Cellulose-Based Porous Membrane for Suppressing Li Dendrite

Aug 25, 2016 - and from a metallic-lithium anode from an organic-liquid electrolyte is demonstrated. A porous cellulose-based membrane used for skin c...
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Cellulose-based Porous Membrane for Suppressing Li Dendrite Formation in Li-Sulfur Battery Byeong-Chul Yu, Kyusung Park, Ji-Hoon Jang, and John B. Goodenough ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00209 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on August 29, 2016

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ACS Energy Letters

Cellulose-based Porous Membrane for Suppressing Li Dendrite Formation in Li-Sulfur Battery Byeong-Chul Yu, Kyusung Park, Ji-Hoon Jang and John B. Goodenough*

Dr. B.-C. Yu, Dr. K. Park, Dr. J.-H Jang, Prof. J. B. Goodenough Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

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ABSTRACT

Dendrite-free, reversible plating/stripping of Li into/from a metallic-lithium anode from an organic-liquid electrolyte is demonstrated. A porous cellulose-based membrane used for skin care absorbs the electrolyte and is wet by metallic lithium. The membrane separator was tested in symmetric lithium cells and in Li(S) rechargeable coin cells assembled in the charged state.

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In a rechargeable battery cell with an organic-liquid electrolyte, dendrites form and grow from a solid alkali-metal anode during charge; the anode dendrites can grow across the electrolyte to the cathode to create an internal short-circuit with incendiary, even explosive, consequences. Rechargeable batteries of high energy density at high rates of charge/discharge will, nevertheless, use an organic-liquid electrolyte until a chemically stable solid or ionic-liquid electrolyte with a comparable working-ion conductivity can be identified. Safety requirements have, therefore, resulted in the fabrication of a Li-ion battery in a discharged state with no metallic lithium in the anode either before or during a charge; the Li+ are either inserted reversibly into a host or are reacted with a particle that undergoes reversible alloy formation or a conversion reaction with lithium. The known anode hosts are carbon or a transition-metal oxide. Only a 0.2 V loss of voltage occurs on the insertion of Li into carbon, but too fast a charge raises the charging voltage high enough to plate metallic lithium on the carbon. On the other hand, insertion of Li+ into an oxide host without formation of a solid-electrolyte interphase (SEI) passivation layer costs a loss of over 1.2 V in the cell voltage. Moreover, the capacities of these hosts are limited. Although an intermediate loss of voltage for a safe fast charge can be obtained with an alloy anode, a huge anode volume change on charge/discharge cycling requires use of small active particles of large surface area requiring passivation by an SEI that is permeable to Li+. The SEI takes its working ion from the cathode to reduce the cathode capacity and, therefore, further reduce the cell energy density. The ability to plate reversibly a solid metallic-lithium anode from an organic-liquid electrolyte would not only increase both the energy density and the rate of charge; it would also provide a safe battery since low-cost separators are known that can block any unexpected anode dendrite from reaching the cathode. In this paper, we report plating of a dendrite-free metallic-lithium

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anode from an organic-liquid electrolyte through a low-cost cellulose-based porous membrane separator that acts as the SEI at the anode/electrolyte interface. The porous cellulose membrane was obtained from a Mask Pack that had been used for skin care. We demonstrate dendrite-free plating/stripping of Lithium with a symmetric Li/Li cell and its use in Li-S full cell. The development of the sulfur cathode of a Li-S cell has been motivated by the availability of sulfur and its high theoretical specific capacity, 1675 m ah g-1. 1-3 However, intermediate polysulfides of the S8 + 16Li = 8Li2S electrode reaction are soluble in an organic-liquid electrolyte, which leads to a shuttle phenomenon, 4-6 and the soluble polysulfides react with a Li anode to corrode it. Various approaches have been employed to confine the sulfur and/or trap the soluble intermediate polysulfides, 7-9 different electrolytes and additives have been employed to reduce the solubility of the polysulfides, 10-12 and modified separators that block shuttling to the anode have been investigated. 13-15 In addition, the voltage of a Li-S cell is low enough that a metallic-lithium anode is required. Several attempts to avoid dendrite formation on a plated solid-lithium anode15-18 have focused on morphological control of the plated lithium to avoid dendrite formation. By replacing the Celgard 2500 separator with a porous cellulose-based separator, we not only confirm that control of the surface morphology of a surface SEI that is wet by the solid lithium metal is effective in suppressing dendrite formation and growth during charge, but also demonstrate that the polysulfides are largely blocked by the membrane from corroding the lithium anode. Figure 1a displays a real image of the porous cellulose membrane and the lithium-anode configuration (left) as well as the Fourier transform infrared (FTIR) spectrum of the cellulose separator that was used for the anode of each cell. The FTIR absorption peaks at 3348 and 1639 cm-1 can be attributed to the OH- stretching vibrations; the weak peaks at 1508 and 1424 cm-1 to

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the C = C aromatic skeletal vibrations and in-plane C-H bending. The strong absorption peak is

Figure.1 (a) Configuration of a Li-S cell with used Cellulose based porous membrane (upper graph : EDS mapping images of carbon paper/S cathode, right graph : FTIR of Cellulose based porous membrane (b) Electrolyte storage capacities of Celgard 2500 and Cellulose based porous membrane with DOL/DME electrolyte and thermal stability of two separators; (c) Celgard 2500 at 25oC(left) and 150oC for 12h(right) (d) Cellulose based porous membrane at 25oC(left) and 150oC for 12h(right).

due to the C-O stretching vibration. This FTIR spectrum is similar to that of the cellulose Mask Pack. 19 The electrolyte storage capacities of the two separators, Figure 1b, were measured by their weight loss with time of a DOL/DME electrolyte under ambient conditions. The uptakes ((Wafter – Wbefore)/Wbefore x 100) were 130 % in Celgard 2500 and 250 % in the cellulose-based membrane, respectively. After 120 s, the percent of remaining electrolyte in the cellulose

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membrane was about 38% whereas that of Celgard 2500 was totally evaporated. This difference in the storage capacities would be much larger in a sealed coin cell. The hydrophilicity of the porous cellulose membrane improves the electrolyte wetting as previously reported. 20-22 Figures 1c and d compare the thermal stabilities of the two separators. The condition of each is shown at 25°C (left) and after 12 h in an oven at 150°C (right). The cellulose membrane remained unchanged, showing good thermal stability, while the Celgard 2500 separator shrank strongly.

Figure.2 FESEM images of (a-c) Celgard 2500 and (d-f) Cellulose based porous membrane, pore distribution of (g) Celgard 2500 and (h) Cellulose based porous membrane.

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Figure 2 compares the morphologies of the two separator membranes obtained by fieldemission scanning electron microscopy (FESEM Hitachi S5500). The thickness of the Celgard 2500 is seen to be about 40 µm and the pore size ranged from 100 to a 300 nm, Figures 2a-c; the cellulose membrane, Figures 2e-f, was only 30 µm thick with a pore size between 50 and 100 nm, significantly smaller than that of Celgard. The pore size distributions of the two separators are displayed in Figures 2g and h. A Mask Pack product that can store more liquid for a longer time has been developed.

Figure.3 (a) CV profiles of a Li/stainless-steel cell with a porous cellulose-based membrane as separator. (b) Voltage profiles of Lithium symmetric cells with Celgard 2500 and Cellulose based porous membrane as separators at a current density of 2.4mA/cm2 (4.8mAh/cm2) (c) Firstcycle voltage profiles of a Li-S cell with Celgard 2500 and with Cellulose based porous membrane as separators. (d) Cycle performance and Coulomb efficiency of a Li-S cell with the Celgard 2500 or mask-pack membranes as separators.

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Figure 3 compares electrochemical test results for a symmetric Li/Li and full Li-S coin cells with a Celgard-2500 versus a porous-cellulose membrane as separator. The anode and reference electrodes of the coin cells were Li foil and the liquid electrolyte was 1 M LiCF3SO3 and 0.5 M lithium nitrate (LiNO3) in 1:1 volume of dimethoxyethane, 1,3 dioxolane (DME/DOL). The cyclic voltammetry (CV) of Figure 3a used a stainless-steel cathode. The CV shows the porouscellulose separator is stable from 0 to 4.5 V versus lithium; the peaks near 0 V for 3 cycles signal reversible plating/stripping of lithium. To confirm Li metal stability during Li plating/stripping, voltage profiles were obtained from Li symmetric cells with two different separators in Figure 3b. The tests were conducted at high current density of 2.4 mA/cm2 (4.8 mAh/cm2). The symmetric cell with a cellulosed-based porous membrane shows a stable voltage plot of Li plating/stripping for 200 h, indicating a uniform Li-anode growth with stable SEI, while the cell with Celgard 2500 died with a big voltage change in a few cycles. This stable Li metal anode with high theoretical specific capacity could be employed in next-generation energy storage systems. It has been well-documented that the separators with a larger pore size permit internal short-circuits by dendrite growth during charge from a lithium anode to the cathode20. Tests of the Li-S full cells having sulfur-loaded carbon paper as the cathode were performed galvanostatically between 1.5 and 2.8 V at 0.2 C rate (335 mA g-1). Figure 3c shows the firstcycle voltage profiles of the Li-S cells with the two separators; the curves exhibit characteristic lithium insertion/extraction plateaus at 2.0 and 2.4 V for the Li-S reaction as previously reported for half-cells. 7-9 With Celgard 2500 as separator, the first discharge and charge capacities were 1322 and 1388 m Ah g-1, corresponding to an initial Coulomb efficiency of 105%. The Li+ of the SEI layer came from the anode and the large charge capacity of the first cycle was caused by the shuttle of polysulfide. 23 In contrast, the Coulomb efficiency with the porous-cellulose separator

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was close to 100% showing the soluble polysulfides from the cathode are largely blocked by the separator, penetrating only a little to the anode through the membrane pores. Figure 3d compares the cycling performance with the two separators; the porous-cellulose separator clearly enhances the specific capacity, retaining 947 mAh g-1 after 100 cycles at 0.2 C and 460 mAh g-1 after 1000 cycles. The capacity fade was the result of failure to recombine the soluble Li2Sx species on recharge; it was not a problem with the anode plating. The full coin cells were disassembled after 100 and 1000 cycles in an Ar-filled glovebox and the electrodes were washed with DME and dried under vacuum for the ex situ FESEM images of Figure 4. The flat surface of the Li-metal foil before cycling is shown in Figure 4a. After 100 cycles with Celgard 2500, Figure 4b shows irregular Li dendrites were found on the surface of the Li metal. However, with the porous-cellulose separator, the surface of the Li-metal anode remained smooth after 100 and 1000 cycles, Figures 4c and d. These results support the Li symmetric cell data as shown in Figure 3b. The elemental EDS mapping images of the cathode are shown in Figures 4e-h; sulfur is seen to remain well-distributed over the carbon-paper cathode after 1000 cycles. Figure 4i is a schematic comparison of the metallic-lithium surfaces after 1000 cycles with Celgard 2500 and with the cellulose-based porous membrane. The nanosized pores of the cellulose membrane may be critical for suppressing dendrite formation to give a uniform current density across the anode-electrolyte interface during cycling20. Based on the above ex situ SEM analysis, the electrochemical properties can be attributed to plating of a metallic-lithium anode without dendrite formation and growth. Suppression of dendrite formation during charge of a metallic-lithium anode provides cell safety with optimization of the cell energy density for a given cathode by increasing the voltage and anode capacity. Growth of dendrites creates new anode surface area on cycling on which new SEI is formed, which causes

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capacity fade. Blocking of soluble sulfides on discharge from reaching the anode also increases cycle life.

Figure.4 Ex-situ FESEM images (inset : high magnification images of each sample): (a) Li metal before cycling (b) Li metal after 100 cycles with Celgard 2500 separator (c-d), Li metal after 100 cycles and 1000 cycles with the cellulose-based porous membrane as separator, (e-h) elemental EDS mapping images of carbon paper/S cathode after 1000cycles with cellulose-based porous membrane. (i) Schematic images of Li metal after cycling with Celgard 2500(left) and Cellulose based porous membrane(right) after many cycles.

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In summary, replacing Celgard 2500 with a cellulose-based membrane having a high concentration of nanopores and nanofibers as the separator in a rechargeable battery cell allows safe plating of a dendrite-free metallic-lithium anode. The commercially available Mask-Pack membrane is low-cost, holds an organic-liquid electrolyte longer than Celgard 2500, has better thermal stability, and is wet by metallic lithium. Cells with the Mask-Pack membrane as separator show excellent, safe electrochemical performance of Li-anode cells and organic-liquid electrolytes for up to 1000 cycles. Dendrite-free metallic-lithium plating was demonstrated by ex situ FESEM imaging.

Experimental Section

We obtained from Etude House Co. the cellulose-based porous membrane as Mask Pack that had recently been used for skin care (Biojelly coconut Gel Mask). The cellulose membrane containing nanopores and nanofibers was originally designed to absorb much water and skin care ingredients. The used Mask Pack was washed with ethanol and deionized water several times before drying overnight at 100°C. The membrane was then punched into ½-inch discs for use as the separator of a coin cell. The symmetric cells were assembled with two lithium metal sheets and two different separators. The discharge and charge lasted for 2h at each step. The cathode of the Li-S cell was prepared by dropping a solution of sulfur dissolved in CS2 onto commercial carbon paper (SIGRACET GDL 10 BA from Ion Power Inc); this sample was held at 155°C for 12 h to obtain a uniform distribution of sulfur on the carbon paper. The sulfur loading was 1 to ~ 1.5 mg/cm2.

AUTHOR INFORMATION

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Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0005397.

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