, C. F.; Cheng, Y.; Liu, J. A Wider Temperature Range Polymer Electrolyte for All-Solid-State Lithium Ion Batteries. RSC Adv. 2013, 3, 10722−10730. (16) http://www.autolib.eu/en/our-commitment/bluecar-menu-en/ 100-electric/ (accessed 2014-10-29). (17) Zhang, H.; Liu, C. Y.; Zheng, L. P.; Xu, F.; Feng, W. F.; Li, H.; Huang, X. J.; Armand, M.; Nie, J.; Zhou, Z. B. Lithium Bis(fluorosulfonyl)imide/Poly(ethylene oxide) Polymer Electrolyte. Electrochim. Acta 2014, 133, 529−538. (18) Zheng, L. P.; Zhang, H.; Cheng, P. F.; Ma, Q.; Liu, J. J.; Nie, J.; Feng, W. F.; Zhou, Z. B. Li[(FSO2)(n-C4F9SO2)N] versus LiPF6 for Graphite/LiCoO2 Lithium-Ion Cells at both Room and Elevated Temperatures: A Comprehensive Understanding with Chemical, Electrochemical and XPS Analysis. Electrochim. Acta 2016, 196, 169−188. (19) Ma, Q.; Tong, B.; Fang, Z.; Qi, X. G.; Feng, W. F.; Nie, J.; Hu, Y.-S.; Li, H.; Huang, X. J.; Chen, L. Q.; Zhou, Z. B. Impact of Anionic Structure of Lithium Salt on the Cycling Stability of Lithium-Metal Anode in Li-S Batteries. J. Electrochem. Soc. 2016, 163 (8), A1776− A1783. (20) Zhou, S. S.; Han, H. B.; Nie, J.; Armand, M.; Zhou, Z. B.; Huang, X. J. Improving the High-Temperature Resilience of LiMn2O4 Based Batteries: LiFNFSI an Effective Salt. J. Electrochem. Soc. 2012, 159 (8), A1158−A1164. (21) Fang, Z.; Ma, Q.; Liu, P.; Ma, J.; Hu, Y.-S.; Zhou, Z. B.; Li, H.; Huang, X. J.; Chen, L. Q. Novel Concentrated Li[(FSO2)(nC4F9SO2)N]-Based Ether Electrolyte for Superior Stability of Metallic Lithium Anode. ACS Appl. Mater. Interfaces 2016, DOI: 10.1021/ acsami.6b03857. (22) Aravindan, V.; Gnanaraj, J.; Madhavi, S.; Liu, H.-K. Lithium-Ion Conducting Electrolyte Salts for Lithium Batteries. Chem. - Eur. J. 2011, 17, 14326−14346.

variable. This may be expected from the participation of C4F9group (in LiTNFSI) in forming stable interphases with the Li metal, which can inhibit the side reactions between the polysulfides and Li metal (Figure 14). Detailed discussion can be seen in Figure S5. This would be a reason for sustaining the long-term cycling performance of Li−S cells, which is critical for the large-scale application of Li−S cells.

4. CONCLUSIONS In summary, we first propose novel perfluorinated sulfonimide salt (LiTNFSI)-based SPEs for rechargeable Li batteries, which can exhibit a ionic conductivity as high as 3.69 × 10−4 S cm−1 at 90 °C and an anodic electrochemical stability at ca. 4.0 V vs Li+/Li, and sufficient thermal stability (>350 °C). More importantly, the LiTNFSI-based SPEs could deliver not only the long-term cycling stability (>400 h) for metallic Li, but also relatively high initial discharge specific capacities for the Li| LiFePO4 (146.0 mA h g−1 at 0.1C) and Li−S (851.4 mA h g−1 at 0.05C) cells at 60 °C. Moreover, the LiTNFSI-based SPEs can also afford good cycling and current-rate performances for the Li|LiFePO4 cell and long-term cycling performances for the Li−S cell, compared with LiTFSI-based SPEs. This would be due to the participation of the C4F9-group (in LiTNFSI) in forming stable interphases with the electrodes. All of the above results demonstrate that the LiTNFSI-based SPEs would be promising electrolytes for application in high-energy solid-state Li batteries.

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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/acsami.6b10597. Tables of phase transition behavior, ionic conductivity, and Li-ion transference number; electrochemical impedance spectra; cycling performances (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 10 82649808. Fax: +86 10 82649046. *E-mail: [email protected]. Tel.: +86 27 87559427. Fax: +86 27 87543632. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51222210, 51472268, 51172083). REFERENCES

(1) Tarascon, J.-M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (2) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29. (3) Xu, W.; Wang, J. L.; Ding, F.; Chen, X. L.; Nasybulin, E.; Zhang, Y. H.; Zhang, J.-G. Lithium Metal Anodes for Rechargeable Batteries. Energy Environ. Sci. 2014, 7, 513−537. (4) Ma, Q.; Fang, Z.; Liu, P.; Ma, J.; Qi, X. G.; Feng, W. F.; Nie, J.; Hu, Y.-S.; Li, H.; Huang, X. J.; Chen, L. Q.; Zhou, Z. B. Improved Cycling Stability of Lithium-Metal Anode with Concentrated Electrolytes Based on Lithium (Fluorosulfonyl) (trifluoromethanesulfonyl)imide. ChemElectroChem 2016, 3, 531−536. 29711

DOI: 10.1021/acsami.6b10597 ACS Appl. Mater. Interfaces 2016, 8, 29705−29712

Research Article

ACS Applied Materials & Interfaces (23) Ji, X. L.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries. Nat. Mater. 2009, 8, 500−506. (24) Evans, J.; Vincent, C. A.; Bruce, P. G. Electrochemical Measurement of Transference Numbers in Polymer Electrolytes. Polymer 1987, 28, 2324−2328. (25) Abraham, K. M.; Jiang, Z.; Carroll, B. Highly Conductive PEOlike Polymer Electrolytes. Chem. Mater. 1997, 9, 1978−1988. (26) Mohan, V. M.; Raja, V.; Bhargav, P. B.; Sharma, A. K.; Rao, V. V. R. N. Structural, Electrical and Optical Properties of Pure and NaLaF4 Doped PEO Polymer Electrolyte films. J. Polym. Res. 2007, 14, 283− 290. (27) Park, C.-W.; Ryu, H.-S.; Kim, K.-W.; Ahn, J.-H.; Lee, J.-Y.; Ahn, H.-J. Discharge Properties of All-Solid Sodium-Sulfur Battery Using Poly (ethylene oxide) Electrolyte. J. Power Sources 2007, 165, 450− 454. (28) Feng, S. W.; Shi, D. Y.; Liu, F.; Zheng, L. P.; Nie, J.; Feng, W. F.; Huang, X. J.; Armand, M.; Zhou, Z. B. Single Lithium-Ion Conducting Polymer Electrolytes Based on Poly[(4-styrenesulfonyl) (trifluoromethanesulfonyl)imide] anions. Electrochim. Acta 2013, 93, 254−263. (29) Park, C.-W.; Ryu, H.-S.; Kim, K.-W.; Hur, B.-Y.; Cho, K.-K.; Ahn, J.-H.; Lee, J.-Y.; Ahn, H.-J. Effect of Sulfur Electrode Composition on the Electrochemical Property of Lithium/PEO/ Sulfur Battery. Met. Mater. Int. 2004, 10, 375−379. (30) Ni’mah, Y. L.; Cheng, M.-Y.; Cheng, J. H.; Rick, J.; Hwang, B.-J. Solid-State Polymer Nanocomposite Electrolyte of TiO2/PEO/ NaClO4 for Sodium Ion Batteries. J. Power Sources 2015, 278, 375− 381. (31) Park, C. H.; Sun, Y.-K.; Kim, D.-W. Blended Polymer Electrolytes Based on Poly(lithium 4-styrene sulfonate) for the Rechargeable Lithium Polymer Batteries. Electrochim. Acta 2004, 50, 375−378. (32) Liu, Y. B.; Cai, Z. J.; Tan, L.; Li, L. Ion Exchange Membranes as Electrolyte for High performance Li-ion batteries. Energy Environ. Sci. 2012, 5, 9007−9013. (33) Zhang, P. F.; Li, M. T.; Yang, B. L.; Fang, Y. X.; Jiang, X. G.; Veith, G. M.; Sun, X.-G.; Dai, S. Polymerized Ionic Networks with High Charge Density: Quasi-Solid Electrolytes in Lithium-Metal Batteries. Adv. Mater. 2015, 27, 8088−8094. (34) Zhou, W. D.; Gao, H. C.; Goodenough, J. B. Low-Cost Hollow Mesoporous Polymer Spheres and All-Solid-State Lithium, Sodium Batteries. Adv. Energy Mater. 2016, 6, 1501802. (35) Girolamo, D. D.; Panero, S.; Navarra, M. A.; Hassoun, J. Quaternary Polyethylene Oxide Electrolytes Containing Ionic Liquid for Lithium Polymer Battery. J. Electrochem. Soc. 2016, 163 (7), A1175−A1180. (36) Chen, J. Z.; Han, K. S.; Henderson, W. A.; Lau, K. C.; Vijayakumar, M.; Dzwiniel, T.; Pan, H. L.; Curtiss, L. A.; Xiao, J.; Mueller, K. T.; Shao, Y. Y.; Liu, J. Restricting the Solubility of Polysulfides in Li-S Batteries Via Electrolyte Salt Selection. Adv. Energy Mater. 2016, 6, 1600160. (37) Zheng, J. M.; Gu, M.; Chen, H. H.; Meduri, P.; Engelhard, M. H.; Zhang, J.-G.; Liu, J.; Xiao, J. Ionic Liquid-Enhanced Solid State Electrolyte Interface (SEI) for Lithium-Sulfur Batteries. J. Mater. Chem. A 2013, 1, 8464−8470. (38) Bai, S. Y.; Liu, X. Z.; Zhu, K.; Wu, S. C.; Zhou, H. S. MetalOrganic Framework-Based Separator for Lithium-Sulfur Batteries. Nat. energy 2016, 1, 16094−16099. (39) Li, Z.; Zhang, J. T.; Lou, X. W (David). Hollow Carbon Nanofibers Filled with MnO2 Nanosheets as Efficient Sulfur Hosts for Lithium-Sulfur Batteries. Angew. Chem., Int. Ed. 2015, 54, 12886− 12890. (40) Fei, L. F.; Li, X. G.; Bi, W. T.; Zhuo, Z. W.; Wei, W. F.; Sun, L.; Lu, W.; Wu, X. J.; Xie, K. Y.; Wu, C. Z.; Chan, H. L. W.; Wang, Y. Graphene/Sulfur Hybrid Nanosheets from a Space-Confined “Sauna” Reaction for High-Performance Lithium-Sulfur Batteries. Adv. Mater. 2015, 27, 5936−5942.

(41) Xu, Y. H.; Wen, Y.; Zhu, Y. J.; Gaskell, K.; Cychosz, K. A.; Eichhorn, B.; Xu, K.; Wang, C. S. Confined Sulfur in Microporous Carbon Renders Superior Cycling Stability in Li/S Batteries. Adv. Funct. Mater. 2015, 25, 4312−4320. (42) Qie, L.; Manthiram, A. High-Energy-Density Lithium-Sulfur Batteries Based on Blade-Cast Pure Sulfur Electrodes. ACS Energy Lett. 2016, 1, 46−51. (43) Chen, L.; Shaw, L. L. Recent Advances in Lithium-Sulfur Batteries. J. Power Sources 2014, 267, 770−783. (44) Ma, L.; Hendrickson, K. E.; Wei, S. Y.; Archer, L. A. Nanomaterials: Science and Applications in the Lithium-Sulfur Battery. Nano Today 2015, 10, 315−338. (45) Du, W. C.; Yin, Y. X.; Zeng, X. X.; Shi, J. L.; Zhang, S. F.; Wan, L. J.; Guo, Y. G. Wet Chemistry Synthesis of Multidimensional Nanocarbon-Sulfur Hybrid Materials with Ultrahigh Sulfur Loading for Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces 2016, 8, 3584− 3590. (46) Mikhaylik, Y. V.; Akridge, J. R. Polysulfide Shuttle Study in the Li/S Battery System. J. Electrochem. Soc. 2004, 151 (11), A1969− A1976. (47) Suo, L. M.; Hu, Y.-S.; Li, H.; Armand, M.; Chen, L. Q. A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481−1489. (48) Zhang, S. S. A New Finding on the Role of LiNO3 in LithiumSulfur Battery. J. Power Sources 2016, 322, 99−105.

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