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Advanced Lithium Ion Sulfur Battery Based on Spontaneous Electrochemical Exfoliation/Lithiation of Graphite in Nonaqueous Electrolytes Pengcheng Shi,† Xin Zhou,† Yong Wang,† Xin Liang,† Yi Sun,† Sheng Cheng,† Chunhua Chen,‡ and Hongfa Xiang*,† †

School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, China CAS Key Laboratory of Materials for Energy Conversions, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China

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S Supporting Information *

ABSTRACT: An advanced lithium ion sulfur battery is constructed on the foundation of spontaneous electrochemical exfoliation and lithiation of graphite in nonaqueous electrolytes, in which to produce a high-quality graphene matrix for the sulfur cathode and lithiated graphite for the anode, respectively. On one hand, the high-quality graphene, obtained via spontaneous electrochemical exfoliation of graphite in propylene-carbonate-based electrolyte, works as a matrix to improve the conductivity of sulfur and trap the polysulfides. On the other hand, the lithiated graphite anode, obtained by spontaneous electrochemical lithiation of graphite in ethylene-carbonate-based electrolyte with 5 wt % fluoroethylene-carbonate additive, provides ample lithium ion sources and increases the safety coefficient of the lithium ion sulfur battery. Meanwhile, the fluoroethylene-carbonate additive can derive a LiF-rich solid electrolyte interphase on a lithiated graphite anode, which can in turn mitigate the notorious “shuttle effect”. More attractively, the spontaneous electrochemical exfoliation and lithiation method can produce high-quality graphene and lithiated graphite in large scale and without electric energy consumption. Therefore, this approach shows great potential for the configuration of an advanced lithium ion sulfur battery with enhanced safety and high energy density. KEYWORDS: electrochemical exfoliation/lithiation, lithium sulfur batteries, lithium ion sulfur batteries, shuttle effect, solid electrolyte interphase



INTRODUCTION The ever-growing market of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has required both academia and industry to develop lithium (Li) ion batteries (LIBs) with high safety and high energy density. For instance, in 2009, IBM launched the “Battery 500” project at the aim of developing advanced batteries to support a target range of 500 miles per charge and a cycle life of 600 cycles for EVs.1,2 To that aim, the development of alternative electrode materials with high capacity and long cycle life is of primary importance.3,4 Recently, sulfur (S), which is notable for its ultrahigh theoretical specific capacity of 1675 mAh g−1, has been regarded as a promising cathode material. Additionally, S is naturally abundant, nontoxic, and low cost.5,6 Therefore, it is highly desired to find a suitable anode for a S cathode. Generally, the most desired anode for a S cathode would of course be metallic Li in regard to battery energy density.7 However, safety problems arising from the growth of Li dendrites, which may pierce through the separator and result in an internal short circuit, have long prevented the application of Li−S batteries.8 For this reason, lithium ion sulfur batteries (LISBs) that replace Li with other anode materials show an © 2019 American Chemical Society

attractive perspective on improving the battery safety coefficient. Various alternative electrode materials such as Sn9,10 and Si3,11−14 have been employed to pair with S and be assembled as LISBs. Unfortunately, huge volume expansion of these materials during cycling always results in the pulverization of electrodes, the loss of active Li+, and the continuous growth of solid electrolyte interphase (SEI). Consequently, capacities of the as-assembled LISBs decay rapidly.15,16 Meanwhile, the complex synthetic routes also further prevented them from application. On the contrary, graphite seems to be a promising candidate, as it has been successfully applied in state-of-the-art LIBs for decades because of its limited volume expansion and stable SEI. These features are also beneficial for enhancing the cycle life of LISBs.17−19 Therefore, it holds great promise to investigate graphite−S full cells. In order to achieve high-capacity and high-energy-density graphite−S full cells, the electrochemical performances of both Received: March 5, 2019 Accepted: May 2, 2019 Published: May 2, 2019 3798

DOI: 10.1021/acsaem.9b00480 ACS Appl. Energy Mater. 2019, 2, 3798−3804

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ACS Applied Energy Materials

Figure 1. Configuration of LISB.



the graphite anode and S cathode should also be optimized. For the S cathode, the main challenge is the poor electronic conductivity, significant volume expansion, and the notorious “shuttle effect”.20−22 To tackle these issues, high-quality graphene has been proven to be a promising candidate because of its excellent conductivity, good mechanical flexibility, and high specific surface area.23,24 For the graphite anode, because of the lack of Li+, it must be prelithiated before being assembled into LISBs. Meanwhile, the SEI layer formed on the prelithiated graphite is thick, which can not sufficiently constrain the “shuttle effect” to a low level, consequently leading to a short cycle life and low Coulombic efficiency (CE) of the LISB. Therefore, in order to configure advanced graphite−S cells, it is still highly desired to develop an effective strategy to prepare a high-quality graphene matrix and minimize the notorious “shuttle effect”. Recently, our group reported a nonelectrified electrochemical method to produce high-quality graphene.25 It is via direct contact between graphite and Li in propylenecarbonate (PC)-based electrolyte to form a tremendous number of Li||graphite microcells, which would then spontaneously exfoliate graphite into high-quality graphene. After 8 h of reaction, the graphite is obviously exfoliated according to our previous investigation.25 Herein, as shown in Figure 1, we further develop this nonelectrified electrochemical method to prepare a high-quality graphene matrix and lithiated graphite (LG) anode for the configuration of an advanced LISB. The high-quality graphene, obtained via spontaneous electrochemical exfoliation of graphite in PC-based electrolyte, worked as a host matrix for S and and a trap for the polysulfides. Meanwhile, the LG electrode, obtained via spontaneous electrochemical lithiation of graphite in ethylene-carbonate (EC)-based electrolyte with 5 wt % fluoroethylene-carbonate (FEC) additive, severed as an anode to provide ample Li+ sources for the LISB. In addition, the FEC additive can derive a LiF-rich SEI on LG anode, which can in turn reduce the notorious “shuttle effect”. More attractively, the LG anode also has a safety advantage over conventional metallic Li. Last but not least, these kinds of spontaneous electrochemical reactions between graphite and Li can produce high-quality graphene and LG in large scale and without consumption of electric energy. Therefore, this method shows great potential for the configuration of an advanced LISB with high safety.

EXPERIMENTAL SECTION

Material Synthesis. The electrochemical exfoliation and lithiation of graphite are tightly associated with the SEI film formation abilities of electrolytes. For spontaneous electrochemical exfoliation, the electrolyte is 1.0 M LiPF6/PC.25 The collected graphene is further used to prepare S@graphene composite. Typically, 0.075 g of graphene, 1.2 g of Na2S2O3, and 0.5 mL of Trixon-100 were dissolved in 500 mL of deionized H2O under magnetic stirring. After 1 h, 60 mL of HCl (0.5 M) was slowly dropped. The product was then filtered and washed with deionized H2O. For spontaneous electrochemical lithiation, the electrolyte is 1.0 M LiPF6 in EC, ethyl methyl carbonate (EMC) (4:6 by weight ratio), and 5 wt % FEC. Typically, the graphite electrode is in direct contact with Li in the electrolyte to obtain an LG electrode. In order to form a good SEI film on the LG electrode, the lithiation process takes 5 h. The graphite electrode (the mass loading of graphite is controlled at ∼3.4 mg) consists of 85% MCMB, 5% KS-6 (conductive carbon additive, obtained from TIMCAL), 5% acetylene black, and 5% poly(vinylidene fluoride) (PVDF). Additionally, both the exfoliation and lithiation procedures were carried out in an argon-filled glovebox with both O2 and H2O contents controlled below 0.1 ppm. Electrochemical Measurements. The S@graphene cathode was made by mixing 70% S@graphene composite, 20% ketjen black, and 10% PVDF in N-methyl-2-pyrrolidone (NMP) solution; after ball milling for 30 min, it was then casted on Al foil. The electrode was then punched into disks with diameters of 10 mm after vacuum drying. The mass loading of S on each disk is controlled at about 0.8 mg cm−2. The S@graphene cathode and LG anode are further employed to construct the LISB (Coin 2032 type). The separator is a Celgard 2400 polypropylene membrane, whereas the electrolyte is 1.0 M lithium bis(trifluoromethane sulfonimide) (LiN(SO2CF3)2, LiTFSI) in a mixture of 1,3-dioxolane (DOL) and dimethyl ether (DME) (1:1 by volume ratio) with 2 wt % LiNO3. To standardize the testing, the electrolyte content is 80 μL per mg of S in each cell. The LISB was cycled between 1.8 and 2.8 V. For comparison, conventional Li−S batteries on the foundation of a S@graphene cathode and metallic Li anode have also been assembled and tested in the same way. The capacities of both batteries are calculated on the basement of S loading content. Characterization. Electrochemical impedance spectra (EIS) were performed on a CHI604D workstation, with a voltage perturbation of 10 mV and a frequency range of 0.1−105 Hz. X-ray diffraction (XRD, Cu Kα) was collected on an X’Pert PRO MPD instrument. The morphology and energy dispersive spectroscopy (EDS) mapping was obtained by scanning electron microscopy (SEM, Zeiss) and transmission electron microscopy (TEM, JEM-2100F), respectively. X-ray photoelectron spectroscopy (XPS) of the SEI layer is analyzed by a Thermo ESCALAB 250 instrument. Thermogravimetric (TG) analysis was performed in N2 atmosphere with a heating rate of 5 °C per min from 25 to 800 °C. 3799

DOI: 10.1021/acsaem.9b00480 ACS Appl. Energy Mater. 2019, 2, 3798−3804

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Figure 2. (a) XRD of the S@graphene composite. (b) TG curve of the S@graphene composite.



RESULTS AND DISCUSSION The key point for building an advanced battery is to successfully synthesize a stable and high-capacity cathode as well as a high-safety anode. In this work, we demonstrate the configuration of an advanced LISB by using a S@graphene cathode and LG anode, which was prepared on the basis of spontaneous electrochemical reactions between graphite and Li in nonaqueous electrolytes. Additionally, materials for the LISB are easily available as well as low cost, and the approach is facile and economic. First, high-quality graphene (shown in Figure S1), obtained via spontaneous electrochemical exfoliation of graphite in PCbased electrolyte, was used to prepare the S@graphene cathode of LISB. Characteristics of the as-prepared S@ graphene composite were first investigated by XRD. As shown in Figure 2a, the diffraction peaks of the S@graphene composite coincide well with the standard XRD pattern of S (JCPDS 00-008-0247),26 suggesting S has been successfully loaded on graphene. Notably, no peaks of graphite were observed, indicating the graphene still remained in a substantially exfoliated state. TG measurement is further used to identify the loading of S content of the S@graphene composite. As shown in Figure 2b, the weight loss within the temperature range between 150 and 250 °C of the TG curves indicates the S content is 69.5%. Such high loading of S content is satisfying for practical applications. The morphology and S distribution of S@graphene composite were further investigated by SEM and TEM. As shown in Figure 3a, S was wrapped efficiently by graphene sheets. Therefore, the graphene is helpful for preventing the dissolution of polysulfides and accommodating the volume expansion of S according to previous studies.27 The TEM image and EDS mapping in Figure 3b−d further revealed the homogeneous distribution of S on graphene. Hence, the graphene matrix can provide sufficient active sites for S. Benefitting from these advantages, the S@graphene composite would be favorable for achieving excellent electrochemical performances, discussed as follows. As discussed above, the low conductivity of S and dissolution of polysulfides is effectively solved by graphene wrapping. Another critical issue of the Li−S battery is the safety problems arising from the growth of Li dendrites as well as high intrinsic chemical reactivity of metallic Li in commonly used electrolytes. For this reason, an alternative configuration of an LG anode, which was prepared by spontaneous electrochemical lithiation of graphite in EC-based electrolyte,

Figure 3. Characteristics of S@graphene composite: (a) SEM, (b) TEM, and EDS mapping for the elements of (c) C and (d) S.

was used as anode for LISB. As shown in the inset image in Figure 4, the color of the graphite turned gold after lithiation, suggesting Li+ has been successfully intercalated into graphite. The lithiation of graphite can further be proven by XRD.28 In Figure 4, the peaks between 20−25° are ascribed to the Li−

Figure 4. XRD pattern and optical image (inset) of the LG electrode. 3800

DOI: 10.1021/acsaem.9b00480 ACS Appl. Energy Mater. 2019, 2, 3798−3804

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Figure 5. Electrochemical performances of the LG||Li cell in 1.0 M LiTFSI/DOL-DME (1:1, v/v) with 2% LiNO3. (a) Cycling performance and the corresponding Coulombic efficiency at 0.2C. (b) EIS curves after 50 cycles.

Figure 6. Electrochemical performances of the LISB and Li−S batteries. (a) Initial charge−discharge curves at 0.1C, (b) corresponding dQ/dV curves, (c) cycling performance and CE at 0.2C, (d) EIS curves after 130 cycles.

Li cells shows a distinct Li+ intercalation plateau. Therefore, LiNO3 can construct a stable SEI on an LG electrode for reversible Li+ intercalation/deintercalation. Meanwhile, the LG||Li cell in the electrolyte with 2% LiNO3 shows an initial charge capacity of 340 mAh g−1 and a high Coulombic efficiency (CE) of 97%. Notably, the capacity and CE remain at 340 mAh g−1 and 100% in Figure 5a, respectively. After 50 cycles, the resistance is only 5 Ω in Figure 5b, indicating that the structure of LG electrode remains stable in the electrolyte containing LiNO3. Therefore, the LG electrode yields a good compatibility with ether-based electrolyte containing LiNO3 and can preserve sufficient Li+ for the S@graphene cathode, which finally guarantees the possibility and the potential advantages of replacing Li by LG to construct an LISB with high safety. On the basis of the aforementioned results, the S@graphene cathode and LG anode are employed to construct an LISB. The electrochemical performances of the LISB are shown in Figure 6. For comparison, conventional Li−S batteries on the foundation of the S@graphene cathode have also been

graphite intercalation compound (stage 1 of LiC6 and stage 2 of LiC12). Additionally, the CV curve in Figure S2 also indicated Li+ has been successfully intercalated into graphite. Therefore, the LG anode has a high lithiation degree and can supply ample Li+ sources for LISB. Generally, the state-of-the-art electrolytes for Li−S batteries are based on ether solvents (e.g., DOL, DME). Unfortunately, these ether-based electrolytes are not suitable for a graphite electrode, because the cointercalation of Li+−solvent complexes would exfoliate graphite and consequently lead to the breakdown of graphite electrode.17 Nevertheless, the exfoliation can be suppressed by construct a stable SEI. Hence, it is essential to investigate the compatibility between the LG electrode and the 1.0 M LiTFSI/DOL-DME electrolyte. In Figure S3a,b, the initial charge curves of both cells in the electrolyte without and with 2% LiNO3 coincide well. Nevertheless, the corresponding discharge curves are different. The cell in the electrolyte without LiNO3 shows a plateau at ∼0.5 V, suggesting the exfoliation of graphite. By contrast, when 2% LiNO3 is adopted, the initial discharge curve of LG|| 3801

DOI: 10.1021/acsaem.9b00480 ACS Appl. Energy Mater. 2019, 2, 3798−3804

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Figure 7. SEM images of (a) Li and (b) LG electrode after cycling.

Figure 8. XPS spectra of various anodes after 130 cycles. (a) Li 1s; (b) F 1s; (c) C 1s; (d) O 1s.

anodic peak at ∼2.3 V, which may relate to the intercalation of Li+ into graphite. Therefore, Li+ can reversibly intercalate/ deintercalate from the LG electrode in a practical LISB. The cycling performance and CE of each cell is further compared in Figure 6c. The initial discharge capacity of the LISB and Li−S cell is 1193 and 1291 mAh g−1, respectively. The higher discharge capacity of Li−S cell is mainly because metallic Li can supply more Li+ for the S@graphene cathode as compared to that of LG anode. After 130 cycles, the discharge capacity of LISB is 714 mAh g−1 whereas that of the Li−S battery is 603 mAh g−1. The average CE of both batteries is about 99%. Additionally, the polarization of LISB is relatively lower as compared with that of the Li−S battery, as shown in Figure S4. The enhanced electrochemical performance can be attributed to the SEI on the surface of LG, which is more effective in suppressing the “shuttle effect”.Table S1 shows a comparison of the configuration and cycling performances of various LISBs.3,12,30−34 The performance of this battery is comparable with those of state-of-the-art LISBs. Nevertheless, only this

assembled and tested in the same way. In Figure 6a, both batteries exhibit the typical voltage profiles of two discharge plateaus and a charge plateau. Nevertheless, it is worth noting that the plateaus of LISB are ∼0.1 V lower than that of a Li−S battery. This is primarily because graphite has a higher Li+ intercalation/deintercalation potential of ∼0.1 V than that of Li. Typically, the discharge plateaus at 2.1−2.3 and 1.9−2.1 V correspond to the reduction of S to a long-chain polysulfide (Li2Sn, 4 ≤ n ≤ 8) and then to a short-chain polysulfide (Li2Sn, n ≤ 2). The charge plateau at 2.3 V is related to the oxidation of short-chain polysulfide to S.29 dQ/dV curves were further employed to understand the electrochemical process of the LISB. In Figure 6b, the cathodic peaks (2.0−2.3 V) of both cells correspond to the multiple-step reduction processes of S into short-chain polysulfide, whereas the anodic peaks at 2.15 and 2.3 V are assigned to the conversion of short-chain polysulfide to S for LISB and Li−S batteries, respectively. These peaks coincide well with the plateaus of the first curves in Figure 6a. Nevertheless, the LISB appears to have a new 3802

DOI: 10.1021/acsaem.9b00480 ACS Appl. Energy Mater. 2019, 2, 3798−3804

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as the anode to provide ample Li+ sources and increase the safety of LISB. Meanwhile, the LiF-rich SEI layer derived from FEC on the LG anode can mitigate the “shuttle effect” and improve the CE of LISB. Last but not least, this spontaneous electrochemical exfoliation and lithiation method can produce high-quality graphene and LG in large scale and without electric energy consumption. Therefore, this approach shows great potential for the configuration of an advanced LISB with enhanced safety and high energy density.

LISB is prepared from spontaneous electrochemical exfoliation and lithiation of graphite by tuning the electrolytes. Figure 6d shows the EIS curves of batteries after 130 cycles. Obviously, the RSEI of an LISB (56 Ω) is much smaller than that of Li−S batteries (95 Ω), suggesting the SEI on LG is more conductive for Li+ than the SEI layer on Li anode.35 Moreover, the lowfrequency slope angle of LISB is also larger than that of Li−S batteries, indicating that the LG electrode has better Li+ transfer kinetics than Li.36 The rate performance of the LISB and Li−S batteries is further compared. As shown in Figure S5, the advanced LISB does not compromise the rate performance as compared with that of the Li−S batteries. At 0.2C, the LISB can deliver a capacity of 842 mAh g−1. When the current density increased to 0.5 and 1C, the discharge capacity reduced to 728 and 232 mAh g−1, respectively. Nevertheless, when the current density is set back to 0.2C, the discharge capacity can return back to ∼750 mAh g−1. Therefore, the LG electrode is a promising candidate for replacing Li to construct LISB. The surface morphology of both anodes after cycling is further analyzed by SEM. Before the measurements, the electrodes were immersed in DME for 20 h and then rinsed with fresh DME several times to remove Li salts. In Figure 7a, a large amount of dendrites were observed on the surface of the Li anode. On the contrary, the LG electrode is dendritefree and covered by a dense and compact SEI layer in Figure 7b. Therefore, the LG electrode has a higher safety coefficient than a conventional Li anode. XPS was further carried out to accurately analyze the chemical composition of the SEI films on various anodes. The differences of the SEI layer can be obviously seen in the wide XPS scans and the corresponding element ratios in Figure S6. In Li 1s spectra, both samples contain a peak of LiNxOy (55.8 eV) in Figure 8a, which originated from the reduction of LiNO3.37 Additionally, the peaks located at 55.3 eV are species of LiF, Li2CO3, ROLi, and ROCO2Li.38,39 The difference of the SEI layer can also be proven by F 1s spectra in Figure 8b. Notably, the peak of −CF3 is ascribed to the residual LiTFSI. Compared with metallic Li anode, the LG electrode shows an intensity peak of LiF (684.9 eV),39 which mainly originated from the reduction of FEC during the electrochemical lithiation process. LiF has been proven to be good for improving the properties of SEI and preventing the growth of Li dendrites. Therefore, the LG electrode is good for reducing the “shuttle effect” and increasing the battery safety coefficient, in turn improving the performances of LISB. In C 1s spectra in Figure 8c, the peaks located at 288.9 and 285.1 eV are ascribed to Li2CO3 and ROCO2Li, respectively.37 The peaks located at 284.8 and 290.0 eV are corresponding to element of C and PVDF in LG anode.40 In O 1s spectra in Figure 8d, the peak located at 535.6 eV is LiNxOy, whereas peaks located at 531.4 and 531.7 eV are consistent with the presence of C−O and C=O containing species.41



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00480.



TEM image of the exfoliated graphene. CV curve of LG|| Li cell. Initial charge and discharge curves of the LG||Li cells in 1.0 M LiTFSI/DOL-DME electrolyte without and with 2% LiNO3. Charge and discharge curves of LISB and Li−S batteries. Rate performance of the LISB and Li−S batteries. Wide XPS scans and the corresponding element content. Electrochemical performance of various LISBs (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-62901457; E-mail: [email protected]. ORCID

Chunhua Chen: 0000-0001-9589-6329 Hongfa Xiang: 0000-0002-6182-1932 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This study was supported by the National Science Foundation of China (Grant Nos. 51372060, 21676067, and 21606065), the Opening Project of CAS Key Laboratory of Materials for Energy Conversion (KF2018003), the Anhui Provincial Natural Science Foundation (Grant No. 1708085QE98), and the Fundamental Research Funds for the Central Universities (Grant No. JZ2017HGTB0198, JZ2017YYPY0253).



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CONCLUSION An advanced LISB is constructed by using a S@graphene cathode and LG anode, which is prepared on the foundation of spontaneous electrochemical exfoliation and lithiation of graphite in nonaqueous electrolytes. On one hand, graphene, obtained from electrochemical exfoliation of graphite in PCbased electrolyte, worked as a conducting matrix to improve the conductivity of S and trapped polysulfides. On the other hand, the LG electrode, obtained by electrochemical lithiation of graphite in EC-based electrolyte with FEC additive, worked 3803

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DOI: 10.1021/acsaem.9b00480 ACS Appl. Energy Mater. 2019, 2, 3798−3804