Nanocomposite single ion conductor based on organic–inorganic hybrid

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  • r based on organicinorganic hybrid

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    However, conventional single ion conductors showed

    Recently, the effect of the nanosized ceramic additives on

    with the propane lithium sulfonate and the cross-linked

    (2004ion dissociation energy. Recently, it was reported that the

    aluminate polymer based single ion conductors showed

    relatively high ionic conductivity of 10 510 6 S/cm[6,7] and that the ionic conductivity of the single ion

    conductor could reach 10 4 S/cm by introducing theplasticizer with a high dielectric constant [8].

    Besides high ionic conductivities, the interfacial stability

    sized silica end-capped with the lithium sulfonate was

    prepared. The effect of the modified silica, which was

    introduced as the lithium ion sources, on the ion conduc-

    tivity and the interfacial stability of the cross-linked nano-

    composite single ion conductor was investigated.low ionic conductivities (f 10 S/cm) due to their large 7 nanocomposite single ion conductor containing the nano-Solid polymer electrolytes have received special attention

    more than two decades due to their potential for application

    in a variety of solid state electrochemical devices such as

    lithium secondary batteries, sensors, and electrochromic

    displays [13]. One of the interesting fields in solid

    polymer electrolytes is the single-ion conductor in which

    only the cation can be mobile under an electric field and

    which does not suffer from the concentration polarization

    caused by accumulation of anions on the electrode [4,5].

    the ion conductivity and the interfacial stability was exten-

    sively investigated for solid polymer electrolyte based on

    PEO [913]. The nanosized ceramic materials such as lithi-

    ated aluminate (g-LiAlO2), fumed silica (SiO2), alumina(Al2O3), and BaTiO3 have been introduced into polymer elec-

    trolyte as a minor component to improve the electrochemical

    properties of the polymer electrolyte. It will be also interest-

    ing to consider the hybrid materials based on inorganic par-

    ticles and organic compounds as a new functional lithium salt.

    In this work, the nanosized-fumed silica was modified1. IntroductionKeywords: Nanocomposite single ion conductor; Nanosized fumed silica; Sulfonation; Ion conductivity; Interfacial stabilityNanocomposite single ion conducto

    Nam-Soon Choi, Yong Min Lee, Baik

    Department of Chemical and Biomolecular Engineering, Korea Advanced In

    373-1, Guseong-dong, Yuseong

    Received 16 June 2002; received in revis

    Abstract

    New nanosized silica with the propane lithium sulfonate was

    conductor. The cross-linked single ion conductor based on the PEG

    was prepared by UV-curing. Ionic conductivity and interfacial stabil

    ion conductor were investigated by varying the modified silica co

    showed maximum trend with the modified silica content. The ionic

    silica was 2.2 10 4 S/cm at 25 jC. Interfacial stability betweenelectrode was enhanced by introducing the modified silica.

    D 2003 Elsevier B.V. All rights reserved.

    Solid State Ionics 167toward the lithium electrode is also an essential factor to

    guarantee acceptable performance in the electrochemical

    devices. The interfacial properties of conventional single

    ion conductor with a plasticizer have rarely been studied.

    0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

    doi:10.1016/j.ssi.2003.06.002

    * Corresponding author. Tel.: +82-42-869-3925; fax: +82-42-869-3910.

    E-mail address: pjk@mail.kaist.ac.kr (J.-K. Park).yeon Lee, Je An Lee, Jung-Ki Park*

    of Science and Technology and Center for Advanced Functional Polymers,

    aejeon 305-701, South Korea

    m 16 June 2003; accepted 19 June 2003

    atically synthesized as the lithium ions source of the single ion

    , the modified silica, and the plasticizer (PC/DMSO, 50/50 w/w)

    ward the lithium electrode of the cross-linked nanocomposite single

    . The ionic conductivity of the cross-linked single ion conductor

    ctivity of the cross-linked single ion conductor with 30% modified

    cross-linked nanocomposite single ion conductor and the lithiumwww.elsevier.com/locate/ssi

    ) 2932992. Experimental

    2.1. Materials

    The size and specific area of the nanosized-fumed silica

    (CAB-O-SIL, EH 5) obtained from Cabot were 11 nm and

    380F 30 m2/g, respectively. The surface of the silica was

  • very hydrophilic (100% SiOH). Poly(ethylene glycol)

    dimethacrylate (PEGDMA, Mw = 400) was purchased from

    Polyscience. Potassium tert-butoxide dissolved in THF, 1,3-

    propane sultone, lithium hydroxide monohydrate

    (LiOHH2O) and methacrylic acid (MAA) were purchasedfrom Aldrich. The mixture of propylene carbonate (PC,

    Merck) and dimethyl sulfoxide (DMSO, Aldrich) (50/50, w/

    w) was dried with 3 A silica molecular sieves and was

    stored in a nitrogen-filled dry box to avoid water contam-

    ination before use as a plasticizer. Tetrahydrofuran (THF),

    which was used as a solvent for the modification reaction of

    the fumed silica, was dried with the sodiumbenzophenone

    complex.

    2.2. Preparation of polymer electrolytes

    tion time (T2) was determined using the spin-echo technique

    by applying 9090j pulse sequences.

    2.4. Electrical measurements

    The ionic conductivities of the polymer electrolytes were

    obtained from bulk resistance measured by AC complex

    impedance analysis using a Solartron 1287 frequency re-

    sponse analyzer (FRA) over a frequency range of 10 Hz1

    MHz.

    The lithium transference number was determined by DC

    polarization/AC impedance combination method [14]. A

    constant polarization of 10 mV was applied to the cell.

    The transference number of the lithium cation was calcu-

    lated by the relation, tLi +=[Is(DV IoRo)/Io(DV IsRs)],where Io and Is are the currents at the initial and steady-

    The sulfonation of an organic material, which was

    reported by Sepulchre et al. [15] was performed by the

    N.-S. Choi et al. / Solid State Ionics 167 (2004) 293299294The UV-curable formulation consists of curable mono-

    mer (PEGDMA), plasticizer (300 wt.% based on matrix

    polymer), modified silica end-capped by the propane lithi-

    um sulfonic acid, and methyl benzoylformate as a photo-

    initiator (2 wt.% based on matrix polymer) which is known

    to undergo a fast cleavage upon photolysis to generate free

    radicals. The above curable solution was cross-linked by

    UV irradiation (8 mW/cm2, 365 nm) for 10 min. The

    resulting film of the cross-linked nanocomposite single ion

    conductor showed a good elastic property.

    2.3. Characterization

    The thermal analysis of the cross-linked nanocomposite

    single ion conductor containing the plasticizer (PC/DMSO

    mixture) with change of the modified silica content was

    done by using a differential scanning calorimeter (DuPont

    TA 2000 DSC). Each sample was scanned at a heating rate

    of 10 jC/min within an appropriate temperature range undernitrogen atmosphere. Solid state 7Li static NMR experi-

    ments and 29Si cross-polarization magic angle-spinning

    (CP/MAS) NMR experiments were performed on a Bruker

    DSX 400 MHz NMR spectrometer. The spinspin relaxa-Fig. 1. Schematic illustration for the reaction of thereaction of 1,3-propane sultone with the CH(CH3)2

    (PEG)nOK+. Contrary to this, in this work the organ-

    icinorganic hybrid material was newly designed by the

    sulfonation of the nanosized fumed silica. The reaction

    scheme of the nanosized fumed silica with the 1,3-propane

    sultone is shown in Fig. 1. Potassium tert-butoxide dis-

    solved in THF as a catalyst was added to the dispersedstate, Ro and Rs are the interfacial resistances at initial and

    steady-state, respectively. The characteristics of the interface

    between the cross-linked single ion conductor and the

    lithium electrode were examined by monitoring the time

    dependence of the impedance of the symmetrical Li/single

    ion conductor/Li cells (dimension: 2 2 cm2) over a fre-quency range of 1 Hz1 MHz at room temperature.

    3. Results and discussion

    3.1. Synthesis of nanosized fumed silica end-capping with

    propane lithium sulfonic acidmodified silica with the 1,3-propane sultone.

  • fumed silica in THF and then 1,3-propane sultone was

    introduced. The reaction was allowed to proceed at 80 jCwith stirring for 24 h under nitrogen atmosphere. After the

    reaction was completed, the reaction solution was centri-

    fuged and the precipitated product was washed three times

    with THF to remove the unreacted monomer. The resulting

    product was the nanosized fumed silica with the propane

    sulfonic acid. The sulfonic acid groups in the nanosized

    fumed silica were neutralized with 0.1 N LiOH (aq) solution

    as shown in Fig. 1 and the neutralized silica, which can act

    as the lithium sources in the plasticized single ion conduc-

    tion system, was dried in a vacuum oven at 110 jC.Fig. 2a,b shows the FT-IR spectra of the unmodified

    silica (EH 5) and the nanosized fumed silica end-capped by

    the propane lithium sulfonic acid. The absorption peaks

    appearing in the range of 10001300 cm 1 and around 815cm 1 in Fig. 2a correspond to SiUOUSi antisymmetricstretching and SiUO stretching vibration mode in theunmodified silica. The peaks centered at 1650 and 1470

    cm 1 in Fig. 2b are assigned to the SMO stretching mode oftheUSO3

    Li+ in the surface of the modified silica and to theCH2 scissoring vibration respectively. The absorption peak

    in the range 10001300 cm 1 due to the SiUOUC

    antisymmetric stretching mode was broad and shifted to a

    lower wavenumber.

    Fig. 3 shows the 29Si CP/MAS NMR spectra of the

    unmodified hydrophilic silica and the modified silica end-

    capped by the propane lithium sulfonate. For the unmodi-

    fied silica (EH 5), the resonance peaks centered at 92, 100, and 110 ppm in Fig. 3 are assignable to silicon ofC (Q2, geminal silanols, O2Si(OH)2), B (Q

    3, free silanols,

    O3Si(OH)), and A (Q4, siloxane, O4Si) sites in silica,

    Fig. 3. Solid state 29Si CP/MAS NMR spectra for (a) the unmodified silica

    and the modified silica end-capped by the propane lithium sulfonate; (b)

    peak resolutions for the silicones under different environment; (c) silicones

    with different environments.

    N.-S. Choi et al. / Solid State Ionics 167 (2004) 293299 295Fig. 2. FT-IR spectra for the unmodified silica (fumed silica, EH 5) and the

    modified silica: (a) full region; (b) SiUOUSi antisymmetric stretching

    region. respectively [16,17]. The difference in the peaks in the

  • range from 70 to 120 ppm between the modified andunmodified silica is due to the introduction of theUSO3

    Li+

    in the surface of the hydrophilic silica. The relative fraction

    of the USO3Li+ in the modified silica could be calculated

    by resolving the resonance peaks in the 29Si CP/MAS NMR

    spectra. Three silicon peaks in a different environment in

    Fig. 3b,c was represented as D, E, F, and G. The fraction of

    peak D (siloxane: Q4 site), peak E (free silanol: Q3 site), and

    peak F, G (silicone attached in UO(CH2)3SO3Li+) was

    16.3%, 33.7%, and 50.0%. The fraction of the free silanol

    and geminal silanol groups in the surface of the unmodified

    silica was about 84%. After the modification of fumed

    silica, the conversion of the silanol groups is found to be

    about 60%. From the above calculation, the lithium content

    of the modified silica was 5.4 mmol/1 g silica.

    3.2. Effect of modified silica on ion conductivity

    The ion conductivity of the modified silica in PC/DMSO

    was 4.3 10 4 S/cm at room temperature as shown in

    the initial increase of the modified silica content and then

    slowly decreased with a further increase of the modified

    silica content. For the initial increase of the ion conductivity

    up to a maximum, ion conductivity is strongly dependent

    on the number of free ions in the cross-linked single ion

    conductor. Since the increase of the modified silica content

    in the cross-linked single ion conductor can produce a

    larger number of free ions as shown in Table 1, it can lead

    to a higher ion conductivity. However, after the highest

    conductivity was reached, the ion conductivity was deter-

    mined by the mobility of free ions and the ion conduction

    pathway tortuosity rather than number of free ions. Accord-

    ing to the previous study of Chen et al. [18], the more ions

    that exist in the solution the higher ion conductivity.

    However, after the highest conductivity is reached, the

    conductivity no longer depends on the number of ionic

    carriers in the solution. They reported that at higher salt

    concentration, the ion conductivity is determined by the

    mobility of ionic carriers. The mobility of the free ions in

    the cross-linked nanocomposite single ion conductor could

    be obtained from T2 (spinspin relaxation time) of7Li

    NMR experiment. Table 1 shows that T2 of the lithium ions

    Fig. 5. Ion conductivities of the cross-linked nanocomposite single ion

    conductor for (a) Arrhenius plot; (b) room-temperature conductivities as a

    function of the modified silica content.

    N.-S. Choi et al. / Solid State Ionics 167 (2004) 293299296Table 2. To investigate the effect of the modified silica on

    the ion conductivity in the polymeric single ion conduction

    system, the cross-linked polymer was used as a matrix. The

    cross-linked nanocomposite single ion conductor films

    containing the modified silica were flexible and semi-

    transparent as shown in Fig. 4. The schematic representa-

    tions for the microstructure of the cross-linked nanocom-

    posite single ion conductor and lithium cation dissociated

    from the modified silica surface were also illustrated in Fig.

    4. The ion conductivities of the cross-linked nanocomposite

    single ion conductors with a different modified silica

    content were plotted against a reciprocal absolute temper-

    ature in Fig. 5a and the ion conductivities as a function of

    the modified silica content was presented in Fig. 5b. The

    ion conductivities increased up to a maximum value with

    Fig. 4. Photographs for the physical state of the single ion conductor film

    and schematic representation for the microstructure of the cross-linkednanocomposite single ion conductor. decreased with increasing the modified silica content. This

  • modified silica in the single ion conductor. To affirm effect

    of the modified silica on the crystallization behavior of the

    plasticizer, DSC thermograms of the cross-linked nano-

    composite single ion conductor were obtained as shown in

    Fig. 6a. Fig. 6b represents a plot of DHm (heat of fusion)

    Table 1

    Mole concentration of lithium ion, transference number, T2 and n parameter

    of the cross-linked nanocomposite single ion conductor with 300 wt.% PC/

    DMSO (50/50, w/w) as a function of the modified silica content

    Modified silica

    content (wt.%

    based on

    polymer matrix)

    Mole concentration

    of lithium/1 g polymer

    matrix (mmol/1 g)

    TLi + T2 (ms) n Parameter

    0a 5.20b 1.00 0.98

    5 0.27 28.30 0.98

    10 0.54 0.98 27.48 0.97

    20 1.08 26.06 0.95

    30 1.62 16.52 0.94

    40 2.16 11.49 0.92

    50 2.70 0.97 1.40 0.89

    a Conventional cross-linked single ion conductor (reference system)

    consisting of PEGDMA-based cross-linked matrix with LiMA and PC/

    DMSO plasticizer.b

    Fig. 6. (a) DSC thermograms of the cross-linked nanocomposite single ion

    coductor; (b) DHm of the cross-linked nanocomposite single ion coducto...

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