An Anode-Free Sodium Battery through In-Situ Plating of Sodium Metal

Transcription

1 Supporting Information An Anode-Free Sodium Battery through In-Situ Plating of Sodium Metal Adam P. Cohn 1, Nitin Muralidharan 2, Rachel Carter 1, Keith Share 2, and Cary L. Pint 1,2 * 1 Department of Mechanical Engineering, Vanderbilt University, Nashville, TN Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, TN * Corresponding Author, electronic mail: cary.l.pint@vanderbilt.edu 1

2 Experimental details: A. Electrochemical measurements. Carbon films were assembled on Al foil using a mixture of conductive carbon black (TIMCAL Super C45) and sodium carboxymethyl cellulose (CMC) with a ratio of 8:2, respectively. Triton X wt% in deionized water was used at the solvent. Slurries were then doctor bladed onto Al foil to obtain carbon films with ~400 µg/cm 2. FeS 2 electrodes were processed similarly using a ratio of 8:1:1 for FeS 2 (325 mesh): carbon black : CMC. FeS 2 electrodes were tested with active mass loading of ~5mg/cm 2. Electrochemical testing was performed at room temperature in CR2032 coin cells using Celgard 2325 separators. Half cell testing was performed using ~20 mg of flattened sodium metal (Strem Chemicals, 99.95%) as the reference and counter electrode. The 1M NaPF 6 in diethylene glycol dimethyl ether (99.5%, Sigma-Aldrich) electrolyte was prepared after NaPF 6 salt, acquired from Strem Chemicals with a purity of 99%, was dried at 100 C for 24 hours in Ar. Prior to plating/stripping testing, all devices were initially galvanostatically cycled 10 times at 0.4 ma/cm 2 from 0.01 to 1.0 V vs. Na/Na + to remove any surface contamination. Plating/stripping testing was performed using a stripping cutoff voltage of 100 mv vs. Na/Na +. Coulombic efficiencies were calculated as the capacity ratio of the Na removed / Na deposited. The voltage hysteresis for each cycle was calculated as the difference between the average voltage measured for corresponding plating and stripping steps. We note that Coulombic efficiency values exceeding 100% for individual cycles may be attributed to the stripping of sodium metal that was left behind after previous cycles. Electrochemical impedance spectroscopy (EIS) was performed on 0.25 mah/cm 2 of plated sodium (0.5 ma/cm 2 for 30 minutes) after the 1 st, 2 nd, 3 rd, 4 th, 5 th and 10 th cycles in half cell configurations with a Na metal reference/counter electrode. EIS was performed using a Metrohm Autolab multichannel electrochemical workstation. B. Sodium imaging. In order to image the plated Na metal, plating was performed in a split-flat cell in an Ar glovebox connected to a single-channel Metrohm Autolab. After plating, electrodes were removed from the glovebox, sealed between two glass slides using a greased O-ring secured with binder clips. To perform the SEM imaging, a pop-top transfer cell was made in the lab utilizing a taught rubber membrane positioned underneath a needle, so that the membrane bursts when placed under vacuum in the SEM loading chamber to expose the sample to the electron beam in a similar fashion to the cell reported in Ref. 1. A Zeiss MERLIN with GEMINI II SEM was used for imaging. C. Anode-free full cells. Prior to assembling full cells, FeS 2 electrodes were pre-sodiated in shorted cells with Na metal, a Celgard 2325 separator, and 1M NaPF 6 diglyme electrolyte for 24h. The presodiated FeS 2 electrodes were then dried and paired with a carbon/al negative electrode using a Celgard 2325 separator and 1M NaPF 6 diglyme electrolyte and assembled into CR2032 coin cells. After cell assembly, full cells were galvanostatically charged to 3.0 V prior to cycling. Energy density calculations were based on the weight of the carbon black on the negative side and the pre-sodiated FeS 2 on the positive side, assuming a stoichiometry of Na 1.5 FeS 2, which would correspond to a FeS 2 specific capacity of ~335 mah/g. In comparison, if the mass of the active Na is not accounted for, the energy density would be calculated to be ~500 Wh/kg. 2

3 Figure S1. Raman spectroscopic characterization of the carbon layer before and after initial sodiation performed using a green (2.33 ev) laser. The D and G peaks labeled correspond to modes originating from defective sp 3 carbon bonding and sp 2 carbon bonding, respectively. The blue-shifting of the G peak may be due to cointercalation of Na ions and diglyme into graphitic domains as we have reported in our previous work [Ref. 32 in main text]. Right, SEM micrograph shows the carbon nucleation layer after sodiation. 3

4 Figure S2. Comparing initial cycling performance for bare Al electrodes and carbon/al electrodes. We observe higher initial Coulombic efficiency for the carbon/al electrodes followed by more stable performance. Testing was performed at 0.5 ma/cm 2 for 30 min plating times. 4

5 Figure S3. Testing the bare Al electrodes at high rates, we see device failure transitioning from 2.0 ma/cm 2 to 4.0 ma/cm 2. In contrast, carbon/al electrodes demonstrated stable performance at 4.0 ma/cm 2 as shown in Figure 2a. 5

6 Figure S4. Comparing the plating hysteresis of carbon/al electrode to recent report on bare Cu electrode [Ref. 2]. Both use 1M NaPF 6 in diglyme electrolyte. It is also worth noting that the low hysteresis for the carbon/al electrode is shown to be stable over 1000 cycles whereas the hysteresis for bare Cu electrodes is reported to increase with cycling (from 13.3 mv to 18.4 mv over 300 cycle). 6

7 Figure S5. Cycling of carbon/al electrodes with different loading times from 30 minutes to 8 hours performed at a current of 1.0 ma/cm 2. 7

8 Figure S6. Bare Al electrode (10 mm diameter) with 2mAh/cm 2 of plated sodium metal performed at a rate of 0.5 ma/cm 2 (4 hour plating duration). 8

9 Figure S7. Top-down SEM images of carbon/al electrode with 0.5 mah/cm 2 of plated sodium metal performed at a rate of 0.5 ma/cm 2 (1 hour plating duration). Right image shows the surface of the sodium metal. We attribute the lightly pitted morphology observed on the surface to be a result of brief exposure to air during the transfer process. 9

10 Figure S8. Carbon/Al electrode (10 mm diameter) plated with 2mAh/cm 2 of plated sodium metal performed at a rate of 4 ma/cm 2 (30 minute plating duration). 10

11 Figure S9. Cross-sectional SEM images of carbon/al electrode with 0.5 mah/cm 2 of plated sodium metal performed at a rate of 0.5 ma/cm 2 (1 hour plating duration). 11

12 Figure S10. Sodium metal (1 mah) plated from pre-sodiated FeS 2 on carbon/al electrode during the first charging of the device. The image shows that, as expected, sodium metal is formed during charging for the anode-free full cells. To open this cell without shorting the device, testing was performed in a split-flat cell in the glovebox for easy disassembly. 12

13 Figure S11. Cycling of FeS 2 full cell over the first 40 cycles including Coulombic efficiency (CE%) (top) and capacity (bottom). We speculate drops in CE% to be attributed to a thick FeS 2 cathode architecture. 13

14 SUPPORTING REFERENCES 1. Gaume R. M., Joubert M.-L., Rev. Sci. Instrum. 2011, 82, Seh, Z. W.; Sun, J.; Sun, Y.; Cui, Y. ACS Cent. Sci. 2015, 1,