Supporting Information for Fluorinated Ethylene Carbonate as Electrolyte Additive for Rechargeable Na Batteries

Transcription

1 Supporting Information for Fluorinated Ethylene Carbonate as Electrolyte Additive for Rechargeable Na Batteries Shinichi Komaba,* a Toru Ishikawa, a Naoaki Yabuuchi, a Wataru Murata, a Atsushi Ito, b and Yasuhiko Ohsawa b a Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo , Japan * komaba@rs.kagu.tus.ac.jp b Nissan Research Center, Nissan Motor Co., Ltd., 1 Natsushima-cho, Yokosuka, Kanagawa , Japan 1

2 Experimental The reagent grade sodium metal, poly(vinylidene fluoride) (Wako Pure Chemical Industries, Ltd.), battery grade ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), vinylene carbonate (VC), NaClO 4 (containing ca. 100 ppm H 2 O), NaPF 6, NaN(SO 2 CF 3 ) 2 (Kishida Chemical Co., Ltd.), fluoroethylene carbonate (FEC), t-di fluoroethylene carbonate (DFEC) (Kanto Denka Kogyo Co., Ltd.) and commercially available hard-carbon (Carbotron P(J), Kureha Co.) (ref. 4) are used. All chemicals are handled in an Ar-filled glove box (whose dew point ranges from 85 to 70ºC). The NaNi 1/2 Mn 1/2 O 2 was synthesized as we previously described (ref. 4). For the electrode preparation, hard-carbon (90 wt%) was mixed with PVdF binder (10 wt%) in N-methylpyrrolidinone. The obtained slurry was doctor-bladed onto Al or Cu foil. The NaNi 1/2 Mn 1/2 O 2 (80 wt%) and acethylene black (10 wt%) were mixed with PVdF (10 wt%) in N-methylpyrrolidinone, and thus obtained slurry was cast onto Al foil. The electrodes were dried at > 80ºC in a vacuum prior to use. The coin-type cells (20 mm diameter, 3.2 mm thickness) with hard-carbon or NaNi 0.5 Mn 0.5 O 2, and excess Na metal electrodes, electrolyte, and a glass fiber filter (thickness: 0.38 mm, Toyo Roshi Kaisha, Ltd.) as a separator were fabricated in the glove-box. The electrodes were observed by using a scanning electron microscope (SEM, S-5000, Hitachi), transmission electron microscope (TEM, 200 kv, H-9500, Hitachi), and X-ray photoelectron spectrometer (XPS, JPS-9010MC, JEOL) equipped with monochromatic Mg Kα X-ray source with depth profiling by argon ion beam sputtering. The electrolyte solution within separator was analyzed by attenuated total reflection-fourier transform infrared spectroscopy (ATR FT-IR, NICOLET6700, Thermo Scientific. Co. Ltd.). 2

3 PC FEC DFEC VC ES Figure S1. Molecular structures of electrolyte additives used in Na cells. 3

4 Voltage / V a) b) Cell type Initial efficiency Beaker 80 % Coin 90 % Beaker-type Coin-type Capacity / mah g -1 Capacity / mah g Coin-type Beaker-type Cycle Number Figure S2. (a) The initial reduction/oxidation curves (inset: initial efficiency defined as Q oxidation /Q reduction ) and (b) reversible capacity variation for hard-carbon electrodes in a 1 mol dm -3 NaClO 4 PC solution comparing results in beaker-type or coin-type Na cells at a rate of 25 ma g -1. 4

5 Voltage / V vs. Na a) b) PC:DMC:FEC PC:DMC Q / mah g -1 Q / mah g PC:DMC PC:DMC:FEC c) Cycle number / - Figure S3. (a) The initial reduction/oxidation curves and (b) reversible capacity variation for hard-carbon on Al foil in 1 mol dm -3 NaClO 4 PC:DMC (1:1) solutions with and without 2 vol% FEC additive tested at a rate of 25 and + 25 ma g -1 in coin-type Na cells. (c) A photo of separator tested in the FEC-free PC:DMC after 50 cycles. 5

6 Voltage / V vs. Na 2 1 a) b) 2 % DFEC 0.2 % DFEC DFEC-free Q / mah g % DFEC DEEC-free 2 % DFEC Q / mah g Cycle number / - Figure S4. (a) The initial reduction/oxidation curves and (b) reversible capacity variation for hard-carbon electrodes in 1 mol dm -3 NaClO 4 PC solutions with and without 0.2 and 2 vol% DFEC examined at a rate of 25 and + 25 ma g -1 in coin-type Na cells. 6

7 2 a) b) Voltage / V 1 0 PC:VC (98:2) VC free Q / mah g -1 Q / mah g VC free PC:VC (98:2) Cycle number Figure S5. (a) The initial reduction/oxidation curves and (b) reversible capacity variation for hard-carbon electrodes in 1 mol dm -3 NaClO 4 PC solutions with and without 2 vol% VC examined at a rate of 25 and + 25 ma g -1 in coin-type Na cells. 7

8 a) b) c) VC-free Na/hard-carbon 2 vol% VC Na/hard-carbon 2 vol% VC hard-carbon/nani 1/2 Mn 1/2 O 2 Figure S6. Photographs of the used glass-separators in coin-type cells of Na//hard-carbon (half cell) configuration with 1 mol dm -3 NaClO 4 in a) PC and b) PC:VC (98:2 volume) and c) hard-carbon//nani 1/2 Mn 1/2 O 2 (Na-ion full cell) configuration with the 2 vol% VC solution after the first galvanostatic cycle. 8

9 PVdF(-CF 2 -) ester linkage C-O-C Na 2 CO 3 NaO-COO-R -CH 2 - graphite R-O-(CO)O-Na R-O-Na Na 2 CO 3 e) d) e) d) c) c) Binding Energy / ev b) a) Binding Energy / ev b) a) Figure S7. (left) C 1s and (right) O 1s X-ray photoelectron spectra for hard-carbons: a) pristine and hard-carbon electrodes after the first galvanostatic cycle test in 1 mol dm -3 NaClO 4 PC b) without and with c) 0.5, d) 2, and e) 10 vol% FEC addition in Na cells. 9

10 C-F PVdF NaF e) d) c) b) a) Binding Energy / ev Figure S8. F 1s XPS spectra for hard-carbons: a) pristine and hard-carbon electrodes after the first galvanostatic cycle test in 1 mol dm -3 NaClO 4 PC b) without and with c) 0.5, d) 2, and e) 10 vol% FEC addition in Na cells. 10

11 a) b) c) FEC-free (5 cycles) 2% FEC Additive (5 cycles) 2% FEC Additive (> 100 cycles) Figure S9. Photographs of the used glass-separators in coin-type cells of Na//hard-carbon cell with 1 mol dm -3 NaClO 4 in a) PC after 5 cycles, and b) PC:FEC (98:2 in volume) after b) 5 and c) 133 cycles. 11

12 c) PC:FEC after 10 cycles b) PC after 10 cycles a) PC before cycle Dry glass filter Wavenumber / cm -1 Figure S10. FT-IR spectra of the electrolyte solutions held in glass-separators a) before and after electrochemical tests in coin-type cells of Na//hard-carbon cell with 1 mol dm -3 NaClO 4 in b) PC after 10 cycles, and c) PC:FEC (98:2 in volume) after 10 cycles. Arrows indicate the peaks of decomposition products such as sodium alkoxides and/or sodium alkyl carbonates. A result of pristine glass-separator without any electrolyte solutions is shown for comparison. 12

13 CO 2 asym. stretching 1650 st CO sym. stretching 1419 st CO sym. stretching 1467 sm, 1343 st CH 3 rocking 1297 m CO stretching 1093 st CO 3 stretching 830 st -PC after 10 cycles -PC before cycle W avenum ber / cm -1 Figure S11 Comparison of FT-IR spectra of the electrolyte solutions held in glass-separators (black) before and (red) after 10 cycle tests in coin-type cells of Na//hard-carbon cell with 1 mol dm -3 NaClO 4 in PC. Additional peaks due to decomposition product are indexed as sodium propyl carbonate according to the literatures. 13

14 0.006 c) Current / ma a) b) Voltage / V vs. Na Figure S12. Cyclic voltammograms at the third cycle for Al foil electrodes (10 mm in diameter) at 0.2 mv s -1 in 1 mol dm -3 NaClO 4 in a) PC, b) PC:FEC (98:2 in volume), and c) the PC used in hard-carbon//na cell after 10 galvanostatic cycles in coin-type Na cells. 14