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PVP-Assisted Synthesis of Uniform Carbon Coated Li2S/CB for High Performance Lithium Sulfur Batteries Lin Chen, Yuzi Liu, Fan Zhang, Caihong Liu, and Leon Shaw ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07331 • Publication Date (Web): 03 Nov 2015 Downloaded from http://pubs.acs.org on November 7, 2015

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ACS Applied Materials & Interfaces

PVP-Assisted Synthesis of Uniform Carbon Coated Li2S/CB for High Performance Lithium Sulfur Batteries Lin Chen,a,b Yuzi Liu,c Fan Zhang, a,b Caihong Liua,b and Leon L. Shaw*,a,b a

Wanger Institute for Sustainable Energy Research, Illinois Institute of Technology, IL 60616 b

Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, IL 60616 c

Center for Nanoscale Materials, Argonne National Laboratory, IL 60439

ABSTRACT The lithium sulfur (Li-S) battery is a great alternative to the state-of-the-art lithium ion batteries due to its high energy density. However, low utilization of active materials, insulating nature of sulfur or lithium sulfide (Li2S), and polysulfide dissolution in organic liquid electrolyte lead to low initial capacity and fast performance degradation. Herein, a facile and viable approach to address these issues is proposed. This new approach entails synthesis of Li2S/carbon black (Li2S/CB) cores encapsulated by a nitrogen-doped carbon shell with PVP assistance during synthesis. Using EFTEM elemental mappings and FTIR measurement, it is confirmed that the as-synthesized material has a structure of a Li2S/CB core with a nitrogen-doped carbon shell (denoted as Li2S/CB@NC). The Li2S/CB@NC cathode yields an exceptionally high initial capacity of 1020 mAh/g based on Li2S mass at 0.1 C with stable Coulombic efficiency of 99.7% over 200 cycles. Also, cycling performance shows the capacity decay per cycle as small as 0.17%. Most importantly, to further understand the materials for battery applications, FETEM and elemental mapping tests without exposure to air for Li2S samples in cycled cells are reported. Along with the first ever FETEM and FESEM investigations of cycled batteries, Li2S/CB@NC cathode demonstrates the capability of robust core-shell nanostructures for different rates and improved capacity retention, revealing Li2S/CB@NC designed here as an outstanding system for high performance lithium sulfur batteries. Keywords: Lithium sulfide, core shell, lithium sulfur batteries, cycled cells, post testing * Corresponding author: Leon Shaw, [email protected]

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1. Introduction Environment pollution issues and energy demands urge the increased development of sustainable energy, such as wind energy and solar power.1-4 Rechargeable batteries to store energy are critical for these intermittent energy sources and essential for electric vehicles.5-7 The rechargeable batteries can thus be power sources to provide electricity for portable devices, electric vehicles, 810

grid system11 and even house uses. Lithium ion batteries (LIBs), due to their high energy density

and rechargeable properties,2 have been studied extensively and developed for commercial applications in the last 30 years. However, the capacity and energy density of state-of-the-art LIBs (such as LiCoO2/graphite) achieved so far have been close to theoretical values, leaving little room for further evolution. Seeking for cost-effective and outstanding alternatives to LIBs is therefore urgent to meet the increased demand on energy while alleviating environment pollutions. The lithium sulfur (Li-S) battery, based on sulfur mass and assumption of complete reactions to lithium sulfide (Li2S), has a theoretical specific capacity of 1673 mAh/g, five times that of its Liion counterpart (274 mAh/g based on LiCoO2).12 Also, its superior specific and volume energy densities, which are 2500 Wh/kg and 2800 Wh/L,13,14 respectively, have attracted extensive interests in academia and industry.15 However, some tricky issues have to be addressed; for example, insulating nature of sulfur makes electron transport during electrochemical processes hard to reach individual particles, resulting in low utilization of active materials. It has been well established that nanoscale and nanostructured materials enable reduced pathway for electron transport and ion diffusion and thus favor the kinetic process. Another challenge in the cathode side is the relocation of sulfur based materials and dissolution of polysulfides,16 causing loss of active materials, shuttling behavior,17,18 and capacity fading.15 Lithium sulfide (Li2S) has a theoretical specific capacity of 1166 mAh/g.19 It, as the starting cathode material, possesses advantages of no volume expansion because the first electrochemical reaction of Li2S is delithiation. By confining the delithiation and lithiation of Li2S within micronsized particles encapsulated with a conductive shell, it is supposed that a stable interface between particles can be established, leaving no mechanical stress to the binder, avoiding electrode pulverization and thus preventing fast performance degradation. In addition, Li2S cathodes enable 2 ACS Paragon Plus Environment

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pairing with high capacity anode without lithium metal,20 which is the source of lithium dendrite and hazard safety problems. Nevertheless, the aforementioned insulating nature and polysulfides dissolution challenges of S-based cathodes are also present in Li2S-based cathodes. In order to solve these issues, recently our group reported a processing method to synthesize submicron-sized Li2S encapsulated by nitrogen-doped carbon. Such an engineered core-shell structure exhibits an ultrahigh initial discharge specific capacity (1029 mAh/g based on Li2S mass).21 Unfortunately, fast capacity decay was observed during electrochemical cycles, suggesting that the quality of the nitrogen-doped carbon shell needs improvement to suppress polysulfides dissolution. Polyvinylpyrrolidone (PVP) has been widely applied as dispersing agent22 and capping agent23 in applications including rechargeable batteries. The polymer is easily soluble in water with a hydrophilic head and a long hydrophobic tail, which could form steric hindrance and disperse the materials it attaches to. Lee, et al. used PVP to stabilize and disperse colloidal nanoparticle clusters through repulsive steric interactions and directed the clusters into a more ordered structure.24 It was also employed as a structure directing agent when the uniformity and range of particle sizes is hard to control.25 Herein, we have designed a novel approach with PVP acting as a capping and dispersing agent when fabricating Li2S/CB@NC. The final composite has a mass content of 70% Li2S and a uniform and thick nitrogen-doped carbon shell, which serves as a protection to mitigate out-flow of polysulfides into electrolytes. A small decay of 0.17% per cycle and highly stable Coulombic efficiency over 99% with 200 cycle life have been realized. Further, the microscopy investigations of cycled cells show unambiguously sustainable core-shell nanostructures of Li2S/CB@NC even after fast charge-discharge processes at 2 C, and illustrate the excellent capability of this material for long cycle stability.

2. Experimental 2.1. Synthesis of samples 1 g Li2S (Alfa Aesar) was mixed with 20 g steel balls and 0.75 g carbon black (CB) in a canister sealed in an Ar-filled glove box (both of H2O and oxygen levels lower than 0.1 ppm). The mixture was mechanically milled using a SPEX 8000M Mixer/Mill machine for 6 hours. The materials were collected and marked as Li2S/CB. 0.1 g Li2S/CB was subsequently loaded into a quartz 3 ACS Paragon Plus Environment


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crucible with 0.015 g PVP, and 0.6 mL pyrrole was subsequently added to the crucible with gently stirring at room temperature for around an hour. Then the crucible was put in a stainless steel autoclave and sealed in the Ar-filled glove box followed by heat treatment at 600 °C for 4 hours. The as-prepared materials were obtained after being cooled down and marked as Li2S/CB@NC. All of these procedures were completed in the glove box or Ar atmosphere without contact of oxygen and moisture. For comparison, we also obtained a similar type of Li2S/CB@NC in the same conditions but without PVP assistance during the synthesis. To avoid confusion, Li2S/CB@NC represents the one with PVP assistance during the synthesis in this paper, whereas a clear claim for Li2S/CB@NC without PVP assistance will be made.

2.2. Materials Characterization X-ray diffraction (XRD) patterns were determined from 10–80 degree using a Bruker D2 diffractometer with CuKα radiation. Li2S particles were sealed in capillary tubes to avoid contact of air during XRD data collection. Scanning electron microscope (SEM) characterizations with both copper and polished aluminum as substrates were obtained from JEOL field-emission SEM 7500F in the Center for Nanoscale Materials (CNM) at Argonne National Laboratory (ANL). The core-shell structure of the as-synthesized composites was confirmed with a transmission electron microscope (TEM) of JEOL 2100F at CNM. Samples were dispersed in 1,3-dioxolane (DOL) and sonicated for 20 minutes, then dropped onto grids in the glovebox before being loaded into TEM. The elemental distribution was confirmed by energy-filtered TEM (EFTEM) elemental mapping. A Gatan environmental transfer holder was employed to keep the TEM specimen from air and moisture. Fourier transform infrared (FTIR) analysis was conducted using a Nicolet, Nexus-IR 470 spectrometer in the ATR mode. As Li2S is not IR active, the FTIR analysis here is to survey the nitrogen doped carbon. To prepare FTIR samples, the as-synthesized Li2S/CB@NC powder was placed in a fume hood to allow Li2S to react with air, eliminating the attack of hydrogen sulfide (H2S) produced from Li2S to FTIR instrument and human being. After several hour exposure, the powder was added onto the ATR crystal with full and intimate contact and the range of the spectrum was set from 4000 to 1000 cm-1. XPS investigations protected in a special environmental holder from air were performed with a Kratos Axis-165 XPS instrument equipped with an Al source (1486.6 eV).


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For post-test microscopy investigations, the cell cycled with different rates was studied. The cell was opened using a crimping machine in an Ar-filled glovebox and the Li2S/CB@NC cathode on aluminum foil was pictured and cut with a small part for SEM analysis. The remaining cathode was washed with DOL for three times, followed by heating at a vacuum oven for 6 hours to remove residue organics. A small piece of this washed and dried cathode was cut for SEM analysis as well. The uncycled cathodes were also examined using SEM for comparison. The time of exposure to air for every sample transfer into SEM vacuum chamber was less than 10 seconds. The samples for post-test TEM analysis were directly attached from the cycled electrodes onto TEM grids and then sealed completely in vials. FETEM and EFTEM elemental mapping studies for cycled Li2S/CB@NC were not exposed to air at all thanks to the Gatan transfer holder.

2.3. Electrochemical Measurements The Li2S/CB@NC powder was mixed with polyvinylidene fluoride (PVDF) and CB (8:1:1, mass ratio) in an Ar-filled glove box followed by using N-Methyl-2-pyrrolidone (NMP) to prepare a slurry and then casted on an aluminum foil as the current collector. The typical mass loading of active materials for assembled coin cells was about 0.8 mg cm-2. The electrode for the control sample without PVP assistance during the synthesis was prepared in the same way and the Li2S loading was kept the same as Li2S/CB@NC. The drying process of cathodes was completed at 110 °C in a vacuum oven for 12 hours. Lithium metal was used as the counter electrode and copper foil was applied as the current collector in the anode side. The electrolyte contained 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt in 1,2-dimethoxyethane (DME) and 1,3dioxolane (DOL) solvents (1:1 v/v) with LiNO3 (1 wt%) as additives, 20 µL of which was used for each cell. For assembly of coin cell CR2032, Celgard 2325 membrane was employed as the cell separators. All the specific capacities presented in this study were based on the mass of lithium sulfide in cathodes.

Electrochemical impedance spectroscopy (EIS) of coin cells with ball milled Li2S, Li2S/CB@NC without PVP assistance and Li2S/CB@NC with PVP assistance were evaluated through Parstat 4000 (Princeton Applied Research) in the frequency range of 1 MHz to 0.1 Hz upon open circuit state of batteries. Cyclic voltammetry (CV) data were obtained using the same Parstat 4000 with coin cells initially charged from the open circuit voltage (OCV) to 4.0 V and then the voltage was 5 ACS Paragon Plus Environment


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switched between 1.6 and 2.9 V at a scan rate of 0.025 mV/s. Fresh cells were fabricated and cycling performances were measured employing Arbin BT2000. All charge-discharge processes were started with a slow charge rate of 0.05 C to 4 V as the cut-off voltage20 and then cycled between 1.6 and 3.0 V at the desired current rate.

3. Results and Discussions Figure 1a shows the schematic diagram of PVP assisted synthesis of Li2S/CB@NC. As illustrated, the first step of the process is to form submicron-sized Li2S/CB cores through high-energy ball milling. During this step nanosized CB (~50 nm) will be occluded into submicron-sized Li2S particles because of the repeated fracture and cold welding of Li2S during high-energy ball milling.21 The subsequent mixing of Li2S/CB with PVP and pyrrole will result in Li2S/CB composite cores coated with PVP and pyrrole. Here, PVP functions as a capping and dispersing agent to disperse Li2S/CB particles uniformly in liquid pyrrole and bridge tightly between Li2S/CB and pyrrole. PVP, as shown in the diagram, contains two parts, a hydrophilic head and a hydrophobic tail. Upon mixing, the hydrophilic head would interconnect with Li2S while the hydrophobic tail attaches to pyrrole in the liquid state so that pyrrole can form a uniform layer on the surface of the Li2S/CB core. Figure 1b and 1c display FESEM images of Li2S/CB@NC on polished aluminum and commercial copper foils as substrates, respectively. Most particles exhibited are in the submicrometer range (100 to 500 nm), confirming that high-energy ball milling has reduced micrometer-sized Li2S particles to submicrometers. It is noted that some particles as small as 80 nm are also present, as indicated by arrows in Figure 1b and 1c. Particles of such small sizes (80 to 500 nm) can effectively reduce the distance for both electron and ion transport. Also, the CB occluded inside the Li2S/CB composite particles can offer a conductive network, enabling fast transfer of electrons and thus promoting high utilization of Li2S. It is well known that particle morphology derived from high-energy ball milling is irregular. However, it is interesting to see that many sphere-like particles are observed in SEM images, which, we believe, is because PVP enhances uniform interconnections of pyrrole with the core composite readily.


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Figure 1. (a) Schematic diagram of PVP-assisted synthesis of Li2S/CB@NC; (b) FESEM image of Li2S/CB@NC using a polished Al as the substrate, and (c) FESEM image of Li2S/CB@NC using a copper foil as the substrate. Arrows indicate particles of ~ 80 nm.

Figure 2a shows XRD results of the as-prepared materials in a capillary tube which is for protection of Li2S from oxygen and moisture. The data is consistent with Li2S pattern (JCPDS Card No. 261188)26 and there is no other peak presented, demonstrating that Li2S is completely unchanged during synthesis and handling processes. It also illustrates that the CB in the composite core and the nitrogen-doped carbon are amorphous. FTIR spectra to investigate nitrogen groups within 7 ACS Paragon Plus Environment


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carbon shell are depicted in Figure 2b. Given that lithium sulfide is not IR active and our objective is focusing on nitrogen-doped carbon, the ball-milled Li2S and Li2S/CB@NC were treated by exposure to air for sufficient time. Li2S/CB@NC displays peaks at ~1250 cm-1 and ~1640 cm-1, corresponding to C-N27 and C=N28,29 stretching bonding, respectively, whereas Li2S/CB without nitrogen doped coating does not have these peaks, revealing that the carbon coating from pyrrole is nitrogen-doped. It has been widely reported that nitrogen is favorable to electron migration,30,31 readily enhancing electronic conductivity of the sample.

Capillary Tube

Figure 2. (a) XRD patterns of Li2S/CB@NC; (b) FTIR for samples of Li2S/CB with and without nitrogen-doped carbon coating; (c) XPS spectra of C1s of Li2S/CB@NC; and (d) XPS spectra of N1s of Li2S/CB@NC.


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The chemical bonding of carbon (Figure 2c) and nitrogen (Figure 2d) of Li2S/CB@NC is further examined using XPS. The C1s signal can be divided into three different peaks: the C-C bond (284.7 eV) of sp2 carbon, C-N group (286 ev), and C=N bond (290.1 ev).32,33 The N1s spectrum contains two contributions with binding energies at 398.4 eV and 400.8 eV, which are assigned to pyridinic-N and pyrrolic-N, respectively, suggesting that nitrogen is doped well in Li2S/CB@NC and the results are in good accordance with FTIR. The content ratio of nitrogen over carbon is 5.36%, the calculation of which can be found in Table S1 (Supporting Information).

Figure 3. (a) FETEM image, (b) carbon mapping, (c) sulfur mapping, and (d) combination of EFTEM elemental mappings.



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Figure 4. (a) FETEM image of a Li2S/CB@NC particle with size of ~80 nm; (b) EFTEM carbon mapping; (c) EFTEM sulfur mapping; and (d) combination of elemental mappings of the particle.

TEM was employed in order to confirm the nanostructure. Many particles with sizes ranging from 100 to 400 nm are viewed in Figure 3a. Some of the particles, especially those without overlapping, exhibit a dark center with light edges. Further, the dark center is uniformly encapsulated by light edges of ~22 nm in thickness. EFTEM elemental mapping results shown in Figure 3b and 3c display carbon and sulfur mapping, respectively. The bright contrast in each figure indicates the presence of the element of interest in the particles. The brighter the contrast, the higher concentration of the element of interest. Signals of carbon element are very strong around edges, illustrating undoubtedly the uniform presence of a carbon shell. It should be noted that some carbon signals are displayed in the center, which is not a surprise since TEM images are two dimensional 10 ACS Paragon Plus Environment

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projections from 3D particles and thus these signals would come from elements on the top and bottom of the particles under the electron beam. Furthermore, the embedded CB within Li2S/CB composite particles could contribute to the carbon signal in the center as well. The combined elemental mapping of Figure 3d definitely declares that Li2S is coated by carbon.

Particles with a diameter of ~80 nm (Figure 4), in agreement with SEM observations, are also found under TEM. It is visible that this particle demonstrates a core shell structure, the architecture and compositions of which are ascertained by the EFTEM elemental mapping shown in Figure 4d. Despite of consistence with SEM results, it is still interesting for us to observe such small particles with uniform carbon coating. We assume that during stirring process some embedded CB are separated from Li2S, resulting in fine Li2S particles without CB being coated by pyrrole. With such small scale, distances of electron transfer and ion diffusions are reduced hugely, and thus it enables improved kinetic reactions. It is interesting to observe some weak sulfur signal (Figure 4c) on carbon structure in the grid. This is probably because we prepared TEM samples dissolved in DOL followed by sonication, resulting in the solution with some sulfur. The charge-discharge performance of Li2S/CB@NC was evaluated at 0.1 C and results are displayed in Figure 5a. The battery delivers an exceptionally high initial specific capacity of 1020 mAh/g, reaching 87.4% of the theoretical capacity. We attribute this remarkable initial capacity to high utilization of Li2S because of the presence of uniform nitrogen-doped carbon coating due to PVP assistance, along with engineered CB networks inside the cores. Figure 5b shows specific capacities in different cycles. It is noted that there is a voltage hump for each charge curve in the early stage of charge. This over potential is attributed to the energy barrier of inactivated lithium sulfide. The overpotential is about 0.2 V for the 1st cycle (Figure S1) and 2nd cycle (circled in Figure 5b), but decreases drastically in the following cycles, indicating that lithium sulfides have been activated after several cycles. Figure 5b also reveals that the capacity decays from 40th to 80th and from 80th to 140th are almost the same. This means slower capacity degradation after SEI on the anode side is formed and the carbon coating helps retain polysulfide, rendering less loss of the active materials. The capacity maintains at 665.4 mAh/g after a long life of 200 cycles and stable Coulombic efficiency over 99% is achieved (Figure 5a). Moreover, this electrochemical performance exhibits the capacity decay per cycle as small as 0.17%, showing great performance N



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Figure 5. (a) Cycling performance of the Li2S/CB@NC cathode at 0.1 C; (b) voltage-capacity curves of the Li2S/CB@NC half cells at different cycles with the current at 0.1 C; (c) Nyquist plots and equivalent circuit model of different electrodes at open circuit state (Cell 1, Cell 2 and Cell 3 represent ball milled Li2S/CB, Li2S/CB@NC without PVP assistance and Li2S/CB@NC with PVP assistance as the cathodes, respectively); and (d) CV results of the Li2S/CB@NC cell. All values of specific capacities are calculated based on the Li2S mass.

stability. We used the CV to investigate the electrochemical reactions upon charge/discharge processes. The lithium sulfide cathode was swept up to 4.0 V in the first scan for activation. It can be seen that multiple peaks show up indicating several side reactions. However, beyond the first cycle side reactions disappear and there is one prominent anodic peak that can be assigned to the electrochemical reaction from short-chain polysulfides to long-chain polysulfides. For the discharge reactions, the two cathodic peaks are associated to the change from elemental sulfur to long-chain lithium polysulfides (peak at 2.35 V) and the reduction of long-chain to short-chain 12 ACS Paragon Plus Environment

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polysulfides (peak at 2.0 V). All of the results are highly consistent with charge/discharge voltage profiles exhibited in Figure 5b.

To further establish the relationship between cycling performance and electrode kinetics of Li2S/CB@NC with and without PVP assistance during synthesis, electrochemical impedance spectroscopy (EIS) measurements of different cells (Figure 5c) were conducted. Cell 2 and Cell 3 represent cathodes of Li2S/CB@NC without PVP assistance and with PVP assistance, respectively, while Cell 1 is made of ball milled Li2S/CB without carbon coating for comparison. All Nyquist plots of three batteries have a single depressed semicircle in the high-medium frequency region and an inclined line at the low frequency. These continuous lines are fitted with the equivalent circuit displayed in the inset of Figure 5c. The elements in this equivalent circuit include ohmic resistance of electrolyte and cell components (Re), surface film resistance (Rsf), charge-transfer resistance at the interface of electrode and electrolyte (Rct), a constant phase element (CPE(sf+dl)) (surface film (sf) and double layer (dl)) used instead of pure capacitance due to the depressed semicircle,34,35 Warburg impedance (Zw), and intercalation capacitance (Cint).36 Based on the fitting of this equivalent circuit, the value of Re is about 5 Ω for all cells, indicating that batteries have been properly assembled.34 Since all cells were tested in open circuit states and no cycling was applied prior to EIS measurements, their Rsf is zero. The fitting parameter of R(sf+ct) is much lower for Cell 3 (50 Ω) compared to Cell 2 (64 Ω) and Cell 1 (110 Ω), which means that PVP-assisted synthesis of Li2S/CB@NC electrode has lower charge-transfer resistance than others. According to these discussions, Li2S/CB@NC prepared with PVP have less resistance and better electrode kinetics than others. It should be emphasized that all of these cells were prepared with the identical amount of the electrolyte (20 L) and the same loading of Li2S in the electrodes, making the direct comparison among these cells reasonable.

The electrochemical performance comparison of Li2S/CB@NC prepared with PVP assistance and without PVP assistance was evaluated with 0.5 C (Figure 6a). Clearly the capacity retention of Li2S/CB@NC prepared with PVP assistance is better than that without it. Compared to results obtained at 0.1 C, the initial specific capacity decreased to 935 mAh/g. However, the capacity retention after 150 cycles is 64.9% and extremely high and stable Coulombic efficiency of over 99% is realized. These results are better than many recently published reports. 20,21,37-41 Rate 13 ACS Paragon Plus Environment


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capability of Li2S/CB@NC synthesized with PVP and without PVP is compared in Figure 6b. The first 11 cycles of Li2S/CB@NC with PVP are performed with 0.1 C and behave similarly with constant current rates. After being switched to different rates, the specific capacities decreased accordingly. It is worthy of mentioning that the capacities at high current rates of 0.5 C, 1.0 C and 2.0 C stay stable and no degradation occurs. We postulate that the carbon is uniformly coated on the core composites for reductions of polysulfides dissolution and attainment of electrochemical reactions with good kinetics. After the current rate is switched back to 0.1 C, the capacity is recovered, also demonstrating excellent carbon coating fabricated on the core. As for the control sample, the outcome of cycling performance is similar, but the capacity retention is definitely less than that of Li2S/CB@NC with PVP assistance. These data further illustrate the improved rate capability as a result of a more uniform and thicker carbon coating in Li2S/CB@NC prepared with PVP.

Figure 6. (a) Cycling performances of Li2S/CB@NC prepared with and without PVP assistance at 0.5 C; and (b) rate capability of Li2S/CB@NC synthesized with and without PVP at different current rates.

To better understand the sustainable properties of nanostructured Li2S/CB@NC under charge/discharge cycles at high and different current rates, the cycled cell was opened for microscopy studies. The inserts in Figure 7 are the electrodes on aluminum foils used for SEM investigations. Figure 7a shows the uncycled Li2S/CB@NC cathode where lots of particles are uniformly distributed and connected to each other realized by PVDF and carbon black. The cycled electrode without washing is displayed in Figure 7b and there are obviously some thin films on the surface of the electrode, probably due to electrolyte residues. After the washing and heating treatment, it is so interesting to observe almost unchanged morphology (Figure 7c) in comparison 14 ACS Paragon Plus Environment

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with the uncycled electrode (Figure 7a), indicating that the core-shell nanostructure is robust and can withstand different and high cycling rates. The present study also offers clear evidence that washing and heating to remove electrolyte residues are essential processes in making comparison between uncycled and cycled electrodes.

Figure 7. FESEM images for electrodes with corresponding photographs in the inserts: (a) uncycled Li2S/CB@NC cathode; (b) Li2S/CB@NC after 61 cycles at different rates without washing by DOL; and (c) Li2S/CB@NC after 61 cycles at different rates after being washed using DOL for 3 times, followed by heating at a vacuum oven for 6 hours.

TEM characterization was also performed to directly confirm the nanostructures of Li2S/CB@NC after charge/discharge processes. It should be emphasized that in this post-test TEM analysis the sample had never been exposed to air during sample handling and transfer and thus the result reflects the genuine characteristics of the particle. Figure 8 presents TEM images of a stack of multiple particles (Figure 8a) and EFTEM elemental mappings. The projection of these particles onto a 2D plane creates a carbon-rich edge, which unambiguously exhibits Li2S/CB@NC coreshell structure after cycling and thus proves sustainable properties of this nanostructure. To the best of our knowledge, this is the first time that the post-test TEM analysis for Li2S-based materials has been accomplished and reported.

4. Conclusions Li2S/CB@NC with PVP assistance in synthesis has been successfully engineered with excellent carbon coating on the Li2S/CB composite core. Such a composite core with a conductive shell can serve as cathodes for lithium sulfur batteries with good electrochemical performance. Through multiple characterization techniques including FTIR, FETEM and EFTEM elemental mapping, 15 ACS Paragon Plus Environment


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Figure 8. TEM images of Li2S/CB@NC after 61 cycles at different rates with washing using DOL for 3 times after cycling, followed by heating at a vacuum oven for 6 hours: (a) Li2S/CB@NC particle; (b) EFTEM sulfur mapping; (c) EFTEM carbon mapping; and (d) combination of EFTEM elemental mappings.

the uniform nitrogen-doped carbon coating is confirmed. Furthermore, the electrode material demonstrates great performance when cycled at fast charge-discharge processes. Assembled cells yield extremely high initial capacity and very stable Coulombic efficiency of 99.7% over 200 cycles is achieved at 0.1 C with the capacity decay per cycle as small as 0.17%. Most importantly, it is the first time that TEM analysis of cycled cells without exposure to air has been accomplished and reported. Coupled with SEM studies of samples before and after cycling, the reliable results confirm the robust capability of core-shell nanostructures after being cycled at different rates. 16 ACS Paragon Plus Environment

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Included in the paper are detailed investigations and battery performance tests stating PVP-assisted synthesis of Li2S/CB@NC as an outstanding system for high performance lithium sulfur batteries.

Acknowledgements The use of the Center for Nanoscale Materials (CNM) was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. The kindly offer of the Gatan vacuum holder for TEM and EFTEM characterization by Dr. Xiao-Min Lin at CNM is much appreciated.

ASSOCIATED CONTENT Supporting Information Available: The voltage profile for the first charge and the XPS analysis of the nitrogen-doped carbon coating. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents Graphic

Cycling performances of Li2S/CB@NC prepared with and without PVP assistance at 0.5 C. Li2S/CB@NC with PVP displays better capacity retention because of its more uniform and thicker carbon coating.


PVP-Assisted Synthesis of Uniform Carbon Coated ... - ACS Publications

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