Chapter 3. Design of organic single-ion conductor based on electrophoretic
3.1. Electrophoretic regulation of polycations for artificial solid-
3.1.2. Experiment
Synthesis of the DADMA-TFSI. DADMA-TFSI was synthesized by two step processes as follows.
50 mole of allyldimethylamine (TCI) was dissolved in 100 mL of acetonitrile (MeCN, Aldrich) and it was cooled to 0℃. Thereafter, 55 mole of allyl bromide (Aldrich) was slowly added to round bottom for 30 min. Resulted mixture was then heated to room temperature and stirred for 12 h. After reaction was completed, MeCN was removed by reduced pressure using a rotary evaporator. Crude product was then purified via extraction with 20 mL of ethyl acetate (Aldrich) and followed by 20 mL of diethyl ether (Aldrich) in order to remove organic impurities remaining in crude product. Finally, drying it in vacuum oven for 12 h gave intermediate, DADMA-Br (1H NMR δ 3.31 (s, 6H), 4.42 (d, J = 8.0, 4H), 5.68 (d, J = 12.0, 2H), 5.83 (d, J = 16.0, 2H), 6.20 (m, 2H), see Figure. S13). 50 mole of DADMA-Br was dissolved in 100 mL of distilled water. Thereafter, 50 mole of LiTFSI (3M) was added to solution and it was stirred for 12 h to allow anion metathesis reaction. After reaction was completed, bottom layer was recovered by extraction with 50 mL of dichloromethane (Daejung) for two times and recovered dichloromethane-based solution was purified by short-column chromatography using a neutral aluminum oxide (Aldrich) as a stationary phase. Thereafter, dichloromethane was removed by reduced pressure and further drying in vacuum oven at room temperature afforded desired product, DADMA-TFSI (1H NMR δ 3.22 (s, 6H), 4.14 (d, J = 8.0, 4H), 5.73 (d, J = 8.0, 2H), 5.76 (d, J = 16.0, 2H), 6.20 (m, 2H), see Figure. S13).
Preparation of the pSEI-Li. ETPTA and 2-hydroxy-2-methylpropiophenone (HMPP) were purchased from Sigma-Aldrich and used as received. The pSEI paste was composed of ETPTA/DADMA-TFSI = 30/70 (w/w) solution with 5 wt% HMPP as a photoinitiator. To fabricate the pSEI-Li, the pSEI paste was printed on the Li metal via doctor blade coating followed by UV irradiation (Hg UV-lamp (Lichtzen) with an irradiation peak intensity of approximately 2000 mW cm-2.
Characterization of the pSEI-Li. The surface and cross-sectional morphologies of the pSEI-Li were characterized using a field emission scanning electron microscope (FE-SEM, S-4800 (Hitachi)) in conjunction with an energy-dispersive X-ray spectrometer (EDS). The structural evolution of the pSEI and electrostatic interaction between positively charged pSEI and free anion were traced by using a Fourier transform infrared spectrometer (FT-IR, Alpha Platinum ATR (Bruker)). The elastic modulus of the pSEI was investigated by Nanoindentation (TS1, Hysitron). The local ionic topology and resistance of pSEI-Cu was characterized using a scanning electrochemical workstation (M470, Biologic) connected with a Pt dual microelectrode (LEIS scanning probe, Biologic) and a potentiostat (SP-300, Biologic). The pSEI-Cu was subjected to Li plating and stripping for 5 cycles at current density
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of 0.5 mA cm-2 with plating and stripping capacity of 1.0 mAh cm-2. The precycled pSEI-Cu was attached on an Au working electrode of a beaker-type analytic cell (μTriCell, Biologic) filled with a liquid electrolyte (1 M LiPF6 in ethylene carbonate (EC)/propylene carbonate (PC) = 1/1 (v/v)). The local impedance area scan was measured at a fixed frequency of 5×104 Hz and an applied amplitude of 10 mV over an area of 1000 μm × 1000 μm with 10 μm spacing. To explore the hydrophobicity of the pSEI-Li, 6 μL of water was dropped onto the surface of the samples and contact angles were analyzed using a contact angle analyzer (Drop Shape Analysis System DSA100, Kruss). The surface tarnish of the pSEI-Li was characterized by optical image analysis using digital camera under a controlled atmosphere. The obtained surface tarnish results were normalized by the open source software ImageJ using the previously reported protocol.16 The surficial oxidation was further elucidated by Raman spectroscopy (Alpha 300S, WITec) with 633 nm excitation.
Electrochemical measurement of the pSEI-Li. The electrochemical performance was investigated using 2032-type coin cells. A liquid electrolyte (1 M LiPF6 in EC/diethyl carbonate (DEC) = 1/1 (v/v), without any additives) was used. To analyze the ion conduction behavior, the pSEI film was swelled in the liquid electrolyte for 1 day. 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 using a potentiostatic polarization method.17 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 tLi+:
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 cyclic voltammetry (CV) was conducted with an asymmetric cell (pSEI-Cu/Li) under a sweep rate of 10 mV s-1 in a voltage range of -0.5 – 3.0 V (vs. Li/Li+). The galvanostatic Li extraction of the moisture-exposed pSEI-Li toward a Cu foil was conducted using an asymmetric cell of Li/Cu at current density of 1 mA cm-2 with a cut-off voltage of -0.75 V. To make Li metal full cells, a LiCoO2 (LCO) cathode was fabricated by casting a slurry mixture (LCO/polyvinylidene fluoride (PVDF)/carbon black = 95/3/2 (w/w/w) in N-methyl-2- pyrrolidone) on an Al foil with areal active material loading of 7.7 mg cm-2. The electrochemical performance of the full cell (LCO/pSEI-Li) was examined using a cycle tester (PNE Solution Co., Ltd) in a voltage range of 3.0 – 4.25 V at charge/discharge current density of 0.2 C/0.5 C. The galvanostatic
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intermittent titration technique (GITT) analysis was conducted with the full cell at current density of 78.5 mA g-1 and interruption time between each pulse of 1 h.
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