Carbonized cellulose paper as an effective interlayer in lithium-sulfur batteries

Carbonized cellulose paper as an effective interlayer in lithium-sulfur batteries

Accepted Manuscript Title: Carbonized Cellulose Paper as an Effective Interlayer in Lithium-Sulfur Batteries Author: Shiqi Li Guofeng Ren Md Nadim Fer...

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Accepted Manuscript Title: Carbonized Cellulose Paper as an Effective Interlayer in Lithium-Sulfur Batteries Author: Shiqi Li Guofeng Ren Md Nadim Ferdous Hoque Zhihua Dong Juliusz Warzywoda Zhaoyang Fan PII: DOI: Reference:

S0169-4332(16)32355-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.10.208 APSUSC 34302

To appear in:

APSUSC

Received date: Revised date: Accepted date:

30-8-2016 30-10-2016 31-10-2016

Please cite this article as: Shiqi Li, Guofeng Ren, Md Nadim Ferdous Hoque, Zhihua Dong, Juliusz Warzywoda, Zhaoyang Fan, Carbonized Cellulose Paper as an Effective Interlayer in Lithium-Sulfur Batteries, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.10.208 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Carbonized Cellulose Paper as an Effective Interlayer in LithiumSulfur Batteries Shiqi Li1, Guofeng Ren1, Md Nadim Ferdous Hoque1, Zhihua Dong2, Juliusz Warzywoda3, Zhaoyang Fan1* 1. Department of Electrical and Computer Engineering and Nano Tech Center, Texas Tech University, Lubbock, Texas 79409, USA. 2. Hangzhou Dianzi university, No. 1158, 2nd Street, Xiasha Higher Education District, Hangzhou City, Zhejiang Province, China. 3. Materials Characterization Center, Whitacre College of Engineering, Texas Tech University, Lubbock, Texas 79409, USA. *Contact Email: [email protected] Highlights 

A facile and economical method to fabricate interlayer for high-performance lithiumsulfur battery was demonstrated.



The performance of lithiumsulfur batteries without and with interlayer was compared.



The mechanism for the function of interlayer was explained.

Abstract One of the several challenging problems hampering lithiumsulfur (LiS) battery development is the so-called shuttling effect of the highly soluble intermediates (Li 2S8-Li2S6). Using an interlayer inserted between the sulfur cathode and the separator to capture and trap these soluble intermediates has been found effective in diminishing this effect. Previously, most reported

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interlayer membranes were synthesized in a complex and expensive process, and might not be suitable for practical cheap batteries. Herein, a facile method is reported to pyrolyze the commonly used cellulose filter paper into highly flexible and conductive carbon fiber paper. When used as an interlayer, such a carbon paper can improve the cell capacity by several folds through trapping the soluble polysulfides. The enhanced electronic conductivity of the cathode, due to the interlayer, also significantly improves the cell rate performance. In addition, it was demonstrated that such an interlayer can also effectively mitigate the self-discharge problem of the LiS batteries. This study indicates that the cost-effective pyrolyzed cellulose paper has potential as interlayer for practical LiS batteries. Keywords: Li-S battery, shuttling effect, self-discharge

Introduction Lithiumsulfur (LiS) battery technology is attracting a fresh surge of interests for a new generation of high-capacity and low-cost battery technology that promises to be commercially viable successor to lithium ion battery [1]. Sulfur has long been considered a promising cathode material with a theoretical specific capacity of 1675 A h kg-1 and LiS battery demonstrates an energy density of 2600 W h kg-1, 3–5-fold higher than those of state-of-the-art transition-metal oxides based intercalation chemistry [2, 3] . Owing to its earth abundance and low production cost, sulfur will also help reduce the battery cost. With these merits, the development of LiS battery, however, has been held back for decades due to numerous challenges [4-6]. The insulating nature of sulfur and its reduction compounds prevents the full utilization of the active material. The remarkable volume 2

expansion and contraction (80%) during the electrochemical conversion leads to severe pulverization of the electrode. Perhaps the most challenging problem that has been affecting LiS battery development is the dissolution and diffusion of lithium polysulfides (Li2Sn, n > 2) formed during charge and discharge cycles, which gives rise to irreversible loss of active materials, capacity fade, corrosion of the lithium anode, and self-discharge. Since the report that mesoporous carbon framework could constrain sulfur nanofiller growth and trap the formed polysulfides [7], many innovative approaches have been investigated [8-14]. Particularly, the so-called interlayer, a conductive and porous membrane inserted between the sulfur cathode and the separator, has been found to be an effective strategy to mitigate some of the problems. The porous interlayer can retard the polysulfide shuttling by intercepting the soluble lithium polysulfides, while its conducting nature also enhances the conductivity of the cathode. It deserves to be mentioned that a separator with a modifying layer can simultaneously function as separator and interlayer [15]. A variety of materials have been investigated as an interlayer or a modifying layer of the common separator with demonstrated performance enhancement. These include polypyrrole nanotube film [16], acetylene black mesh [17], graphene film [15, 18-20] and many others [21-24]. The recent progress on interlayer and advanced separator for LSBs has been summarized in a review [25]. Some of these nanomaterial based interlayers, with a delicate structure and a complex synthesis process [16, 18, 19], might be unfavorable for production of cost competitive LiS batteries, even though they may perform very well in a testing cell. Thus, economical, facile and eco-friendly methods for fabricating interlayer deserve to be further explored for practical battery. Herein, we report the application of carbonized cellulose paper as a cheap and effective 3

interlayer for LiS batteries. Cellulose, the main structural component of the cell walls of plants or certain bacteria, is the most common polymer on earth with an estimated 10 12 tons of production annually [26]. The biomass feedstock from agricultural/forest crops or residues provide a sustainable source to extract natural fibers. Considering its abundance, low cost, and high content of carbon, cellulose fibers are expected to be an excellent candidate precursor for producing carbon fibers. In recent years, cellulose nanofibers have caught great interests [27, 28], including for battery applications [22, 29, 30]. However, considering the extensive energy consumption to extract nanofibers from the cellulose macrofibers and the low productivity of bacterial cellulose, in this study, common filter paper made from cellulose marcofibers was selected as a model precursor, which is directly pyrolyzed to obtain carbon paper and then used as an interlayer. In fact, the paper precursor might be extended to include any cellulose paper that is used in the printing industry for fabrication of carbon paper via pyrolysis. To demonstrate the effectiveness of such simple interlayer, sulfur coated nickel foam (S-NF) was directly used as the sulfur cathode in this study.

Results and Discussion The pristine cellulose filter paper (Fisherbrand Quantitative Q8) was directly pyrolyzed in argon atmosphere at 800 ℃. Cellulose polymer has a linear chain of ringed glucose molecules with a flat ribbon-like conformation. In this high temperature pyrolysis process, cellulose decomposes and converts into amorphous carbon and graphite [31]. As shown in Figure 1 (a) and Figure S1, with the massive loss of oxygen and hydrogen elements during pyrolysis, the paper size is reduced with its diameter decreasing from 3 cm to 1.6 cm and its thickness 4

decreasing from 190 μm to 70 μm. The yield of carbon paper from pyrolysis of cellulose paper was 5.3%. The morphology of the cellulose paper and the carbonized paper is compared using scanning electron microscopy (SEM) images in Figure 1 (b, c). The cellulose paper consists of fibers with diameters ranging from approximately several hundred nanometers to 20 μm. After pyrolysis, the cross-linked network morphology is maintained, as shown in Figure 1 (c). However, the resulting carbon fiber network has very different morphology with a fiber diameter of a few micrometers. The conductivity of the carbonized paper was measured with four-probe method, showing an excellent conductivity of 250 S m-1. The as-obtained carbonized paper exhibits excellent flexibility, as illustrated in the inset of Figure 1 (a). It should be emphasized that the flexibility is crucial for a membrane to be used as an interlayer. This will facilitate the cell assembling and particularly prevent the interlayer pulverization during cycling.

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The structure and composition of these papers were characterized by X-ray diffraction 6

(XRD) and X-ray photoelectron spectroscopy (XPS). As shown in Figure 1(d), the filter paper exhibits characteristic XRD peaks of cellulose [32, 33]. After carbonization, the paper shows two broad peaks at 21° and 43° 2, corresponding to the typical 002 and 100 peaks of graphite, respectively [34]. XPS data in Figure 1 (e) show that the cellulose paper consists of approximately 55 at% carbon and 45 at% oxygen, while after pyrolysis, the paper consists of approximately 96 at% carbon and 4 at% oxygen, indicating an effective carbonization. N and O heteroatoms with extra pair of electrons, have been explored to immobilize the lithium polysulfides, thereby alleviating the shuttle effect [35]. Although N doping has been extensively discussed, the role of tiny amount of oxygen heteroatom in carbonaceous materials has not caught enough attention [36]. Herein, the 4 at% oxygen in the carbonized paper was considered capable of facilitating the anchoring of polysulfides via dipole-dipole electrostatic interaction [35]. Raman spectroscopy analysis was employed to further study the structure of the carbonized paper. As illustrated in Figure 1(f), the Raman spectrum shows two bands at approximately 1350 and 1580 cm-1, corresponding to the structural defects and disorders in the carbon (D band) and E2g vibration mode of the sp2-bonded carbon atoms (G band), respectively [19]. The presence of these two bands indicates the successful conversion of cellulose into carbon after pyrolysis, which is in consistence with the XRD pattern and the XPS spectrum. In addition, the ID/IG of 1.0 indicates its partial graphitization [37], which is conductive to the transport of electrons between the carbon and the sulfur species. To evaluate the properties of the carbonized paper as an interlayer in LiS batteries, coin cells were assembled with or without the interlayer by using S-NF as the cathode and lithium foil as the anode. The carbonized paper was sandwiched between the cathode and the separator. 7

The S-NF electrode is shown in Figure S2 of the Supplemental Information. The widely open structure of the nickel foam, with pore diameters of 50-200 m, cannot effectively trap the soluble polysulfides, and therefore, using such an electrode can effectively test the functionality of the interlayer. The electrochemical performances of the assembled cells were characterized and are presented in Figure 2. The cyclic voltammetry (CV) was studied at a scan rate of 0.05 mV s −1 (Figure 2 (a)). There are three cathodic peaks (CP) for both cells. CP1 (2.30 V and 2.33 V for the cell without and with interlayer, respectively) corresponds to the conversion of sulfur to high-order polysulfides. CP2 (2.13 V and 2.15 V for the cell without and with interlayer, respectively) is ascribed to the reduction of higher-order polysulfides to medium-order polysulfides. CP3 (2.02 V and 2.03 V for the cell without and with interlayer, respectively) is due to the further reduction of polysulfides to sulfides. As shown by the peak positions of the two cells, the delayed reaction (larger polarization) in the cell without interlayer results from a higher kinetic barrier for the reduction of sulfur to lithium polysulfides and then to Li 2S. In other words, the insertion of the conductive carbonized paper as an interlayer can reduce sulfur reduction barrier and hence the polarization. In the anodic scan, the two adjacent anodic peaks (AP) at 2.31 and 2.43 V are attributed to the oxidation of Li2S to lithium polysulfides and then to sulfur. Likewise, the slightly higher anodic voltage for cells without interlayer than with interlayer implies the effectiveness of the carbonized paper in diminishing the oxidation polarization. It is also noted that the loop area of the cell without interlayer is smaller. Since the initial S-loading is the same for both cells, the difference of the CV loop area suggests that with the interlayer, more sulfur can be utilized for energy storage. Figure 2 (b) and 2 (c) shows the discharge/charge curves of the two types of cells at 0.2 C 8

(1 C = 1675 mA h g −1) between 2.6 and 1.8 V versus Li+/Li. They are consistent with the CV measurements. Specifically, the discharge plateaus of cells with interlayer are higher than those of cells without interlayer, while the charge plateaus are opposite, i.e., ΔE for cells without interlayer is larger than that for cells with interlayer. These results further confirm that the conductive interlayer mitigates the redox polarization in the cells. The discharge plateaus of cells without interlayer are very short. This indicates that the polysulfides formed in the discharge process detached from the cathode and dissolved in the electrolyte. The cells with interlayer show more stable upper plateaus than those without interlayer, and exhibit high upper plateau (QH) discharge capacities of 330 mA h g

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at a 0.2 C rate, approaching 79% of the

theoretical value corresponding to the upper plateau (419 mA h g −1) [38]. In addition, the long lower discharge plateaus (QL) for cells with interlayer suggest effective intercept and reutilization of lithium polysulfides. The cycling performances of these cells were evaluated at a 0.2 C rate (Figure 2 (d)). The cells without and with interlayer deliver an initial discharge capacity of 265 and 961 mA h g −1, respectively. Here, all the specific capacity is based on the sulfur loading. Thus, with the carbonized paper as interlayer, the discharge capacity is still low; however, the carbonized paper’s functionality is obvious, as indicated by the initial discharge capacity increase of more than 260%. This increase implies that the interlayer successfully intercepts soluble lithium polysulfides during the first discharge. The cells with interlayer retain a capacity above 830 mA h g −1 (86.4% of the initial capacity) after 130 cycles. Additionally, the coulombic efficiency for cells with interlayer is >97% after the third cycle. This is higher than the coulombic efficiency of cells without interlayer (95%), and further demonstrates the effect of interlayer in 9

intercepting soluble lithium polysulfides and mitigating shuttle effect. It is noted that in the initial several cycles, the discharge capacity of the cell with interlayer slightly increased. This could be ascribed to the activation process in this cell. Sulfur particles at electrochemically inactive sites become electrochemically active after neighboring sulfur transformed to soluble lithium polysulfides, which are trapped in the interlayer and are then reutilized. On the contrary, the cell without interlayer has no mechanism to collect and reuse soluble lithium polysulfides.

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Figure 2. (a) Cyclic voltammograms of LiS cells without and with interlayer at a potential sweep rate of 0.05 mV s-1. The charge/discharge profiles of LiS cells without interlayer (b) and with interlayer (c). (d) Cycling performance of LiS cells without and with interlayer at 0.2 C. (e) Rate performance of LiS cells without and with interlayer.

Insertion of an interlayer also improves rate capability of the LiS cells. Different cells were discharged / charged at various current densities ranging from 0.1 C to 2 C rates, for 5 cycles at each rate. As shown in Figure 2 (e), cells with interlayer exhibited stabilized discharge capacities of 1200, 970, 840, and 720 mA h g

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Information, the polarization of cells with interlayer is less severe than that of cells without interlayer at large charge/discharge rates. Better rate performance for cells with interlayer could be attributed to the fact that the interlayer not only prohibits the soluble polysulfides from dissolving in the electrolyte and diffusing to the lithium anode, but also improves the conductivity in the sulfur cathode [25, 39, 40].

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Figure 3. SEM and EDS characterization of the interlayer after 130 cycles at 0.2 C: (a) SEM image of the cycled interlayer (Scale bar=200μm), and (b) C map and (c) S map for the region shown in image (a).

To confirm the polysulfide trapping in the interlayer, element mapping by energy dispersive X-ray spectroscopy (EDS) was conducted. After cycling, the coin cell was disassembled and the interlayer was thoroughly washed with 1, 2-dimethoxyethane to remove soluble species prior to EDS characterization. As presented in Figure 3 (a) and 3 (b), the integrity of microstructure of the interlayer is well maintained, due to its mechanical flexibility. Figure 3 (c) indicates that the interlayer has intercepted a significant amount of sulfur species.

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The role of the interlayer in LiS batteries was further probed by electrochemical impedance spectroscopy (EIS). Nyquist plots of the cell impedance are shown in Figure 4 (a). In the high-frequency region, the intercept of the impedance curve on the x-axis corresponds to the electrolyte resistance (Re). In the middle-frequency region, the semicircle arises from the charge transfer resistance (Rct), which represents the charge-transfer process at the interface between the electrolyte and electrode. In the low-frequency region, an inclined line denotes the Warburg resistance (Wo), which is related with mass transfer processes. Both cells exhibit typical semicircles at medium-frequency region and inclined lines in the low-frequency region. The cells with interlayer show smaller semicircle diameter (Rct = 2Ω) at medium frequency, and higher slope (low Wo) at low frequency than cells without interlayer. This indicates that the cells with interlayer have better electrochemical environment to promote the conversion of the sulfur to high-order polysulfides. The reduction of the resistance can be attributed to the use of the highly electrically conductive carbon interlayer framework as a second current collector. Simultaneously, the interlayer could effectively reuse the dissolved active materials and mitigate surface aggregation, thus providing better performance. Self-discharge is the phenomenon by which a secondary battery loses capacity when stored for a period of time at a certain temperature. It depends on the battery chemistry, electrode composition, current collector, electrolyte formulation, and storage temperature. Self-discharge will decrease the shelf-life and hinder the practical application of LiS batteries. For LiS batteries, the shuttle mechanism and corrosion of current collector have been blamed for the self-discharge phenomena [41, 42]. The dissolution and diffusion of lithium polysulfides and 13

their irreversible reaction with the lithium anode were considered to be the dominate factor [43]. Although self-discharge behavior is one of the important factors for practical batteries, only a few attempts have been made to overcome the self-discharge issue of LiS batteries, which can be generally categorized as anode/cathode protection and polysulfide anion diffusion suppression strategies [41-44]. Herein, we demonstrate that the interlayer in the LiS battery can effectively mitigate the self-discharge. As shown in Figure 4 (b), the open circuit voltages (OCV) were measured as a function of storage time. The OCV of the cell with interlayer stabilizes at 2.39 V after storage for a period of time. In contrast, severe self-discharge behavior was observed for the cell without interlayer. The OCV of the cell without interlayer was only 2.35 V, which indicated the spontaneous reduction of high-order polysulfides into low-order polysulfides [44]. The interlayer mitigated the parasitic reactions between the lithium polysulfides and the lithium anode by intercepting the lithium polysulfides, thereby retarding the self-discharge. The fact that the interlayer improves the cell efficiency during the charge and discharge process also demonstrated that shuttle phenomenon could be effectively controlled via the interlayer.

Conclusion An economical and environmentally friendly method to fabricate effective interlayer for LiS batteries, based on direct pyrolysis of cellulose paper, has been demonstrated. Cells with the interlayer achieved a discharge capacity of > 830 mA h g−1 and an efficiency of more than 97% at a 0.2 C rate with cycling stability. Such cells delivered a capacity of 710 mA h g−1 at 1 C rate. The interlayer was shown to be effective in mitigating self-discharge problem. The good cycling 14

stability, high capacity retention, high rate capability and low self-discharge indicate that carbonized cellulose paper used as interlayer is a viable approach to improve the performance of LiS batteries. The improved performance is ascribed to the trapping capability of the interlayer to reutilize the dissolved polysulfides and the reduction of charge transfer impedance of the electrode. Our results suggest that pyrolyzed cellulose paper used as an interlayer could be an economical method to suppress the diffusion of lithium polysulfides and enhance electrode conductivity in practical LiS batteries.

Experimental section Synthesis of interlayer and sulfur cathode. The cellulose filter paper (Fisherbrand Quantitative Q8) was pyrolyzed in a tube furnace under argon atmosphere (54 sccm) at 800 °C for 2 hours. Sulfur was electrodeposited on NF. Briefly, NF sheet was pressed to a thickness of around 0.15mm, and then used as electrode for sulfur deposition after cleaning with hydrochloric acid (2 wt %), DI water and isopropanol. The electrodeposition was carried out in 0.1 M sodium sulfide (Na2S) aqueous solution under a current density of 2 mA cm-2. After two hours, a sulfur loading of 1.39 mg cm-2 was obtained. The S-coated electrode was washed using DI water and dried in vacuum for 24h at a temperature of 60 °C. Characterization. The morphology of NF, S-NF and interlayer was characterized by a field emission scanning electron microscope (FE-SEM). X-Ray powder diffraction (XRD) was conducted through a Bruker D5005 X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) data were acquired using PHI 5000 VersaProbe X-ray photoelectron spectrometer. The photoelectrons were excited by monochromatic Al Kα radiation (1486.6 eV) with a step of 0.1 15

eV and pass energy of 23.5 eV. Raman spectra were recorded using a Bruker SENTERRA dispersive Raman microscope spectrometer with an excitation laser beam wavelength of 532 nm. The elemental mapping was accomplished using an energy dispersive X-ray spectrometer (EDS) attached to the Hitachi S-4700 FE-SEM. Electrochemical measurements. 2032-type coin cells were assembled in an argon-filled glove box. A separator was put on the S-NF cathode and 20 μL electrolyte was put on the separator, followed by a Li foil as the anode. For the LiS batteries with interlayer, the carbonized paper was sandwiched between the S-NF cathode and the separator. The electrolyte was a solution of lithium bis(trifluoromethanesulfonyl)imide (1 M) in 1:1 v/v 1, 2-dimethoxyethane and 1, 3DOL containing LiNO3 (1 wt %). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical workstation (EC-Lab SP-150, BioLogic Science Instruments) for the 2032-type coin cells. CV was recorded over the potential window of 1.8 to 2.6 V versus Li+/Li, and the scan rate of CV was 0.05 mV s-1. The EIS was taken by applying 5 mV alternative signal versus the open-circuit voltage in the frequency range of 1 MHz to 0.1 Hz. Galvanostatic cycling was carried out using a LAND CT-2001A instrument (Wuhan, China) from 1.8 to 2.6V versus Li+/Li. Specific capacity values were calculated based on the mass of sulfur in the samples.

Supplementary Information

Carbonized Cellulose Paper as an Effective Interlayer in LithiumSulfur Batteries

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Shiqi Li1, Guofeng Ren1, Md Nadim Ferdous Hoque1, Zhihua Dong2, Juliusz Warzywoda3, Zhaoyang Fan1* 1. Department of Electrical and Computer Engineering and Nano Tech Center, Texas Tech University, Lubbock, Texas 79409, USA. 2. Hangzhou Dianzi university, No. 1158, 2nd Street, Xiasha Higher Education District, Hangzhou City, Zhejiang Province, China. 3. Materials Characterization Center, Whitacre College of Engineering, Texas Tech University, Lubbock, Texas 79409, USA. *Contact Email: [email protected] The sulfur cathodes were prepared via electrodeposition using NF as the substrate and Na 2S aqueous solution as the electrolyte. As shown in Figure S1 (a), the surface of the original NF was composed of some smooth polygons. After electrodeposition, the surface was covered by well-dispersed particles (Figure S1 (b)). To further examine the successful deposition of sulfur, XRD analysis was conducted. As shown in Figure S1 (c), there were not only peaks corresponding to nickel, but also peaks corresponding to sulfur. The electrochemical reaction of sulfur deposition on the NF could be described as below: S2--2e-→S↓(1)

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Figure S2. SEM images of NF before electrodeposition (a) and after electrodeposition (b) (Scale bar=10 μm). (c) XRD patterns of S-NF cathode.

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Figure S3. The charge/discharge profiles of LiS cells without interlayer (a) and with interlayer (b) at different rates.

Acknowledgements S. L. acknowledges a fellowship from the China Scholarship Council (CSC). Z. D. thanks the support from National Natural Science Foundation of China (No. 61306100).

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