Chapter 3. Design of organic single-ion conductor based on electrophoretic
3.2. Interpenetrated polycations for anode-free lithium storage
3.2.2. Experiment
Materials. Pyrrolidine(99%), 11-bromo-1-undecanol(97%), anhydrous dimethyl sulfoxide (DMSO) (99.8%), sodium tetraphenylborate (NaB(Ph)4) (99%), and 1,4-Dazabicyclo[2,2,2]octane (DABCO) (>99%) were purchased from Alfa Aser and used as received. Potassium hexafluorophosphate (KPF6,
>95%), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, >98%), and methylene diphenyl 4,4’- diisocyanate (MDI, 97%) were purchased from TCI and used as received. Tetrahydrofuran (THF) and acetonitrile (MeCN) were purchased from SAMCHUN (Korea) and used as received. Ethoxylated trimethylolpropane triacrylate (ETPTA) and 2-hydroxy-2-methylpropiophenone (HMPP) were purchased from Sigma-Aldrich and used as received.
Preparation of dihydroxyl pyrrolidinium monomers.
N-(11-hydroxyundecyl)pyrrolidine. A mixture solution of pyrrolidine (3.114 g, 43.78 mmol), NaOH 50 wt% solution (4.066 g, 50.82 mmol), 11-bromo-1-undecanol (10.00 g, 39.09 mmol) in THF (50 mL) was refluxed for 24 h. After the reaction mixture was cooled to room temperature, the solvent was removed by a rotoevaporator. The residual mixture was extracted with dichloromethane/water 3 times.
The combined organic layer was washed with water and then was dried over anhydrous MgSO4. Column chromatography through a short silica gel column with THF eluent gave a white solid (7.784 g, 81%).
1H NMR (500 MHz, CDCl3, 23 °C): δ 1.12 (m, 4H), 1.30 (m, 4H), 1.54 (m, 4H), 2.19 (t, J = 8, 2H ), 2.26 (s, 4H), 3.30 (t, J = 7, 2H). 13C NMR (125 MHz, CDCl3, 23 °C): δ 20, 23, 25, 26.4, 26.8, 26.93, 26.96, 26.99, 27, 30.
N,N-di(11-hydroxyundecyl)pyrrolidinium Br-. A solution of 11-bromo-1-undecanol (8.910 g, 35.46 mmol) and N-(11-hydroxyundecyl)pyrrolidine (7.784 g, 32.24 mmol) in MeCN (45 mL) was refluxed for 24 h. After the reaction completed, the solvent was removed by a rotoevaporator. The residue solid was precipitated by THF 3 times. After the filtration, drying in a vacuum oven gave a white crystalline solid (13.03 g, 82%). DSC: Tm = 30 oC (2nd heating) 1H NMR (500 MHz, DMSO-d6, 23 °C): δ 1.24 (m, 28H), 1.38 (m, 4H), 1.59 (m, 4H), 2.03 (t, J = 7, 4H), 3.19 (m, 4H), 3.35 (m, 4H), 3.45 (t, J = 5, 4H), 4.35 (t, J = 5, 2H). 13C NMR (125 MHz, DMSO-d6, 23 °C): δ 21, 22, 25, 26, 28, 29.2, 29.3, 29.4, 29.5, 33, 58, 61, 62.
N,N-di(11-hydroxyundecyl)pyrrolidinium PF6-. The previously obtained bromide salt (2.000 g, 4.060
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mmol) was dissolved in deionized water (10 mL) and KPF6 (1.495 g, 8.120 mmol) was added to the solution with vigorous stirring. The produced precipitate was filtered and the filter cake was washed by deionized water 3 times. Drying in a vacuum oven gave a white crystalline solid (2.015 g, 89%). DSC:
Tm = 82 oC (second heating). 1H NMR (500 MHz, acetone-d6, 23 °C) : δ 1.29 (m, 20H), 1.39 (m, 8H), 1.48 (m, 4H), 1.85 (m, 4H), 2.26 (m, 4H), 3.45 (m, J = 8.5, 4H), 3.51 (m, 4H), 3.71 (t, J = 6.5, 4H). 13C NMR (125 MHz, acetone-d6, 23 °C) : δ 23, 24, 27.7, 27.8, 30, 31.1, 31.2, 31.4 34, 61, 63.4, 63.5, 64.
N,N-di(11-hydroxyundecyl)pyrrolidinium TFSI-. The previously obtained bromide salt (2.000 g, 4.060 mmol) was dissolved in deionized water (10 mL) and LiTFSI (2.331 g, 8.120 mmol) was added with vigorous stirring. After decanting off the upper aqueous layer, the residual viscous liquid was washed with deionized water 3 times. Drying in a vacuum oven gave a yellow viscous liquid (2.128 g, 76%).
DSC: Tg = 51 oC (second heating), no Tm. 1H NMR (500 MHz, acetone-d6, 23 °C) : δ 1.29 (m, 20H), 1.39 (m, 8H), 1.48 (m, 4H), 1.85 (m, 4H), 2.26 (s, 4H), 3.45 (m, 4H), 3.51 (m, 4H), 3.71 (t, J = 6.5, 4H). 13C NMR (125 MHz, acetone-d6, 23 °C) : δ 23, 24, 27.6, 27.9, 30, 31.0, 31.1, 31.3 34, 61, 63.2, 63.3, 64, 118, 120, 123, 125.
N,N-di(11-hydroxyundecyl)pyrrolidinium B(Ph)4-. From the previous experiment, the bromide salt (2.000 g, 4.060 mmol) was dissolved in deionized water (10 mL) NaB(Ph)4 (1.667 g, 4.872 mmol) was added with vigorous stirring. The insoluble precipitate was filtered and the filter cake was washed by deionized water there times. Drying in a vacuum oven gave a white solid (2.511g, 84%). DSC: Tg = - 13 oC (second heating), no Tm. 1H NMR (500 MHz, acetone-d6, 23 °C) : δ 1.30 (m, 20H), 1.36 (m, 8H), 1.50 (m, 4H), 1.75 (m, 4H), 2.11 (m, 4H), 3.27 (m, 4H), 3.47 (m, 4H), 3.52 (m, 4H), 7.25(t, J =7, 4H), 7.39(t, J =7.5, 8H), 7.80(m, 8H) 13C NMR (125 MHz, acetone-d6, 23 °C) : δ 23, 24, 27., 29.7, 30, 31.0, 31.1, 31.3 34, 61,121, 125(q, 2J(B,C) = 2), 136, 164(q, 1J(B,C)= 50)
General synthetic procedures for the cationic polyurathanes (PUs). A mixture of N,N-di(11- hydroxyundecyl)pyrrolidinium salt (1.000 eq.), MDI (1.000 eq.) and DABCO (130 ppm) as a catalyst in DMSO (20 mL) was stirred for 5-7 days at 90 °C under N2 atmosphere. Complete consumption of the isocyanate groups was demonstrated by FT-IR spectroscopy. After the reaction completion, the mixture was cooled to room temperature, and then the formed polymer was precipitated by methanol (or ethyl acetate). Further purification was followed by a Soxhlet apparatus using MeOH for 12 h.
Drying in a vacuum oven at 70 °C gave a solid phase cationic PU.
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Preperation of the QSIC. The QSIC precursor was composed of 1 M LiPF6 in ethylene carbonate:propylene carbone = 1:1 (v/v) without any additives, UV-curable ETPTA (incorporating 5 wt%
HMPP as a photoinitiator), and cationic PU, in which the composition ratio of 1 M LiPF6 in EC/PC:ETPTA:cationic PU = 90:5:5 (w/w/w). To fabricate the QSIC, the precursor was filled in a home-made polypropylene mold followed by UV irradiation (Hg UV-lamp (Lichtzen) with an irradiation peak intensity of approximately 2000 mW cm-2.
Structural characterizations. The structural evolution of the QSIC and electrostatic interaction between positively charged QSIC and free anion were traced by using a Fourier transform infrared spectrometer (FT-IR, Alpha Platinum ATR (Bruker)). The chemical species formed on SEI of cycled Li anodes were investigated by using a X-ray photoelectron spectroscopy (XPS, K-alpha (ThermoFisher)).
The surface and cross-sectional morphologies of the QSIC were characterized using a field emission scanning electron microscope (FE-SEM, S-4800 (Hitachi)) in conjunction with an energy-dispersive X- ray spectrometer (EDS). To investigate pore size distribution, the etched QSIC was prepared by solvent extraction (dimethyl carbonate followed by acetone) of the QSIC. The pore size distribution was investigated by using mercury intrusion porosimetry (Auto Pore IV 9520 (Micromeritics)). A penetrometer of 3 mL total volume and 0.4 mL stem volume was loaded with 0.38 g of etched QSIC.
Mercury intrusion volume was obtained in a pressure range of 0.1-60000 psia. The pore radius was estimated from the pressure (P) by the Washburn equation12:
𝑟𝑃=2𝛾 cos 𝜃 𝑃
where a contact angle (𝜃) is 130° and mercury surface tension (𝛾) is 485 dyn cm-1. The chemical shift of Li salts was recorded by using a nuclear magnetic resonance spectroscopy (600 MHz FT-NMR, VNMRS 600 (Agilent)) with 1.6 mm HXY Fast MAS T3 probe. The chemical shift is referenced to a 1 M aqueous LiCl solution at 7Li (0 ppm).
Electrochemical measurements. The electrochemical performance was investigated using 2032-type coin cell and potentiostat (VSP classic, (Bio-Logic)). A liquid electrolyte (1 M LiPF6 in EC/PC = 1/1 (v/v) without any additives) was used. The ion conductivity was measured with an Li-ion blocking symmetric cell based on an electrochemical impedance spectroscopy (EIS) analysis at a frequency range from 10-2 to 106 Hz and an applied amplitude of 10 mV. The Li+ transference number (tLi+) was evaluated
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using a potentiostatic polarization method.13 The DC polarization through a Li-ion non-blocking symmetric cell and its sequential EIS before and after the polarization was analyzed to determine the Li+ transference number:
𝑡𝐿𝑖+=𝐼𝑠(∆𝑉 − 𝐼𝑜𝑅𝑜) 𝐼𝑜(∆𝑉 − 𝐼𝑠𝑅𝑠)
where ΔV is applied potential, Io and Ro are the initial current and resistance, and Is and Rs are the steady-state current and resistance after the polarization, respectively. The Li metal anode cycle test was conducted with the Li|QSIC-B(Ph)4|Li symmetric cell under a current density of 1 mA cm-2 for 1 hour per cycle at room temperature. For the cathode test, a LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode was prepared by casting a slurry mixture (NCM811:polyvinylidene fluoride:carbon black = 94:3:3 (w/w/w) in N-methyl-2-pyrrolidone (NMP)) on an Al foil with electrode thickness of 10 μm (= areal active material loading of 1.1 mg cm-2). The cathode and anode kinetics of QSIC were investigated by cyclic voltammetry (CV) with cathode (NCM811|QSIC-B(Ph)4|Li) and anode (SuS|QSIC-B(Ph)4|Li) cells under a sweep rate of 0.2 and 1.0 mV s-1, respectively. The electrochemical performance of the half cell (NCM811|QSIC-B(Ph)4|200 μm Li metal) and anode-free cell (NCM811|QSIC-B(Ph)4|copper foil) was examined using a cycle tester (PEBC050.1, (PNE Solution Co., Ltd)) in a voltage range of 3.0–4.25 V vs. Li/Li+. The galvanostatic intermittent titration technique (GITT) analysis was conducted with the cathode half cell at current density of 16.7 mA g-1 and interruption time between each pulse of 1 h.
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