SUPPORTING INFORMATION

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1 SUPPORTING INFORMATION PVDF/ Palygorskite Nanowire Composite Electrolyte for 4V Rechargeable Lithium Batteries with High Energy Density Pengcheng Yao 1, Bin Zhu 1,2, Haowei Zhai 1, Xiangbiao Liao 3, Yuxiang Zhu 1, Weiheng Xu 1, Qian Cheng 1, Charles Jayyosi 4, Zheng Li 5, Jia Zhu 2, Kristin M. Myers 4, Xi Chen 3 and Yuan Yang 1* 1 Program of Materials Science and Engineering, Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, 10027, United States 2 College of Engineering and Applied Science, Nanjing University, Nanjing, , P. R. China 3 Department of Earth and Environmental Engineering, Columbia University, New York, New York, 10027, United States 4 Department of Mechanical Engineering, Columbia University, New York, New York, 10027, United States 5 Jiangsu Qingtao Energy S&T Co.,Ltd, Huai-an, , P. R. China P.Y., B.Z., H.Z., and X.L. contributed equally to this paper. Preparation of polymer and composite electrolyte PVDF (Arkema, Kynar 761) was vacuum dried at 80 C for 12h to remove trapped water thoroughly. Palygorskite was dried in vacuum oven at 150 C to get rid of bound water. PVDF was first dissolved in N, N-dimethylformamide (DMF) with a concentration of 10 wt.%, followed by magnetic stirring at 45 C for 4 h to obtain a uniform solution. Next, lithium perchlorate (LiClO 4, Sigma Aldrich, 99.99%) was added into the solution with the weight ratio PVDF: LiClO 4 : DMF = 3: 1: 26, and continued stirring for 1 h until LiClO 4 was fully dissolved. Then 1, 3 and 5 wt.% palygorskite of PVDF polymer were added to the homogeneous solution. After that, the obtained suspension was undergone magnetic stirring for 12 h and ultrasonic dispersing for 6 hours so that the palygorskite can be disaggregated into single crystals. After that, the dispersed mixture was cast with a doctor blade on a clean glass plate. Finally, the solid composite electrolyte membranes (~100 m in thickness) were obtained by further drying in vacuum oven at 25 C, 60 C, 80 C, 100 C, and 120 C for 24 h to remove part of the DMF solvent. To prepare the composite cathode, 1

2 NMC powders (MSE Supplies), Super C65 (IMERYS), PVDF and LiClO 4 as both binder and electrolyte, were mixed at 8:1:1 weight ratio in N-methylpyrrolidone (NMP) solvent to obtain viscous slurries, which were then coated on Al foil and dried in a vacuum oven for 12 h at 80 o C. The active material mass loading of the composite cathode was controlled to be ~1.5 mg cm -2. The cell was fabricated in an Ar-filled glovebox (O 2 < 5 ppm, H 2 O < 0.1ppm) a b c d Figure S1. (a) PVDF/LiClO 4 dissolved in DMF with a ratio of 3:1:26. (b) Bent PVDF polymer electrolyte showing the excellent flexibility of the film. (c) PVDF polymer electrolyte membrane vacuum dried at 80 o C. (d) PVDF polymer electrolyte vacuum dried at 100 o C. Material Characterization Scanning electron micrographs (SEM) images were obtained with a Zeiss Sigma VP scanning electron microscope. Thermo-Gravimetric Analysis (TGA) was carried out with Ar flowing over the samples heated at a rate of 10 C min -1 with a TA Instruments Q500. X-ray diffraction (XRD) patterns were collected on a Panalytical Xpert 3 (Cu-Kα) diffractometer operated at 40 kv and 200 ma. 1 H NMR measurement on liquid samples was operated on a Bruker 300WB spectrometer. Raman spectra were performed with a Raman spectrometer (XploRA One by HORIBA). Transmission electron microscopy (TEM) images were achieved with an FEI Talos F200X Transmission / Scanning Transmission Electron Microscope (S/TEM). To figure out the origin of reduced thermal stability of the composite electrolyte, PVDF membranes without LiClO 4 were prepared using the same procedure described in main text (vacuum heating at 60 o C), so that the DMF content is ~24% inside. Also, 2

3 DMF-free PVDF/LiClO 4 membranes were prepared in the similar way but the DMF solvent was replaced by acetone, which was fully removed from the membrane during the vacuum drying process. The TGA results are shown in Supplementary Figure S2a. The LiClO 4 -free PVDF/DMF film shows almost the same behavior as PVDF powder, but the DMF-free PVDF/LiClO 4 has high weight loss around 300 o C, similar to PVDF/LiClO 4 /DMF one. The mechanism is likely to be that LiClO 4 is a strong oxidant, so that it starts to oxidize PVDF and release gas (e.g. CO 2, HF, etc) around 300 o C. Figure S2. (a) TGA curves of PVDF powder, PVDF polymer electrolyte without LiClO 4 vacuum dried under 60 o C and PVDF/LiClO 4 membrane without DMF. (b)raman spectra of PVDF polymer electrolyte and PVDF powder. EIS for PVDF polymer electrolytes and CPE membranes PVDF polymer electrolyte and CPE membranes with an average thickness of 100 μm were sandwiched between stainless steel plates and assembled in coin cells in an Ar-filled glovebox for the test of ionic conductivity over the frequency range from 10 Hz to 1 MHz. The ionic conductivity σ data was calculated by the equation σ = L/RS, where R was the bulk resistance, L and S were the thickness and area of the solid electrolyte membrane, respectively. The Nyquist spectra tested at room temperature are presented in Figure S3 in which the semicircle at high and intermediate frequencies is ascribed to the parallel combination of bulk resistance and bulk capacitance and plot at low frequencies corresponds to the double-layer 3

4 capacitance 1. The charge-discharge tests were performed to study the electrochemical performance of the PVDF/palygorskite CPE membranes. CR2032 type coin cells were assembled based on the as-prepared Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 (NMC111) cathode, PVDF/palygorskite CPE, and Li foil (Alfa Aesar) anode (Figure S5a). CPE membrane served as both electrolyte and separator. The full cell performance was tested under galvanostatic conditions between 3V and 4.2V using a battery test system (C2001A, LANDT, China). Figure S3. EIS for (a) 60 o C vacuum dried PVDF polymer electrolyte membrane (b) 80 o C vacuum 4

5 dried PVDF polymer electrolyte membrane (c) 100 o C vacuum dried PVDF polymer electrolyte membrane (d) 1 wt% palygorskite PVDF CPE (e) 3 wt% palygorskite PVDF CPE and (f) 5 wt% palygorskite PVDF CPE. Electrochemical performance of the PVDF/Palygorskite composite electrolyte membrane was measured in the symmetric lithium/lithium cell by periodically charged and discharged for 0.5 h. (Figure S4 b and c) Figure S4. (a) Arrhenius plots of PVDF based composite electrolytes with different content of ceramic fillers. Galvanostatic cycles with a constant current density of (b) 0.05 ma/cm 2 and (c) 0.15 ma/cm 2 for Li PVDF/palygorskite CE Li cells at 25 C. Full cell test of PVDF/Palygorskite CPE cells Figure S5. (a) Schematic illustration of NMC/PVDF-Palygorskite CPE/ Li battery. (b) Discharge 5

6 specific capacity vs cycle number (1C = 150 ma/g) of 1,3 wt % PVDF/palygorskite CPE cell. (c) Cycle performance at 0.3 C of liquid electrolyte cell. (d) Typical charge discharge curves of liquid electrolyte cell. Shorting time experiment and SEM characterizations. To provide better link between dendrite suppression and addition of palygorskite nanowires, shorting time during lithium plating and SEM characterizations are used to unveil the connection. First, pure PVDF polymer electrolyte, 3 wt % Palygorskite/PVDF CPE and 5 wt % Palygorskite/PVDF CPE were assembled in lithium/lithium symmetric cells, and charged with a constant current density of 0.3 ma/cm 2. The shorting time is used as an indicator to estimate the capability to suppress dendrite (Figure S6a-c). With pure PVDF polymer electrolyte, the cell is shorted after only 3.8 hours, which is equivalent to 1.14 mah/cm 2. Meanwhile, the time for 3 wt % Palygorskite/PVDF CPE and 5 wt % Palygorskite/PVDF CPE are 26 h and 36 h, respectively, corresponding to 7.8 and 10.8 mah/cm 2, respectively. This indicates that with the addition of palygorskite nanowires in such CPE can suppress the penetration of lithium dendrite. Second, SEM is used to characterize the surface of the Li electrode at three scenarios: 1) before deposition, 2) after shorting in a Li pure PVDF Polymer electrolyte Li cell at 0.3 ma/cm 2, and 3) at the same time as 2) in a Li 5 wt % Palygorskite/PVDF CPE Li cell at 0.3 ma/cm 2. As shown in Figure R1d, the fresh lithium anode shows a smooth surface, while irregular deposition and protrusions are clearly observed on the surface of the Li anode with pure PVDF polymer electrolyte (Figure S6e and h). In contrast, for the 5 wt % Palygorskite/PVDF CPE cell, no apparent dendrites or defects are observed (Figure S6f and i). These positive results indicate that the Li dendrite growth is effectively suppressed by the Palygorskite/PVDF CPE membrane during Li deposition progress. 6

7 Figure S6. (a-c) Voltage profile of (a) Li PVDF Polymer electrolyte Li cells, (b) Li 3 wt % Palygorskite/PVDF CPE Li cells and (c) Li 5 wt % Palygorskite/PVDF CPE Li cells with a constant current density of 0.3 ma/cm 2. (d-f) SEM images of the surface of the Li electrode (d) before Li deposition, (e) the Li electrode obtained from a Li PVDF Polymer electrolyte Li cell until shorting, and (f) from a Li 5 wt % Palygorskite/PVDF CPE Li cell after applying a current density of 0.3 ma/cm 2 after the same time as that in (e). (g-i) Zoom-out SEM images of the surface of the Li electrode. Measurement of lithium ion transference number (t Li+ ) of PVDF based polymer electrolytes Potentiostatic polarization (PP) method was used to measure the lithium ion transference number (t Li+ ) of PVDF based polymer electrolyte. A small constant potential (8.7mV) was applied on PVDF-based polymer electrolytes between two lithium electrodes which leads to a decrease of the initial current (I 0 ) until a steady-state current (I SS ) flowing through the cell. R 0 and R SS, representing the charge-transfer resistance before and after the polarization of the system respectively, 7

8 which were obtained by impedance spectra of the cell in the frequency range from 1 MHz to 0.1Hz with an oscillation voltage of 10 mv. t Li+ was calculated using Equation(1) 2. t Li + = I ss(δv I 0 R 0 ) I 0 (ΔV I SS R SS ) As shown in Figure S7 (a), the interfacial resistance of PVDF polymer electrolyte increases from 222 to 301 Ohm after polarization. During the polarization process, the steady-state current (I SS ) decreases to a steady value of ma from the initial current (I 0 ) of ma. Therefore, the calculated t Li+ for the PVDF CE is Similarly, the calculated t Li+ for the PVDF/5wt% palygorskite CPE and PVDF/3wt% palygorskite CPE is 0.54 and 0.31 respectively. The great improvement of t Li+ for the PVDF/5wt% palygorskite membrane is owing to the addition of palygorskite nanowires. There may be two specific reasons. First, with the addition of inorganic fillers, the local chains of polymer can be relaxed and the segment motion is promoted under the interaction of inorganic fillers and polymer chains, as a result, the mobility of Li ions and t Li+ can be enhanced 3. Second, there may be interactions between the palygorskite and anions in the lithium salt which immobilize anion and enhance transference number. This has been observed in Mg 2 B 2 O 5 4 and Li 7 La 3 Zr 2 O (1) Figure S7. Current-time profile of (a) Li pure PVDF Li symmetric cell, (b) Li PVDF/3 wt % palygorskite CPE Li symmetric cell and Li PVDF/5 wt % palygorskite CPE Li symmetric cell after applying a DC voltage of 10 mv for determining Li + transfer number. The inset shows the Nyquist impedance spectra of the cell before and after polarization. To understand the mechanism behind the enhanced Li + transfer number, FTIR is used 8

9 to characterize these samples. As shown in Figure S8 (a), ClO - 4 has two peaks at 3559 cm -1 and 1663 cm -1. When palygorskite nanowires are added, the shape of the peak at 3559 cm -1 is changed, and the peak at 1663 cm -1 is shifted to 1652 cm -1. This indicates that palygorskite nanowires interact with ClO - 4. It is likely that the exposed metal - cations on palygorskite nanowire surface act as Lewis acid, which interact with ClO 4 as Lewis base to cause such shift, similar to that in Mg 2 B 2 O 5 nanowires 4. Such interaction slows down the movement of ClO - 4, and thus enhance the transference number of Li +. Figure S8. (a) FTIR spectra of the PVDF polymer electrolyte, PVDF polymer electrolyte without LiClO 4 and PVDF/Palygorskite CPE at cm 1. (b) Zoom-in FTIR spectra of the PVDF polymer electrolyte membrane and PVDF/Palygorskite CPE membrane at cm -1. Cyclic voltammetry of 5 wt % Palygorskite/PVDF CPE Once palygorskite is added, the side reaction is reduced significantly to ~1 μa/cm 2 (Figure S9). Since palygorskite is a well-known absorbent 5, 6, therefore, it can trap DMF so that the electrolyte has better electrochemical stability. 9

10 Figure S9. Cyclic voltammetry curve of 5 wt% Palygorskite/PVDF CPE with a scan rate of 10mV/s. Mechanical tests for the composite films For the mechanical properties, the PVDF polymer electrolyte membrane and different PVDF-based CPE membranes with an average size of ~200 mm 10 mm 0.4mm are measured by a Model 5948 MicroTester Instron machine, with a strain rate of /s until breaking. According to the stress-strain curves, Young s modulus can be evaluated by the ratio of stress to strain in the range of elastic deformation stated by Hooke s law, and the tensile strength was achieved by the maximum stress value of the strain-stress curves. Calculation of effective Young s modulus of Nanowire/PVDF polymer electrolyte composites Based on TEM and image in Figure 4(a), the length and diameter of randomly distributed nanowires are assumed to be 1μm and 50nm, respectively. Here, non-crosslinks are also assumed for simplicity in the PVDF composite. We adopt the Mori-Tanaka theory 7 to estimate its effective elastic modulus as a function of volume fraction of nanowire. Monte Carlo simulations of 2D networks In the representative area (L L) of nanowires network, nanowires as simplified as line segments with the length of l and diameter of d, and their position and 10

11 orientation follow uniform distributions in the range of [0, L) and [0, π). Also, periodic boundary conditions are assumed in the unit. The nanowire volume fraction in the area can be defined as ρ = Nld 2, where N is the number of the nanowire in the L area. Given N, l, d, L, we generate the randomly distributed nanowire network by a pre-process code and observe the number of intersections. Figure S10. The 2D stochastic diagram of randomly distributed nanowire network with the uniform size (length= 1μm; diameter 50 nm) and the 5 wt% ratio in the representative unit Finite element method (FEM) simulation of 2D nanowire networks According to the periodic cell in Figure 4e, the FEM model is built. Its mechanical behaviors are simulated in the software ANASYS. The thickness of the cell is set as the diameter of the nanowire. For simplicity, a linear elastic isotropic behavior is used to simulate the nanowire and beam elements are used in the simulation. The interaction between two crossing nanowires is simplified as a hinge, and periodic boundary conditions are applied on four sides. References 1. Liu, W.; Liu, N.; Sun, J.; Hsu, P.-C.; Li, Y.; Lee, H.-W.; Cui, Y. Nano letters 2015, 15, (4), Evans, J.; Vincent, C. A.; Bruce, P. G. Polymer 1987, 28, (13), Zhang, W. Q.; Nie, J. H.; Li, F.; Wang, Z. L.; Sun, C. Q. Nano Energy 2018, 45, Sheng, O.; Jin, C.; Luo, J.; Yuan, H.; Huang, H.; Gan, Y.; Zhang, J.; Xia, Y.; Liang, C.; Zhang, W. Nano letters Murray, H. H. Applied Clay Science 2000, 17, (5-6),

12 6. Zuo, S. X.; Chen, J.; Liu, W. J.; Li, X. Z.; Kong, Y.; Yao, C.; Fu, Y. S. Carbon 2018, 129, Qiu, Y.; Weng, G. International Journal of Engineering Science 1990, 28, (11),