Sulfur Co-polymer a New Cathode Structure for Room Temperature Sodium-Sulfur Batteries

Transcription

1 Supporting information Sulfur Co-polymer a New Cathode Structure for Room Temperature Sodium-Sulfur Batteries Arnab Ghosh, Swapnil Shukla, Monisha Monisha, Ajit Kumar, Bimlesh Lochab* and Sagar Mitra* Electrochemical Energy Laboratory, Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai , India Department of Chemistry, School of Natural Sciences, Shiv Nadar University, Gautam Budh Nagar, Uttar Pradesh , India 1

2 Experimental section Materials Cardanol (ρ = g cm 3, iodine value 250, acid value max 5, hydroxyl value ) was procured from Satya Cashew Chemicals Pvt. Ltd. (India), paraformaldehyde and aniline, fuming nitric acid from Fisher scientific, chloroform from Finar, anhydrous sodium sulphate from Chemlabs, CDCl 3 from Euriso-top and sulfur (S 8, 325 mesh, reagent grade) from Alfa Aesar, super-p (conducting carbon) from Timcal, polyethylene oxide (PEO), NaClO 4 and NaNO 3 from Sigma Aldrich, acetonitrile, tetrahydrofuran, tetraethyl glycol dimethyl ether (TEGDME) from Merck. All the reagents were used as received. Synthesis of cardanol based benzoxazine (Ca) Cardanol benzoxazine was prepared from cardanol as per the procedure reported previously. 26, 27 Briefly, a mixture of cardanol (10 g, 33.2 mmol), paraformaldehyde (1.99 g, 66.4 mmol), aniline (3.03 ml, 33.2 mmol) was gradually heated from room temperature to 70 o C followed by heating at 80 o C (1 h) and 90 o C (2 h). Thereafter, water was added to the reaction mixture and extraction was carried out with chloroform. Hence, obtained organic layers was combined and dried over sodium sulphate and filtered. Solvent was removed in vacuo to yield cardanol benzoxazine (Ca) as red brown oil. FT-IR (ATR, v cm 1 ): 3008, 2926, 2854,1626, 1595, 1579, 1350, 1257, 1240, 1116, 1030, 995, 950 and 627 cm 1 ; 1 H-NMR (500 MHz, CDCl 3, δ ppm): 0.94 (CH 3, t), 1.35 [(CH 2 ) n, m], 1.85, 2.07 (CH 2 CH=, m), 2.59 (CH 2 Ar, t), 2.89 [CH 2 (CH=) 2, m], 4.57 (s, ArCH 2 N ), 5.05 ( CH=CH, dd), 5.37 (m, CH=, CH 2 =CH, OCH 2 N, HC=CH 2 ), 6.60(ArH, s), 6.72(ArH, d), 6.93 (ArH, m), 7.10 (ArH, d), 7.27 (ArH, m); LCMS (ESI Interface-positive ions, HRMS): Bzc adduct: , , , , [Ca expected for monoene , , ; diene , , ; and triene , , ]. Synthesis of poly sulfur-random-cardanol benzoxazine (CS90) As reported previously, elemental sulfur (1.8 g, mmol) was heated to a temperature of 185 o C in an oil bath equipped with thermostat to form an orange coloured viscous molten phase. Ca (0.2 g, 0.47 mmol) added and reaction was continued at the temperature of 185 o C for 10 minutes. Mixture was cooled down to yield CS90 as a brown solid. 2

3 Synthesis of graphene oxide (GO) The preparation of Graphene oxide (GO) was carried out via Hummer s method. Graphite flakes (2 g) were added to concentrated sulfuric acid (46 ml) and sodium nitrate (1 g) at a temperature of 0 o C with continuous stirring. Thereafter, potassium permanganate (6.0 g) was added gradually maintaining the temperature below 20 o C. Hence, obtained suspension was vigorously stirred to give a brownish suspension in 30 minutes. Distilled water (92 ml) was added to the followed by the treatment with warm water (280 ml) and hydrogen peroxide (3%) to give brown coloured GO. Hence formed GO was washed with water till all the remaining acidic content was removed followed by vacuum drying, producing brown flakes (1.2 g, 60%). Synthesis of reduced graphene oxide, rgo (S) GO (260 mg) was sonicated in deionized water (65 ml) to form a homogenous solution, where after liquor ammonia (5 ml) was added to this aqueous dispersion (ph 12). Gallic acid (2.607 g, 15.3 mmol) was then added to the mixture followed by stirring at 95 o C for 10 h in N 2 atmosphere. The reduced graphene oxide (rgo) was filtered and subjected to washing three times with deionized (3 Х 250 ml) water followed by methanol (50 ml). The as obtained product was dried and weighed to form a black powder (150 mg, 58%). The gallic acid reduced graphene oxide is designated and further referred as rgo (S). Synthesis of CS90-rGO (S) composite A suspension of CS90 (1.46 g) in THF (30 ml) was sonicated for 1 h to ensure a uniform dispersion. This was followed by addition of 2.5 wt % (37.5 mg) rgo (S) after which the mixture was further sonicated for 2 h to ensure uniform distribution of rgo (S). A grey-black powder was obtained after removal of solvent under vacuum. The obtained composite is designated and further referred as CS90-rGO (S). Characterization techniques and instruments Raman spectra were obtained using a Renishaw invia Raman Microscope with 500 mw at 785 nm. FTIR spectra of the samples were recorded in the wavelength range cm 1 using a Thermo scientific FTIR (NICOLET 8700) analyzer with an attenuated total reflectance (ATR) crystal (Diamond) accessory. Bruker AC 400 MHz FT-NMR spectrometer 3

4 was used to record the 1 H-NMR of the samples after filtering the solution through PTFE filters (0.45 µm) to remove particulates, if any. The spectrum was recorded in CDCl 3 using tetramethylsilane as the internal standard. Agilent HRMS 6540 Series Q-TOF was used for mass spectrometry of Ca monomer. Hot stage microscopy (HSM) studies were carried out using a Leica MZ75 polarizing microscope equipped with a P350 heating stage. Leica IM 50 software was used for image grabbing and monitoring of the sample temperatures. Calorimetric studies were performed on a Differential Scanning Calorimeter (TA instruments Q 20). For dynamic DSC scans, samples (10 ± 2 mg) were sealed in aluminium pans, and heated from 50 to 160 o C at the scan rate of 10 o C min 1 under nitrogen atmosphere. Powder X-Ray Diffraction (PXRD) studies were performed on Rigaku SmartLab X-Ray Diffractometer, Cu Kα radiation (λ = Å). X-ray photoelectron spectroscopy (XPS; AXIS Supra, Kratos Analytical, monochromatic Al Kα 600 W X-ray source) studies were carried out to analyse the chemical composition. The improvement of electrical conductivity of sulfur copolymer upon rgo (S) addition was confirmed by a four probe system (MHP-12 & Multi-PT8) at room temperature. For measurements, two different pellets of CS90 copolymer and CS90-rGO (S) composite powder (diameter = 12 mm and thickness = 700 µm) were prepared by applying a pressure of 5 tons for 1 min. The electrical conductivity (σ) of each pellet was calculated using the equation: σ = (ln2/πt) (I/V) S cm 1 (where, t = thickness of sample, I = applied current, and V = measured voltage). ICP-AES studies were carried out on ARCOS, Simultaneous ICP Spectrometer. Device assembly and electrochemical measurements The CS90 copolymer cathode was prepared by ball-milling 70 wt% CS90 copolymer, 20 wt% super C-65 carbon black as conductive additive and 10 wt% polyethylene oxide as binder. The slurry was prepared in acetonitrile and blade cast onto carbon-coated aluminium foil followed by vacuum drying at 50 o C for 48 h. For comparison, two electrodes were fabricated using elemental sulfur and S90-2.5% rgo (S) composite, respectively, following same process. The cathodes were then cut into disks with a diameter of 1 cm. The average active sulfur loading density on all the electrodes was 2.14 mg cm 2. Coin-type cells were assembled in an argon filled glovebox using sodium metal foil as anode. Celgard 2400 was used as separator soaked with 50 µl of electrolyte composed of 1 M NaClO 4 and 0.2 M NaNO 3 in tetraethyl glycol dimethyl ether. Cyclic voltammetry was carried out at a scan rate of 20 µv s 1 within the potential window V vs Na + /Na using Biologic VMP-3. The galvanostatic charge- 4

5 discharge experiments were performed in Arbin BT-2000 keeping the potential range V vs Na + /Na. The capacities were calculated based on the mass of sulfur. All electrochemical measurements were performed at 20 o C. In order to study the amount of active sulfur and intermediate sodium polysulfides leached out from the cathode, the cells were cycled for different cycle number and then disassembled in argon-filled glovebox. The celgard separator, sodium anode and cell package from the respective cell were then washed with fixed volume (2 ml) of TEGDME solvent. The solution containing the sulfur and sodium polysulfides were then diluted to 10 ml. After oxidizing 1 ml of the diluted solution with concentrated nitric acid, the acid solution was diluted to 10 ml with deionized water. The sulfur content was measured by ICP elemental analysis. Figure S1. Raman spectra of cardanol based benzoxazine (Ca), sulfur copolymer (CS90) and elemental sulfur. 5

6 Figure S2. FT-IR spectra of cardanol based benzoxazine (Ca) and CS90 copolymer. 6

7 Figure S3. 1 H-NMR spectra of cardanol benzoxazine (Ca) and CS-90 copolymer. 7

8 Figure S4. DSC scans for elemental sulfur and CS90 copolymer. 8

9 Figure S5. PXRD pattern of elemental sulphur and CS90 copolymer. The diffraction peaks of elemental sulphur matched with that of orthorhombic phase sulphur from JCPDS (card No ). The crystallite size was calculated using Scherrer s equation 1 at (222) reflection, 2θ 23 which is the highest diffraction peak (inset). 9

10 Figure S6. (a) Synthetic steps involved to prepare reduced graphene oxide rgo (S) using gallic acid as reducing agent; (b) UV-vis characterizations of graphene oxide (GO) and reduced graphene oxide rgo (S) ; (c) FT-IR characterizations of graphite, GO and rgo (S) ; (d) SAED pattern of gallic acid reduced graphene oxide. 10

11 Figure S7. Camera images of the solutions after washing the cell components (except cathode) of the cells cycled for different number of charge-discharge cycles for CS90 cathode. Figure S8. Camera images of the solution after washing the cell components (except cathode) of the cells cycled for 20 cycles for pure sulfur and CS90 cathodes. 11

12 Figure S9. Amount of sulfur leached out from the electrode after different charge-discharge cycles at 0.2 A g 1 current rate. 12

13 Figure S10. XRD patterns of pure sulfur cathode before cycling and after 50 cycles (at fully charged state). The appearance of two new intense peaks at o and o (marked with asterisks) seem to be originated from insoluble lower order sodium polysulfides (Na 2 S n, 1 n 3) deposit on sulfur cathode. 13

14 Figure S11. (a) XPS survey scan and (b) S2p spectra of pure sulfur cathode after 50 cycles of charge-discharge (at fully charged state). 14

15 Figure S12. XRD patterns of pure CS90 cathode before cycling and after 50 cycles (at fully charged state). After 50 cycles, the CS90 cathode did not exhibit any intense characteristic peaks of lower order sodium polysulfides (Na 2 S n, 1 n 3), indicating less aggregation of these insoluble lower order polysulfides on the surface of CS90 cathode. 15

16 Figure S13. S2p spectra of CS90 cathode before cycling and after 50 charge-discharge cycles (at fully charged state). Figure S14. EIS spectra of pure sulfur and CS90 cathodes: (a) before cycling and (b) after 50 charge-discharge cycles. 16

17 Figure S15. (a) Cycling performance and (b) second cycle charge-discharge profile of CS90 cathode with 3.92 mg cm 2 of active material loading. Figure S16. SEM image of sodium metal anode before cycling. 17

18 References: 1. Scherrer P. Bestimmung der Grösse und der inneren Struktur von Kolloidteilchen mittels Röntgenstrahlen. Nachr. Ges. Wiss. Göttingen 1918, 26,