The Use of Redox Mediators for Enhancing Utilization of Li 2 S cathodes for. Advanced Li-S Battery Systems

Transcription

1 Supporting information for: The Use of Redox Mediators for Enhancing Utilization of Li 2 S cathodes for Advanced Li-S Battery Systems Stefano Meini 1,, Ran Elazari 2,,, Ariel Rosenman 2, Arnd Garsuch 3 and Doron Aurbach 2,* 1. Chair of Technical Electrochemistry, Technische Universität München 2. Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel 3. BASF SE, Ludwigshafen am Rhein, 67063, Germany Present address: BASF SE, Ludwigshafen am Rhein, 67063, Germany Present address: ICL-IP, Beer Sheva, 84101, Israel * address: Doron.Aurbach@biu.ac.il These authors contributed equally to this work. 1

2 Electrode preparation PVdF bonded Li 2 S electrodes were prepared by spreading using an Eppendorf pipette an ink composed of a mixture of 59% Li 2 S (Aldrich, 99%, as received), 33% carbon black (Printex XE-2) and 8% PVdF binder in N-methyl-pyrrolidone on a 14 mm diameter disk aluminum current collector. The inks were prepared using the following procedure: mixtures of carbon black and Li 2 S (mixed by hand in a mortar for 15 min) were added to a PVdF solution in NMP, and subsequently magnetically stirred for 3 hours in a glass vial. The slurry was spread onto Al disks (100 µl of slurry on each disk), and the solvent was evaporated at room temperature for at least 48 hours. The whole process was performed in an Ar-filled glove box with a MBraun purifying system (O 2 < 1 ppm, H 2 O < 1 ppm). The active mass was 3 mgcm -2. It has to be noticed that the electrode preparation procedure used herein does not involve complex electrode engineering or advanced materials such as carbon nanotubes or nano-sized Li 2 S. As we do not expect excellent rate capability nor fast kinetics of Li 2 S electrochemistry in such electrodes, on the other hand this allows us to prove the effectiveness of redox mediators on an electrode prepared with a procedure that could be easier/more convenient to scale up to larger scale, compared to other more advanced techniques. Electrochemical and XRD characterization of Li 2 S electrodes The Li 2 S cathodes were tested in a two electrodes configuration with coin-type cells (2523, NRC, Canada) vs. lithium metal (Chemetall Foote Corporation, USA) with 60 µl of electrolyte solution and polypropylene membranes (Tonen, Stela) as a separator. The basic electrolyte solution used in this work, referred to as "standard electrolyte solution", was DOL and DME (v:v ratio) with 10% LiTFSI and 2% of LiNO 3 (by weight) to minimize shuttle phenomena. 1,2,3 The RM tested as additives to the standard solution were: Ferrocene Fe(η 5 -C 5 H 5 ) 2, LiI, Decamethylferrocene Fe(η 5 -C 5 Me 5 ) 2, Dibenzenechromium Cr(η 6 -C 6 Me 6 ) 2, and Cobaltocene Co(η 5 -C 5 H 5 ) 2. All additives were employed in 50 mm concentration, except Decamethylferrocene (30 mm) because of its lower solubility. The cells were assembled in the very same 2

3 argon-filled glove box wherein electrode preparation took place, in order to avoid any contact of the electrodes with ambient air. Coin-cells were cycled at 30 C using BT2000 battery cycler (Arbin Instruments, USA). X-ray diffraction (XRD) patterns obtained with a D8 Advance system (Bruker Inc.) using Cu Kα radiation (λ=1.54 Å). Electrochemical characterization of electrolyte solutions by Cyclic Voltammetry The standard electrolyte solution 10 % LiTFSI + 2 % LiNO 3 in DOL:DME 1:1, and the redox mediators additives dissolved to 5 mm concentration in standard solution, were characterized by cyclic voltammetry using a Teflon T-cell with glassy carbon working and counter electrodes, and Li metal as reference electrode (PGSTAT Autolab electrochemical measuring system from Eco Chemie, Inc., The Netherlands). Figure S1 shows the cyclic voltammogram of our standard electrolyte solution recorded at 0.2 mv/s between V Li. An irreversible electroreduction of LiNO 3 can be clearly observed at cell voltage <1.85 V Li. In order to avoid the continuous reduction of LiNO 3 concentration caused by reduction at the cathode surface upon cycling, we set the lower cutoff for cycling to a safe value of 1.9 V Li, above which no other cathodic processes take place rather than those concerning Li-S electrochemistry. That expedient allows us to maintain the benefits of LiNO 3 additive through a higher number of cycles. Figure S1. Cyclic voltammogram (6 th scan) of 10 % LiTFSI + 2 % LiNO 3 in DOL:DME 1:1 electrolyte solution (standard solution), recorded at 0.2 mv/s on a glassy carbon disc working electrode in an Ar-filled 3

4 glovebox. A Teflon T-cell comprising a glassy carbon counter electrode and a Li metal reference electrode was used. Figure S2 shows the cyclic voltammograms of each electrolyte solution we use. All metallocenes show well established reversible behaviour of the Mc + /Mc couples in the whole potential range explored. However, despite its efficiency, LiI doesn't show a fully reversible behaviour, as it was previously reported, which relates to the ability of iodine moieties to build relatively stable ion couples in non-aqueous solvents having moderate donor numbers. 4 With the five additives reported herein, we are thus able to explore the whole potential range of Li-S batteries. Figure S2. Steady-state cyclic voltammograms of all the red-ox additives studied in this work, dissolved to 5 mm concentration in our standard electrolyte solution (that is, 10 % LiTFSI + 2 % LiNO 3 in DOL:DME 1:1): a) Co(η 5 -C 5 H 5 ) 2, b) Cr(η 6 -C 6 H 6 ) 2, c) Fe(η 5 -C 5 Me 5 ) 2, d) LiI, e) Fe(η 5 -C 5 H 5 ) 2. All CVs were recorded at 0.2 4

5 mv/s on a glassy carbon disc working electrode in an Ar-filled glovebox. A Teflon T-cell comprising a glassy carbon counter electrode and a Li metal reference electrode was used. 5

6 Voltage profiles upon cycling of Li-Li 2 S cells Upon cell activation, an overpotential hump attributed to the initial charging of pristine Li 2 S electrodes is shown in Figure S3b. Figure S3. a) Comparison between the voltage profiles in the first charge of Li 2 S electrodes cycled at C/10 rate ( 0.5 ma) using 10% LiTFSI + 2% LiNO 3 in DOL/DME 1/1 (black curve) and different redox mediators: 50 mm LiI (magenta curve), Fe(η 5 -C 5 H 5 ) 2 (red curve), and 30 mm Fe(η 5 -C 5 Me 5 ) 2 (orange curve). High voltage cut-offs were chosen as the minimum value needed for a complete Li 2 S electrooxidation (4.0, 3.2, 3.6, and 3.2 V Li respectively). b) Detail of the initial potential peak of Li 2 S charge profiles shown in Figure S3a (part of the curve enclosed in the dotted black square). 6

7 The maximum voltage peak obtained with the various solutions was found to be correlated to the additive's reduction potential determined by cyclic voltammetry (Figure S2). While the highest peak at 3.2 V Li is measured for the Li/Li 2 S cell charged in an electrolyte solution without any RM, a moderate hump at 2.8 V Li is observed for cells containing Decamethylferrocene as an additive to the electrolyte solution corresponding nicely to its reduction potential (Figure S3b). Sporadic Li-Li 2 S coin cells showed surprisingly higher charge capacities in the activation cycle than the theoretical value expected (1167 mahg -1 Li2S). As an example, the specific charge capacity of a Li-Li 2 S cell using 30 mm Decamethylferrocene additive reached 1700 mahg -1 Li2S (Figure S3a). We believe that these kinds of artefacts can arise when using redox mediators due to shuttling mechanisms additional to the classic polysulfide shuttle phenomenon existing in Li-S cells, in particular at the beginning of cell's life when the SEI layer on the anode is not yet fully formed. Some Li-Li 2 S cells using redox mediators as additives showed unstable voltage values upon charging (Figure S3a, orange curve); a definitive explanation for this phenomenon is not yet available. We speculate that fast RM such as metallocenes, eventually accumulated in cavities within the electrode matrices can release very quickly polysulfides when they come in contact with Li 2 S particles. This can lower the electrode s potential in several hundreds of millivolts to a mixed potential RM/polysulfide, thus resulting in a spike. We suggest that the random nature of this phenomenon has to do with a structural inhomogeneity of the electrodes used herein. Despite those singularities, all cells exhibited the typical cycling behavior of Li-S batteries, showing two potential plateaus upon both discharge and charge (Figure S4). It can be observed that the charge/discharge overpotential (i.e. the potential difference between charge and discharge plateaus) is decreasing in the following order: NO additive > LiI > Ferrocene > Decamethylferrocene. We do not believe that this observation is related to an additional beneficial effect of RM on the battery performance, but rather to the different utilized capacity achieved by the corresponding cells. In fact, since the absolute current (ma) applied to all cells is the same, all cells with lower utilized 7

8 capacity have been cycled at higher charge discharge rates, which results in higher overpotentials. For example, the cells with no additive achieve a maximum capacity of 200 mah/g, 3 times less than the cells with Decamethylferrocene that reach a capacity of 600 mah/g. The presented corollary is that the active material of the cell with no additive is cycled at a 3-fold higher current rate than the cell with Decamethylferrocene, thereof the higher overpotential. Indeed, the capacity trend Decamethylferrocene > Ferrocene > LiI > NO additive is inversely proportional to that observed for charge/discharge overpotentials. Figure S4. Comparison between discharge and charge voltage profiles of Li-Li 2 S cells in the 10 th, 50 th and 150 th cycles, using a) 10% LiTFSI + 2% LiNO 3 in DOL/DME 1/1 and different redox mediators: b) 50 mm Fe(η 5 -C 5 H 5 ) 2 (red curve), c) 50 mm LiI (magenta curve), and d) 30 mm Fe(η 5 -C 5 Me 5 ) 2 (orange curve). Cells are galvanostatically charged at C/10 ( 0.5 ma) up to 4.0, 3.2, 3.6 and 3.2 V Li respectively, and subsequently cycled at C/10 between V Li. 8

9 References (1) Mikhaylik Y.V. Electrolytes for Lithium Sulfur Cells. US 2008/ , (2) Aurbach, D.; Pollak, E.; Elazari, R.; Salitra, G.; Kelley, C.S.; Affinito, J. On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li-Sulfur Batteries. J. Electrochem. Soc. 2009, 156, A694 A702. (3) Elazari, R.; Salitra, G.; Talyosef, Y.; Grinblat, J.; Kelley, C.S.; Xiao, A.; Affinito, J.; Aurbach, D. Morphological and Structural Studies of Composite Sulfur Electrodes upon Cycling by HRTEM, AFM and Raman Spectroscopy. J. Electrochem. Soc. 2010, 157, A1131 A1138. (4) El-Hallag, I.S. Electrochemical Oxidation of Iodide at a Glassy Carbon Electrode in Methylene Chloride at Various Temperatures. J. Chil. Chem. Soc. 2010, 55,