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Facile Formation of a Solid Electrolyte Interface as a Smart Blocking Layer for High-Stability Sulfur Cathode

Junling Guo, Xinyu Du, Xiaolong Zhang, Fengxiang Zhang,* and Jinping Liu*

J. L. Guo, X. Y. Du, X. L. Zhang, Prof. F. X. ZhangState Key Laboratory of Fine Chemicals and School of Petroleum and Chemical EngineeringDalian University of Technology2 Dagong Road, Liaodongwan New District Panjin 124221, P. R. ChinaE-mail: [email protected]. J. P. LiuSchool of ChemistryChemical Engineering and Life Science and State Key Laboratory of Advanced Technology for Materials Synthesis and ProcessingWuhan University of TechnologyWuhan 430070, P. R. ChinaE-mail: [email protected]

DOI: 10.1002/adma.201700273

materials leads to low sulfur utilization and poor rate capability. Many designs have been used to overcome such draw-backs. For instance, (1) the PS binding ability of carbon materials can be improved by nitrogen doping,[14] hierarchical struc-tures,[15] graphene wrapping,[16] etc., and (2) the conductivity of polar materials can be enhanced by hydrogen reduction (hydro-genated TiO2)[17] or by unique carbon/polar material hybrid structures (hollow carbon nanofibers filled with MnO2 nanosheets,[18] carbon spheres coated by TiO2).[19] How-ever, the complexities of these manufac-turing processes reduce their feasibility.[20] Consequently, it is necessary to develop a simple but effective method that can sig-nificantly improve the cycle performance

of sulfur cathodes while maintaining a good rate capability.To minimize the diffusion of dissolved PSs from the sulfur

cathode, one intuitive method is to wrap the PSs with a dense encapsulating layer. However, to the best of our knowledge, this method has rarely been successfully implemented, which is likely due to the following difficulties. If sulfur is encapsu-lated in a completely closed space that impedes the contact of sulfur with the electrolyte, the insulated PSs will not dissolve, and the PSs will be deposited on the interface. This deposition will impede subsequent reactions of sulfur and PSs, which can only occur on the surfaces of the conductive materials.[21–25] If sulfur is encapsulated in a space that is not completely enclosed that allows sulfur to maintain contact with the electrolyte (like in conventional cathodes), the PSs will readily dissolve into the electrolyte and diffuse out of the cathode, leading to poor cycle performance. Based on the above analyses, an effective wrap-ping strategy will depend on the simultaneous encapsulation of sulfur and the electrolyte with a smart blocking layer, which can selectively allow Li ions to pass but prevents PS accumulation and suppresses the migration of the dissolved PSs to the anode. Thus, the blocking layer must be generated in the presence of the electrolyte. According to the literature, a blocking layer can be obtained on the carbon and Li anode surface of recharge-able lithium batteries in a very simple way[26–28] by using a solid electrolyte interface (SEI) generated when charging–dis-charging the anode in the electrolyte below 1.0 V. Such an SEI layer can prevent the anode from contacting the outside electro-lyte and can prevent further irreversible and unfavorable reac-tions between the anode and the electrolyte.[29–32]

Enlightened by the above strategy, we report a novel strategy to improve the cycle performance of the sulfur cathode by

The practical application of lithium–sulfur batteries (LSBs) is hindered by their poor cycle life, which stems mainly from the “redox shuttle reactions” of dissolved polysulfides. To develop a high-performance cathode for LSBs, encapsulation of polysulfides with a blocking layer is potentially straightfor-ward. Herein, a novel strategy is reported encapsulate sulfur and the electro-lyte together in porous carbon spheres by using a solid electrolyte interface (SEI) that can selectively sieve Li+ ions while efficiently avoiding polysulfide accumulation and suppressing undesired polysulfide migration. This strategy is simple, straightforward, and effective. The carbon/sulfur cathode only needs to be cycled a few times within a voltage window of 0.3–1.0 V to form such a smart SEI, allowing the resulting cathode to exhibit superior stability extending 600 cycles. This strategy can be combined with other existing advanced sulfur cathode designs to improve the overall performance of LSBs.

Li-S Batteries

Lithium–sulfur batteries (LSBs) are expected to satisfy various energy storage and conversion demands because their theoret-ical energy densities (2500 Wh kg−1) are much higher than those of existing lithium-ion batteries (≈200 Wh kg−1).[1–4] However, practical application of LSBs is hindered by their poor cycle life, which stems mainly from the well-known “redox shuttle reac-tions” of dissolved lithium polysulfides (PSs) between the sulfur cathode and the lithium anode. Considerable efforts have been devoted to minimizing the diffusion of dissolved PSs out of the sulfur cathode.[5,6] The most popular strategy has been employed nanostructured carbon materials with high specific surface areas for PS trapping via physical confinement effects.[7–9] Another effective method is to use polar materials for PS binding via chemical interactions.[10–13] Although these methods work well, the nonpolar nature of carbon often causes unsatisfactory cycle performance, and the low electrical conductivity of polar

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encapsulating sulfur and the electrolyte together in a porous carbon sphere with an SEI layer, which is generated simply at the carbon surface when charging–discharging the electrode at 0.3–1.0 V. Figure 1a–c illustrates the advantages of such a cathode. As Figure 1a shows, without an SEI, the dissolved PSs can diffuse easily out of the porous carbon spheres. However, if we simply confine sulfur in the carbon spheres without pre-encapsulating the electrolyte inside (Figure 1b), the PSs cannot dissolve and will precipitate at the carbon surface, impeding subsequent cathode reactions. By contrast, with sulfur and the electrolyte simultaneously confined by a complete SEI (Figure 1c), the SEI layer can efficiently prevent the dissolved PSs from diffusing out of the carbon spheres and minimize the shuttling effect. Meanwhile, the pre-existence of electrolyte in the Li-ion conductive SEI-wrapped carbon spheres could keep the cathode reactions progressively moving forward; this advantage, in combination with the high electrical conductivity of carbon, would result in high utilization of sulfur and fast reaction kinetics for the cathode. Consequently, our cathode exhibits ultralong cycle life and good rate capability. To our knowledge, up until now, an SEI layer has only been reported

for anode protection and dendrite preven-tion in rechargeable lithium batteries; an SEI layer has never been reported for applica-tion in a cathode of LSBs. Thus, this report is the first attempt to investigate an SEI on a cathode for polysulfide confinement.

We first used our SEI strategy on porous carbon spheres, which have often been used to accommodate sulfur.[33–36] The porous carbon spheres were synthesized by using a modified sol–gel process.[37] The mor-phology of the carbon spheres is shown in a scanning electron microscopy (SEM) image in Figure 1d and indicates that the carbon spheres have a smooth surface and an average diameter of 450–550 nm. Sulfur was infiltrated into the porous carbon spheres by using a melting–diffusion process. A trans-mission electron microscopy (TEM) image of the resultant sulfur-loaded carbon spheres (Figure 1e) reveals a uniform distribution of sulfur, which may give rise to adequate sulfur/carbon contact and thus improve the utilization of sulfur. X-ray diffraction (XRD) patterns (Figure 1f) further confirm the exist-ence of S8 in the porous carbon spheres, and the sulfur content was determined to be ≈70 wt% based on thermogravimetric analysis (TGA) results (Figure S1, Supporting Information). This level of sulfur loading is similar to that in typical carbon/sulfur (C/S) composite cathodes[38–40] and ensures the high capacity of the cathode.

The SEI was generated at the carbon sur-face by charging–discharging the cathode at 0.3–1.0 V. Before generating the SEI, we recorded the charge–discharge curves (begin-ning section of the voltage–time profile in

Figure 2a) for the pristine cathode within 1.5–3.0 V at a high rate (1 C, 1675 mA g−1) to minimize the loss of dissolved PSs; these curves show regular charge–discharge plateaus, con-firming that the pristine cathode is a standard LSB cathode and works well. After the cathode underwent three charge–dis-charge cycles within 0.3–1.0 V at 1 °C, an SEI layer was formed on the carbon sphere surface, as confirmed below.

Electrochemical impedance spectroscopy (EIS) was generally utilized as direct proof of SEI formation.[41,42] The EIS curves of our porous carbon sphere/sulfur cathode at three different charge–discharge states (marked in Figure 2a) are illustrated in Figure 2b–d. The corresponding equivalent circuits are also dis-played in the insets of these figures, where RSEI and CSEI stand for the resistance and capacitance of the SEI, respectively.[30] As shown in Figure 2b, the EIS curve of the cathode before discharge (state b) is a typical semicircle. After one discharge at 0.3–1.0 V, the cathode shows an EIS curve (Figure 2c; state c) that is significantly different from that shown in Figure 2b, but a typical twin-semicircle feature indicative of SEI forma-tion is not found.[43] After two cycles, the cathode still shows an EIS curve that is similar to the curve after one cycle (Figure S2,

Adv. Mater. 2017, 1700273

Figure 1. The schematic illustration of the PS trapping process in three porous carbon sphere/sulfur cathodes: a) without an SEI, b) without electrolyte inside, and c) with electrolyte inside and an SEI. d) SEM image of the porous carbon spheres. e) TEM image of the porous carbon sphere/sulfur composite. f) XRD patterns of sulfur, the porous carbon spheres and the porous carbon sphere/sulfur composite.



Supporting Information). Only when three cycles of charge–discharge were performed (state d) was a complete and stable SEI layer obtained, as indicated by the presence of two distinct semicircles in Figure 2d. We note that an SEI layer can also be built at other C rates, such as 0.1, 0.5, and 2 C (Figure S3, Sup-porting Information). With low C rates, more cycles are not necessary to establish a stable SEI. However, during longer SEI formation periods, more polysulfides will diffuse out, leading to subsequently lower reversible capacity.

As shown in the SEM images of the cathode at state b (Figure 2e,f) and state d (Figure 2g,h), the surfaces of the carbon spheres become rougher after three charge–discharge cycles within 0.3–1.0 V. Consistent with previously published findings, this observation implies that a surface SEI layer is present,[42–44] which is further indicated by the TEM images (Figure 2i,j)[45,46] and energy-dispersive X-ray spectroscopy (EDX) mapping results (Figure 2k–m). From the TEM images, the thickness of the SEI layer was ≈5–8 nm (Figure 2j), and the distribu-tion of F (a constituting element of the SEI, which is discussed later) on the carbon sur-face was uniform.

X-ray photoelectron spectroscopy (XPS) is also a powerful tool for probing SEI for-mation.[47–49] Figure 3 presents the XPS spectra of three porous carbon sphere/sulfur electrode samples obtained under different conditions: sample 1 was simply immersed in the electrolyte for 3 h; sample 2 experienced four charge–discharge cycles within 1.5–3.0 V (the normal voltage window of LSBs) at 1 C; and sample 3 was cycled three times within 0.3–1.0 V at 1 C. Inter-estingly, sample 3 exhibits a C1s peak at 288.0–292.0 eV (Figure 3a), which is dis-tinct from the other two samples; this peak is deconvoluted into two components at 290.4 and 289.6 eV[22,38] that can be assigned to Li2CO3 and ROCO2Li, respectively (the most common components of an SEI).[22,38] The other deconvoluted C1s peaks are all located at ≈284.0–287.0 eV and were attrib-uted to CC (graphite, 284.4 eV), CO or amorphous carbon (285.0 eV), and COH (286.0 eV).[38] Sample 3 also shows an intense O1s peak at 532.0 eV (Figure 3b), which was not detected in the other two samples; this peak can be assigned to Li2CO3, and the three deconvoluted weak peaks in Figure 3b corre-spond to CO (531.0 eV), CO (532.5 eV), and COH (533.0 eV).[20,38,44,50] The Li1s spec-trum of sample 3 (Figure 3c) can be deconvo-luted into peak components corresponding to LiO (54.0 eV), LiOC (55.0 eV), LiCO3 (55.5 eV), and LiF (56.0 eV). Finally, the F1s spectrum of sample 3 features a sharp peak

at 686.5 eV that can be attributed to LiF, which is another main component of an SEI layer (Figure 3d).[44,50] The above XPS data agree well with those of previously reported SEI layers formed on Li anodes in a 1,3-dioxolane (DOL)/dimethoxymethane (DME) electrolyte.[30] Such an SEI cannot be formed in the cathode that is simply immersed in the electrolyte or cycled within 1.5–3.0 V (see EIS analysis in Figure S4 in the Supporting Information).

Next, the electrochemical performances of the pristine and SEI-wrapped porous carbon sphere/sulfur cathodes were

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studied. The sulfur contents in the two cathodes were com-parable to each other (16.3% vs 15.9%, Figure S5, Supporting Information). The first charge–discharge curve at 0.2 C (Figure 4a) of the SEI-wrapped cathode shows a slightly higher polarization than the pristine cathode due to the insulating nature of the SEI. Fortunately, the SEI-wrapped cathode still shows a rate performance (1205, 824, 631, and 454 mAh g−1 at 0.2, 0.5, 1, and 2 C, respectively; Figure 4b) that is compa-rable to the performance of the pristine cathode (Figure S6, Supporting Information). This comparable performance occurs because carbon can maintain high electrical conductivity, and the well-dissolved but trapped polysulfides can facilitate the cathode reactions. The cycle performances of the two cathodes at 0.5 C are displayed in Figure 4c. For the pristine cathode, a capacity of only 50% is retained after 200 cycles. By contrast, the SEI-wrapped cathode shows 75% capacity retention (relative to the highest capacity at the 9th cycle) after 400 cycles. At the higher rate of 2 C (Figure 4d), only a capacity of 57% is retained after 300 cycles for the pristine cathode, but a capacity of 71% (relative to the highest capacity at the 13th cycle) is maintained

for the SEI-wrapped cathode, even after 600 cycles. The cycle performance of our SEI-wrapped cathode is also impressive relative to the cycle performances recently reported for cathodes of sulfur embedded in a graphene/carbon composite,[16] N-doped carbon,[14] and a metal oxide/carbon composite,[18] as listed in Table S1 in the Supporting Information.

Apart from the above difference in capacity retention, the apparent difference in the capacity fading modes of the two cath-odes is also interesting to note. The capacity of the pristine cathode decreases consistently during the cycle process, but the capacity of the SEI-wrapped cathode remains almost unchanged after a short “increase-and-decrease” period within the first 25 cycles. The SEI-wrapped cathode must be discharged below 1.0 V to generate the SEI before normal charge–discharge operation in the voltage range of 1.5–3.0 V. During the low-voltage discharge, some lithium polysulfides may diffuse out of the carbon spheres and can be activated at the early stages of the sub-sequent normal charge–discharge, leading to an increased capacity in the first few cycles; however, these polysulfides are not confined by the SEI, causing a capacity decay in the following 10–25 cycles. After this process, the cycling of the SEI-wrapped sulfur cathode is relatively stable.

To further understand how the SEI wrapping improves the cycle stability, we recorded TGA curves of the pristine and SEI-wrapped cathodes after different cycles at 2 C (Figure 5). The TGA curves allowed us to determine the remaining ratios of sulfur in the cathodes. In addition, the photos of the corresponding separators are also provided

in the insets of Figure 5. Since the separator is in close contact with the cathode, the dissolution of lithium polysulfides from the cathode will render the separator yellow; thus, the degree of polysulfide dissolution can be indicated from the color of the separator.[19,51] Figure 5a shows that the sulfur content of the pristine cathode determined by TGA decreased continuously during the cycle process, while the sulfur content of the SEI-wrapped cathode remained almost unchanged after 20 cycles (Figure 5b). In accordance with these results, the yellowish area on the separator of the pristine cathode increased during the cycle process, and the color of the separator of the SEI-wrapped cathode changed little after the initial cycles, suggesting that the SEI can efficiently suppress undesired polysulfide migration.

Our SEI wrapping strategy is also versatile and can be extended to other cathode structures, such as commercial carbon nanotube (CNT)/sulfur composites. After charging–discharging, the CNT/sulfur cathode at 1 C within 0.3–1.0 V for two cycles, a similar SEI layer can be readily generated, as evidenced by the EIS and TEM results in Figure S7a–c. As expected, the cycle performance of the SEI-wrapped CNT/sulfur

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cathode is greatly improved (Figure S7d, Supporting Informa-tion). In detail, for the pristine cathode, a capacity of only 48% is retained after 30 cycles, while the SEI–CNT/sulfur cathode shows 84% capacity retention (relative to the highest capacity at the 5th cycle), even after 100 cycles.

Based on the above analysis, the SEI wrapping strategy is very efficient and in general enhancing the cycle stability of sulfur electrodes. We also note that the thin insulating SEI layer may affect the electron transport efficiency in the powder electrode (Scheme S1a, Supporting Information) and thus may slightly deteriorate other properties of the electrode, such as the Coulombic efficiency (Figure 4c; ≈95%; nevertheless, this effi-ciency is still comparable to most previous values).[52] To fur-ther optimize the properties of the sulfur cathode, we designed a 3D CNT array structure on carbon cloth to accommodate sulfur and fabricated an SEI-wrapped CNT/sulfur array cathode (Figures S8 and S9, Supporting Information). This cathode, differing from the powder cathode, allows electron transport from the current collector (carbon cloth) to the CNTs and sulfur without the need to pass through the SEI layer since the CNTs are grown directly on the current collector (Scheme S1b, Sup-porting Information). Consequently, the first charge–discharge

curves of the CNT/sulfur array cathodes with and without SEI show virtually the same polarization (Figure S8i, Supporting Infor-mation). Furthermore, the SEI-wrapped CNT/sulfur array electrode simultaneously demonstrates excellent rate performance (1275, 1081, 889, and 667 mAh g−1 at 0.2, 0.5, 1, and 2 C, respectively; 52.3% of the capacity retained with the rate increased ten times from 0.2 to 2 C), high Coulombic efficiency (≈99% after four activation cycles) and good cycle stability (81.4% capacity retention after 200 cycles; by contrast, only 54.5% retention after 50 cycles was observed for the array cathode without the SEI; Figure S8j,k, Sup-porting Information).

In summary, we have designed a novel strategy to improve the cycle performances of sulfur cathodes by encapsulating sulfur and the electrolyte together in a nanocarbon structure with an SEI layer. Differing from previous designs and fabrication of compli-cated sulfur-confining cathodes, our strategy is simple, straightforward, effective, and easy to scale up. The SEI blocking layer not only prevents polysulfide accumulation and the migration of polysulfides but also selectively sieves Li+ ions. Combined with the merits of highly conductive carbon, the SEI-wrapped carbon/sulfur cathode exhibits an impressive cycle stability and good rate performance. Our work opens up a new avenue for the construction of high-performance sulfur cathodes.

Experimental SectionMaterials Preparation—Porous Carbon Sphere: Porous carbon

spheres were prepared via a templated sol–gel process. First, 0.15 mL of aqueous ammonia solution was added into a solution containing 0.15 g of resorcinol, 0.11 g of cetyltrimethylammonium bromide, 6.0 mL of ethanol, and 15 mL of deionized water. This mixture was stirred vigorously for 30 min followed by the addition of 0.6 mL of tetraethyl orthosilicate and 0.21 mL of formaldehyde solution (37 wt%). The resultant solution was stirred at 25 °C for 24 h and then transferred into a Teflon-lined autoclave. The solution was further heated at 100 °C for 24 h, during which time resorcinol–formaldehyde polymer powders containing silica cores were formed. The powders were collected by centrifugation and washed with ethanol and deionized water. After drying the powders at 60 °C overnight, they were carbonized at 800 °C for 3 h under flowing N2. Finally, the carbonized powders were etched with an aqueous solution of 8 m NaOH for 6 h at 100 °C to remove the SiO2 cores, generating porous carbon spheres.

Materials Preparation—Porous Carbon Sphere/Sulfur and CNT/Sulfur Composites: The porous carbon sphere/sulfur (or CNT/sulfur) composite was synthesized by heating a mixture containing 10 mg of porous carbon sphere (or CNT) and 40 mg of sulfur at 155 °C for 12 h under vacuum. During this process, sulfur melted and diffused into the pores of the carbon spheres.

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Adv. Mater. 2017, 1700273

Materials Preparation—Porous Carbon Sphere/Sulfur and CNT/Sulfur Cathodes: First, the porous carbon sphere/sulfur composite (or CNT/sulfur composite), Super P (conductive agent), and polyvinylidene difluoride (binder) were mixed (8:1:1 by weight) in the presence of N-methyl-2-pyrrolidone to form a homogeneous slurry. The slurry was then casted on aluminum foil and heated at 80 °C for 12 h under vacuum to obtain the porous carbon sphere/sulfur or CNT/sulfur cathode.

Materials Preparation—CNT Array on Carbon Cloth (CC): Nickel nitrate hexahydrate was first dissolved in a 50 mL mixed solution of alcohol and ethylene glycol (1:1, v/v) under stirring. Then, a piece of CC (1 cm x 4 cm) was immersed into the above solution for 1 h. After this, the treated CC was dried and further heated at 800 °C in a tube furnace for 1 h under a flowing N2 atmosphere with an 18 mL solution of ethanol and ethylene glycol (1:5, v/v) placed upstream.

Materials Preparation—CNT/Sulfur Array Cathode: A 1% sulfur solution in toluene was first dropped onto the CNT array on CC, which was then dried on a hot plate for 20 min. After this, the CC was kept at 155 °C for 12 h and finally at 200 °C for 2 h under vacuum to obtain the CNT/sulfur array cathode.

Materials Characterizations: The morphologies of the as-prepared electrodes were studied using an FEI NanoSEM-450 Nova scanning electron microscope with a field emission gun source. XRD (Shimadzu, XRD-7000S) with Cu Kα radiation (λ = 1.5416 Å) was used to probe the composition and phase information. The sulfur content was determined using TGA (SDT, Q600) with a heating rate of 10 °C min−1 from room temperature to 500 °C under a flow of N2. The sulfur distribution in the carbon spheres was investigated using an FEI Tecnai G2 F30 transmission electron microscope. XPS characterization was implemented with an ESCALAB 250Xi ThermoFisher X-ray photoelectron spectrometer.

Battery Assembly and Measurements: Batteries were assembled in an argon-filled glove box. The electrochemical performances were tested using 2025 coin cells with a porous carbon sphere/sulfur (or CNT/sulfur) electrode as the cathode, Li-metal circular foil (0.59 mm thick) as the anode, and a polypropylene membrane as a separator. The electrolyte used in these coin cells comprised of 1 m lithium bis(trifluoromethanesulfonyl)imide in a mixture of DOL and DME

(1:1, v/v) and 0.2 m of LiNO3. A multichannel battery tester (Shenzhen Neware Technology Co., Ltd, China) was used to test the electrochemical performances of the 2025 coin cells at 20 °C. EIS measurements were performed using a CS310 electrochemical workstation with a frequency range from 0.01 Hz to 100 kHz.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by grants from the National Natural Science Foundation of China (Grant Nos. 21276252 and 51672205), the National Key Research Program of China (Grant No. 2016YFA0202602), the Fundamental Research Funds for the Central Universities (Grant No. DUT14RC(3)020), and the Fund of State Key Laboratory of Fine Chemicals Panjin (Grant No. JH2014009).

Conflict of InterestThe authors declare no conflict of interest.

Keywordslithium polysulfide blocking, Li–sulfur batteries, solid electrolyte interfaces

Received: January 13, 2017Revised: March 22, 2017

Published online:

Figure 5. TGA curves and digital photos of the corresponding separators (insets) of a) the pristine and b) SEI-wrapped porous carbon sphere/sulfur cathodes after different cycles. Note that the mass loss between 50–156 °C is attributed to the breakdown of the SEI film, and the mass loss between 455–500 °C corresponds to the decomposition of lithium alkyl oxide in the SEI.[49–51]

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