Nanostructured Li 2 S-C Composites as Cathode Material for High Energy Lithium/Sulfur Batteries

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1 Supplementary Information Nanostructured Li 2 S-C Composites as Cathode Material for High Energy Lithium/Sulfur Batteries Kunpeng Cai 1,, Min-Kyu Song 1,, Elton J. Cairns 2,3, and Yuegang Zhang 1,,* 1 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 2 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 3 Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720, USA 1

2 Materials and Methods Powder preparation Commercial Li 2 S powder (Sigma-Aldrich, St. Louis, MO, USA) was first mixed with carbon black powder (Denka Black, average diameter of 35 nm, New York, NY, USA) at the weight ratio of 75:25 in a stainless steel jar sealed in a glove box filled with Ar gas, using a high-energy ball mill machine (8000M Mixer/Mill, Thomas Scientific, Swedesboro, NJ, USA) at 1,060 cycles per minute. The carbon black powder was dried in a vacuum furnace at 130 o C for 18 hours before mixing with the Li 2 S powder. Zirconia balls with three different diameters (Φ= 10, 5 and 3 mm, Inframat Advanced Materials TM LLC. Manchester, CT, USA) with a weight ratio of 3:4:3 were used as the milling medium without solvents in order to reduce the size of the Li 2 S particles and obtain uniform mixing. The weight ratio between all milling balls and the Li 2 S-C mixture was set to 15:1. To prevent overheating of the system, a rest time of 20 minutes was added after each 20-minute ball milling, and the total milling time was 2 hours. For better capacity retention, multi-walled carbon nanotubes (MWCNTs, Cheap Tubes, Inc., Brattleboro, VT, USA) were introduced into the mixtures at a weight ratio of 75:20:5, (Li 2 S:C:MWCNTs) and ball-milled under same conditions. Toluene, with a weight of 5 times that of the total powder weight, was used when the wet-milling method was used. Electrode fabrication and cell assembly A solution of poly(styrene-co-butadiene) rubber (SBR, Sigma-Aldrich, St. Louis, MO, USA) binder was first prepared by adding SBR to toluene (Sigma-Aldrich, St. Louis, MO, USA) at a weight ratio of 1:30. The solution was ultra-sonicated for 15 min before a proper amount of the as-milled Li 2 S-C nanocomposite was added and ultra-sonicated for another 30 min. The weight ratio between the Li 2 S-C mixture and the SBR was set to 9:1, which gave a slurry with a final weight formula of 67.5:22.5:10 for Li 2 S:C:SBR. When the MWCNTs were introduced, the resulting ratio was 67.5:18:4.5:10 for Li 2 S:C:MWCNTs:SBR. The well-dispersed viscous slurry was then casted using a doctoral blade (Elcometer 3540 Bird Film Applicator, Rochester Hills, MI, USA) onto the surface of a two-side carbon coated aluminum foil (Intelicoat Technology, South Hadley, MA, USA) under an Ar atmosphere. After evaporating the toluene for 30 min at 2

3 room temperature, additional drying at 50 o C for 24 h was carried out to fully eliminate any solvent residue. The electrode was punched into circular pieces with a diameter of 12.7 mm for cell assembly. For the electrolyte, 1 mol/kg Lithium Bis(Trifluoromethanesulfonyl)Imide (LiTFSI, Sigma-Aldrich, St. Louis, MO, USA) in N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR14TFSI, Sigma-Aldrich, St. Louis, MO, USA)/poly(ethylene glycol) dimethyl ether (PEGDME, Sigma-Aldrich, St. Louis, MO, USA) mixture (1:1, by weight) was prepared. CR2032- type coin cells were fabricated with a porous polypropylene separator (2400, Celgard LLC., Charlotte, NC, USA) and a lithium metal foil (99.98%, Cyprus Foote Mineral, USA) as counter/reference electrode in an Ar glove box. Materials characterization X-ray diffraction (XRD) Diffraktometer (D8 Discover, Bruker AXS, Madison, WI, USA) was used to determine the crystalline structure of the Li 2 S, carbon black powders, milled Li 2 S-C mixtures, and electrodes. Their morphology and composition were characterized using a field emission scanning electron microscope (JEOL JSM- 7500F, Thermo Scientific, Pleasanton, CA, USA) coupled with an energy dispersive X- ray spectrometer (EDX). Electrochemical measurements Cyclic voltammetry (CV) experiments were carried out using an Arbin automatic battery cycler (BT-2000, Arbin Instruments, College Station, TX, USA) at the rate of 0.04 mvs 1. Two different cut-off potentials of 2.8 and 4 V were used. Electrochemical impedance spectroscopy analysis was also performed with amplitude of 5mV in the 1 MHz to 0.1Hz frequency range on a Maccor battery cycler to monitor how the imdedance changed during the first charge up to 4V at 0.02C. Constant-current charging and discharging cycling tests were conducted using a battery analyzer (MTI Co. CA, USA) with a constant-current, constant-voltage (CCCV) method. 4V was used only for the first charging for activation purposes and the cells were cycled between 1.5 and 2.8V. Before all electrochemical characterizations, the cells were held at open circuit at room 3

4 temperature for 24 h. All electrochemical characterizations were performed inside a chamber maintained at 30 C. 4

5 Supplementary Discussion Synthesis of high-energy dry ball-milled Li 2 S-C nanocomposites a b Li 2 S Carbon Nanoparticles Ball milling 500nm c Agglomerate 2µm Figure S1. (a) Schematic of the high-energy dry ball milling process. (b) SEM image of commercial carbon black (Denka Black) powder. (c) SEM image of as-milled Li 2 S-C composite agglomerates. Figure S1a shows a schematic of the high-energy dry ball-milling process for preparing Li 2 S-C nanocomposites. Large Li 2 S particles are reduced to a smaller size during ball milling while carbon nanoparticles are being uniformly dispersed and deposited onto the surface of these smaller Li 2 S particles. Under dry ball-milling conditions, the carbon particles become strongly bonded to the Li 2 S particles, which agglomerate, as shown in Figure S1(c). 5

6 2µm Figure S2. SEM image of the surface of the fabricated electrode. Figure S2 shows the surface of the fabricated electrode prepared using a doctor blade with a 50um gap. It is shown that the relatively large agglomerates could be easily casted onto Al foil, which indicates that the agglomerates are not individual Li 2 S particles but rather consist of porous Li 2 S-C nanocomposites. Moreover, the small Li 2 S particles are uniformly mixed with carbon black powder, which can provide the Li 2 S-C composite cathode with improved current collection capability. 6

7 Capacity (mah/g LI 2 S) Capacity [mah/g S] Capacity (mah/g LI 2 S) Capacity (mah/g S) 700 C/ C/ a C/ Cycle number b C/ C/ Cycle number Figure S3. Cycling performance of Li 2 S-C cathodes prepared from (a) wet-milled powders and (b) dry-milled powders. A potential range of V was used for these two experiments without the activation process up to 4V. Figure S3 shows the cycling performance of Li 2 S-C cathodes prepared from wet-milled and dry-milled powders. Without the activation process of charging to 4V, both electrodes needed a few cycles to reach maximum capacity. A cathode made from wetmilled powders needed more cycles to reach maximum capacity, and showed poorer capacity retention than a cathode made from dry-milled powders. 7

8 2µm Figure S4. SEM images of wet-milled Li 2 S-C nancomposites SEM observation of the morphology of the wet-milled Li 2 S-C nanocomposites indicates that the carbon black powder is separated from the Li 2 S powder, implying poor contact and current collection. Additionally, the average size of the Li 2 S is too large which can lead to poor utilization. 8

9 Voltage (V) C/ C/2 C/ C/ Capacity (mah/g LI 2 S) Figure S5. Voltage profiles of Li 2 S-C nanocomposite cathodes, cycled between 1.5 and 2.8V after activation by first charging up to 4V. Voltage profiles for different discharging rates of C/10, C/5 and C/2, the first and last discharge curves at each C-rate are shown. A constant charging current rate of C/10 was used for all cycles, followed by constantvoltage until the current drops below 5% of the initial charging current. 9

10 Table S1. Details of ball-milling conditions and electrode compositions No Composition (wt %) Milling Media Milling condition and Time (h) Maximum discharge capacity (mah/g Li 2 S) 1 Denka Black (45%), Li 2 S(45%), SBR(10%) Dry, 1h 17 (0.002C, 2.8V) 2 Denka Black (22.5%), Li 2 S(67.5%), SBR(10%) Dry, 1h 731 (0.002C, 2.8V) 3 Denka Black (22.5%), Li 2 S(67.5%), SBR(10%) Wet, 1h 719 (0.02C, 2.8V) 4 Denka Black (22.5%), Li 2 S(67.5%), SBR(10%) Dry, 2h 767 (0.02C, 2.8V); 1144 (0.02C, 4V) 5 Denka Black (22.5%), Li 2 S(67.5%), SBR(10%) Dry, 4h 714 (0.02C, 2.8V) 6 C45 (22.5%), Li 2 S(67.5%), SBR(10%) Dry, 2h 422 (0.02C, 2.8V); 963 (0.02C, 4V) 7 MW-CNTs (4.5%), Denka Black (18%), Li 2 S(67.5%), SBR(10%) Dry, 2h 1239 (0.02C, 4V) The detailed fabrication conditions for Li 2 S-C nanocomposite cathodes are shown in Table S1, where the composition, milling media, milling time and condition, as well as maximum discharge capacity, are provided. Although the high-energy ball milling machine was used to prepare all mixtures, only ball-milling conditions 4 and 7 led to the effective reduction of Li 2 S particle size and excellent cycling performance described in this report. It should be noted that, under same ball-milling conditions, different milling time (Condition 2 and 5) resulted in different microstructure of Li 2 S-C composites, which led to much poor electrochemical performance than that of the best electrode fabricated from condition 4 (2 hours dry-milling) we reported in the main text. 10

11 SEM images for dry-milled powders after 1 and 4 hours are given in Figure S6. Based on SEM images, it appears that 1 hour high energy dry ball-milling was not enough to give a good binding between Li 2 S and carbon powders. On the other hand, 4 hour high energy dry ball-milling caused much bigger agglomeration of these two particles. These two microstructures might impose difficulties in obtaining good charge and mass transfer needed for efficient electrochemical reactions. (a) (b) Figure S6. SEM images of Li 2 S-C composites: (a) after 1 hour dry ball-milling and (2) after 4 hour dry ball-milling. By considering the hardness of Li 2 S and C, addition of more Li 2 S could be regarded as increasing the amount of milling media. Hence, smaller Li 2 S particles could be obtained under same milling condition (dry, 1h) by increasing amount of Li 2 S (67.5%), which is confirmed by comparing Figure S6 (a) and Figure S7. It is clearly shown that the composites milled with 45 % of Li 2 S, particle size of Li 2 S are not reduced as much as those shown in Figure S6 (a). Also with higher amount of carbon (45% compared to 22.5% in condition 2), we observed these carbon particles tends to stick together instead of depositing onto the surface of small Li 2 S particles. As a result, with much bigger Li 2 S particle and poor connectivity between Li 2 S and C, the utilization of the electrode fabricated from condition 1 was very poor compared to that of electrode milled with 67.5% 11

12 (condition 2), indicating the importance of microstructure (dimension and connectivity) of Li 2 S-C composites. Figure S7. SEM images of Li 2 S-C composites: after 1 hour milling with 45% Li 2 S. 12

13 Table S2. Data for estimation of the Li 2 S cell specific energy Design Parameters for Calculations of Cell Specific Energy Density Cell part Weight of Material for Li 2 S cell (mg/cm 2 ) Cu Foil (5 microns thick) 4.5 Lithium Electrode (100% excess) 3.6 Electrolyte (50 microns thick) 5 S Electrode (including binder/additives) 6 Al Foil (5 microns thick) 1.4 Total weight 20.5 Cell design parameters used to calculate the cell specific energy (including all components except cell-housing) are provided in Table S2. A sulfur electrode loading of 6 mg/cm 2 is assumed for all data from different literatures to calculate the cell specific energy curves shown in Figure 6. A 100% excess of lithium is provided with respect to the theoretical amount required for the full conversion of S to Li 2 S. For the electrolyte, two layers of separator (polypropylene, porosity 50%, density = 0.9 g/cm 3 ) and organic solvent (density = 1.1 g/cm 3 ) are assumed. 13