Research ArticleMATERIALS SCIENCE

Silica gel solid nanocomposite electrolytes with interfacial conductivity promotion exceeding the bulk Li-ion conductivity of the ionic liquid electrolyte filler

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Science Advances  10 Jan 2020:
Vol. 6, no. 2, eaav3400
DOI: 10.1126/sciadv.aav3400
  • Fig. 1 Macro- and microstructure of the nano-SCE.

    (A) Picture of two nano-SCE pellets (left) synthesized in the vial; after gelation, a transparent pellet is obtained. Note that the pellet is fully transparent and was therefore given a blue hue for visibility. When the ILE is removed, a brittle white pellet remains for the highly porous silica matrix (right). (B) Scanning electron microscopy (SEM) image of the SiO2 matrix that remains after removal of the ILE. (C) Zoom of the picture shown in (B) depicting the mesoporous nature of the matrix material with some macropores. (D) Transmission electron microscopy (TEM) image showing a dense packing of 7- to 10-nm silica nanoparticles as the building blocks of the porous matrix material. (E) The porosity of the matrix structure plotted for different molar ratios of ILE with respect to SiO2 (x value). The dashed line gives the theoretical porosity determined from volume fraction of ILE and silica. The acetone-rinsed samples (black squares) were dried in air, which gives partial collapse of the structure for x > 0.5. Supercritical CO2 drying of ethanol-rinsed nano-SCE (green circles) prevents collapse up to x = 2 for extra slow removal of the CO2 (open circle). BET, Brunauer-Emmett-Teller. Photo credit: Fred Loosen, imec; Akihiko Sagara, Panasonic.

  • Fig. 2 Free versus ice water in the nano-SCE.

    (A) IR spectra of the nano-SCE as dried in vacuum (black) and subsequently further dried in a glove box with 0.0005% RH for 9 days (blue) and exposed to 30% RH for 4 days (red) and to 60% RH for 8 days (green), respectively. a.u., arbitrary units. (B) Cyclic voltammograms of a Li/SCE/TiN stack with x values of 1.0 (blue), 1.5 (green), and 2.0 (red) and of ILE reference (black); the inset shows the current in logarithmic scale. (C) Cyclic voltammograms of Li/SCE (x = 2)/40-nm TiO2 stack (red), ILE (dotted black), and ILE spiked with 5 weight % (wt %) H2O (dash-dotted blue line); in (B) and (C), measurements with ILE and ILE with H2O were done in three-electrode configuration with TiN as a working electrode and Li as counter and reference electrodes. The SCE was dried for 2 days in the glove box after vacuum drying.

  • Fig. 3 Conductivity of the nano-SCE.

    (A) Temperature dependence of the conductivity of nano-SCEs dried for 8 days in the glove box (GB) with x values of 2 (black squares), 1.75 (orange circles), 1.5 (blue triangles), and 1.0 (green triangles) and of ILE reference (open squares). (B) Conductivity of nano-SCEs additionally dried in GB for 0 days (green squares), 10 days (black triangles), and 138 days (blue triangles). (C) Conductivity versus square root of drying time of nano-SCE with x values of 2 (black squares), 1.5 (blue triangles), 1.0 (green triangles), and 0.5 (brown diamonds). (D) Conductivity of nano-SCE with x = 2 (black squares), 1.5 (blue triangles), and 1.0 (green triangles) exposed in a N2-filled humidity chamber.

  • Fig. 4 Model for the adsorbed interfacial layer.

    (A) IR spectra of nano-SCE with an x value of 1.5 (red), ILE reference (black), and SiO2 (blue), showing that the O═S═O group (1231 cm−1) is involved in the interaction with OH-groups on the silica surface. (B) Raman spectra of nano-SCE with x values of 2 (black), 1.5 (red), and 0.5 (blue), showing the presence of ice water bonded on silanol-terminated silica even for nano-SCE near saturation (0.0005% RH) in a glove box (30 days). (C) Proposed model for the interface interaction in the nano-SCE with dissociation Li-TFSI into free Li+ as the TFSI anion shares part of its negative charge with the adsorbed ice-TFSI-BMP layer; the colors represent different elements with purple (silicon), red (lithium), dark yellow (sulfur), orange (oxygen), blue (nitrogen), white (hydrogen), and green (fluorine). The purple dashed lines represent the hydrogen bond between the O═S group of TFSI anion and the OH-groups of hydroxylated silica surface. The Li+ ions set free by the dipole over the adsorbed layer can migrate through subsequent mobile or diffuse ionic liquid layers above the interface layers. Note that depending on the strength of the hydrogen bonds and the equivalent charge on the silica, multiple adsorbed layer could be formed as well. Full spectra are shown in fig. S8.

  • Fig. 5 Free Li+ ions in the nano-SCE.

    (A) Raman spectra of an ionic liquid (IL; dotted blue line) and ILE reference (ILE; dash-dotted line) of as prepared nano-SCE (vacuum dried) with x values of 0.5 (green), 1.5 (yellow), and 2 (brown) and of nano-SCE (x = 1.5) additionally dried in glove box for 30 days or near saturation at 0.0005% RH (red). The vertical lines label the Raman shift for TFSI with its N center coordinated to Li+ (746 cm−1) and not coordinated to Li+ (741 cm−1), respectively. (B) Ratio of free to coordinated Li+ of nano-SCE as synthesized (vacuum dried, black circles) and additionally dried in glove boxs with 0.0005% RH for 30 days (blue diamonds), corresponding to the ratio of the integrated intensity of the Raman peaks (746 cm−1 over 741 cm−1). (C) PFG-NMR–derived Li+ self diffusion coefficient of nano-SCE (red diamonds) and ILE ref. (black squares) as a function of the interval between the gradient magnetic field pulses. The theoretical peaks on Raman spectra were simulated using DFT calculation.

  • Fig. 6 Functionality of nano-SCE as Li-ion electrolyte.

    (A) Cyclic voltammogram of nano-SCE (x = 1.5, as synthesized after vacuum drying) (red) and ILE reference (black) measured in three-electrode configuration with Li as working, counter, and reference electrodes (SEI resistance estimated from IR drop on cathodic current is 0.9 and 1.8 kilo-ohm·cm2 for ILE and SCE, respectively). (B) Galvanic charge/discharge curves of Li/SCE (x = 1)/100-nm thin-film LiMn2O4 cell for five cycles at C-rates of 1C, 5C, and 20C. (C) Cyclic voltammograms of the Li/SCE/40-μm Li4Ti5O12 and Li/SCE/30-μm LiFePO4 powder electrode cells (1 mV/s). (D) Galvanic charge/discharge curves of Li/SCE/40-μm Li4Ti5O12 powder electrode at 1C, 0.1C, 0.2C, and 0.02C. (E) Galvanic charge/discharge curves of the Li/SCE/30-μm LiFePO4 powder electrode at 1C, 0.5C, 0.2C, 0.1C, 0.05C, and 0.01C. (F) Capacity (filled diamonds for delithiation and open squares for lithiation) versus cycle number of the Li/SCE/30-μm LiFePO4 powder electrode; the thickness of the SCE in the cells is about 280 μm. The density of the LFP and LTO cathode is about 1.9 and 11.0 mg/cm2, respectively. (G) Potential versus time curves of a Li/SCE/Li stack cycled at current densities of 0.1, 0.2, 0.5, and 0.1 mA/cm2. (H) The 1st, 10th, 125th, and last polarization of the Li/SCE/Li stack stressed at 0.1 mA/cm2, shown in (G). For (G) and (H), the SCE has a conductivity of 0. 34 mS/cm, and the thickness of the SCE pellet is 0.152 cm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/2/eaav3400/DC1

    Table S1. Structural properties of silica matrix in the nano-SCE for increasing molar fraction of ionic liquid to silica (x value) determined from N2 adsorption/desorption or BET measurements and TEM observations.

    Fig. S1. Normalized conductivity versus equivalent ILE thickness.

    Fig. S2. The effect of ice-like and liquid-like adsorbed water on nano-SCE ion conductivity.

    Fig. S3. Temperature dependence of the conductivity of the nano-SCE.

    Fig. S4. TGA curves for nano-SCE.

    Fig. S5. SEM images of the nano-SCE.

    Fig. S6. N2 adsorption/desorption isotherm of the silica derived from nano-SCE.

    Fig. S7. Solid-state three-electrode setup for cyclic voltammetry on nano-SCE pellets.

    Fig. S8. IR spectra of nano-SCE (x = 1.5), ILE, and silica in the range of 4000 to 400 cm−1.

    Fig. S9. IR spectra of 225-nm nano-SCE (x = 1.5) and Li/SCE thin-film stack.

  • Supplementary Materials

    This PDF file includes:

    • Table S1. Structural properties of silica matrix in the nano-SCE for increasing molar fraction of ionic liquid to silica (x value) determined from N2 adsorption/desorption or BET measurements and TEM observations.
    • Fig. S1. Normalized conductivity versus equivalent ILE thickness.
    • Fig. S2. The effect of ice-like and liquid-like adsorbed water on nano-SCE ion conductivity.
    • Fig. S3. Temperature dependence of the conductivity of the nano-SCE.
    • Fig. S4. TGA curves for nano-SCE.
    • Fig. S5. SEM images of the nano-SCE.
    • Fig. S6. N2 adsorption/desorption isotherm of the silica derived from nano-SCE.
    • Fig. S7. Solid-state three-electrode setup for cyclic voltammetry on nano-SCE pellets.
    • Fig. S8. IR spectra of nano-SCE (x = 1.5), ILE, and silica in the range of 4000 to 400 cm−1.
    • Fig. S9. IR spectra of 225-nm nano-SCE (x = 1.5) and Li/SCE thin-film stack.

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