Fast Ion Conduction in Nanodimensional Lithium Silicate Glasses

Dec 21, 2015 - Nanodimensional glass of composition 35Li2O·65SiO2 was grown within the nanochannels of diameter 5.5 nm of mesoporous silica SBA-15 by...
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Fast Ion Conduction in Nanodimensional Lithium Silicate Glasses Soumi Chatterjee,†,‡ Ramaprasad Maiti,† Shyamal Kumar Saha,‡ and Dipankar Chakravorty*,† †

MLS Professor’s Unit, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata 700032, India Department of Materials Science, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata 700032, India

‡

ABSTRACT: Nanodimensional glass of composition 35Li2O· 65SiO2 was grown within the nanochannels of diameter 5.5 nm of mesoporous silica SBA-15 by suitable heat treatment of the required precursor sols. A dc electrical conductivity of 10−4 S·cm−1 was measured for the nanoglass at room temperature. This also shows a low activation energy of 0.1 eV for the lithium ion migration. This has been explained as arising due to oxygen ion vacancies caused by the presence of Si2+ and Si4+ species in the mesoporous silica at its interface with the nanoglass. This causes a repulsive interaction between the defects and the lithium ions, thereby reducing the attractive electrostatic force between the nonbridging oxygen ions and the lithium ions. These nanocomposites will be suitable for applications in lithium-ion batteries for storage of renewable energy.

1. INTRODUCTION Fast ion conductors have been the subject of intense research for quite some time because of the theoretical challenges posed by them and the wide range of applications in energy, sensors, photoelectrochemical cells, fuel cells, etc.1,2 These materials exhibit a high electrical conductivity at room temperature due to some ion movement. Beta alumina with a chemical formula NaAl11O17 is an example. The high mobility of alkali ions (Na+) in this material arises due to its unique crystal structure, viz., Al2O3 blocks being separated from each other by nanometerscale channels through which Na+ ions can move easily under the influence of an applied electric field. Sodium beta alumina has been used in making solid-state batteries for different applications.3 Lithium-ion batteries have been shown to be essential components in consumer portable electronics.4,5 Recently synthesis of a lithium superionic conductor with a chemical formula Li10GeP2S12 has been reported,6 which showed a conductivity of ∼12 mS·cm−1 at room temperature with an activation energy of 0.25 eV. Ion transport in confined systems like nanocomposites has attracted considerable attention lately.2 It has been shown that interfaces in such systems are so closely spaced that they control significantly the physical properties of the material.2 Very recently, it has been reported that nanoconfined LiBH4 exhibits fast lithium ion conduction with a conductivity of 10−4 S·cm−1 at room temperature.7 Such effects have also been explored in nanocomposites comprising oxide nanoparticles and nanoglasses, and the results have been reported.2,8,11 In particular, it had been shown that nanosilicate glasses containing lithium ions grown within the pores of a semiconductor (CuO or ZnO) nanoparticle compact showed ionic conductivity 2 or 3 orders of magnitude higher than that of the corresponding bulk glass caused by a lowering of activation energy for ionic conduction in the nanoglass. This arose due to the effect of thermally induced asymmetric contraction of the two phases during the cooling operation of © 2015 American Chemical Society

the synthesis, resulting in an increase in free volume of the nanoglass. The artificially created increase in free volume of the nanoglass gave rise to a lowering of activation energy for lithium ion migration.9,10 In the present work, a mesoporous silica template having channels of diameter ∼5.5 nm was used, and a nanoglass of silica containing lithium ions was grown within them. The template silica network had both Si2+ and Si4+ species giving rise to defects like oxygen vacancies. The interface between the mesoporous silica and the nanoglass phase thus controlled the migration of lithium ions. Indeed, electrical conductivity ∼ 10−4 S·cm−1 was observed at around room temperature with a low activation energy of ∼0.1 eV for lithium ion migration. The present work, therefore, relies on the properties of a glass phase rather than those of a crystal and hence will have far reaching impact on the development of other materials with useful applications. The details are reported in this paper. Our work has resulted in the development of a nanoglass of binary components that can be prepared by a simple technique of heat treatment of a suitable sol.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Mesoporous silica SBA-15 was prepared using the procedure reported by Zhao.12 All reagents were purchased from Aldrich. A 4.0 g portion of Tri-Block copolymer Pluronic P-123 (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H) was dissolved in 30 g of water and 120 g of 2 M HCl solution. Tetraethylorthosilicate (TEOS) was used as silicon precursor, and 8.50 g of it was added into this solution, which was stirred for 21 h at 308 K. The resulting solution was transferred into a Teflon-lined stainless steel autoclave for hydrothermal synthesis at 373 K for 24 h. The Received: November 6, 2015 Revised: December 20, 2015 Published: December 21, 2015 431

DOI: 10.1021/acs.jpcc.5b10883 J. Phys. Chem. C 2016, 120, 431−436

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The Journal of Physical Chemistry C solid product was recovered, washed with deionized water, and dried at 333 K overnight. The sample was then calcined to remove the template by slowly increasing the temperature from ambient to 823 K in 4 h and heated at 823 K for 6 h.The temperature was then lowered to 573 K in 3 h, which produced pure mesoporous silica SBA-15. For preparing mesoporous silica SBA-15 nanocomposite with the nanoglass, 1.2056 g of lithium nitrate was added with 18 mL of ethanol, and the mixture was stirred for 1 h. Another solution of 8.50 g of TEOS and 18 mL of ethanol was also stirred for 1 h at room temperature. These solutions were mixed and stirred for 5 h, and product A was made. Now, in another solution, 4.0 g of Pluronic P-123, 30 g of water, and 120 g of 2 M HCl were mixed and stirred for 1 h at room temperature, and product B was made. Products A and B were mixed and stirred for 21 h at room temperature. For hydrothermal synthesis, the resulting solution was poured into a Teflon-lined stainless steel autoclave and kept at 373 K for 24 h. The subsequent steps were identical to those described in the previous paragraph. Bulk glass of the same composition, viz., 35Li2O·65Si2O, was prepared by gelation of the relevant sol. The sol was prepared by mixing two solutions: 0.261 g of lithium nitrate was mixed with 2 mL of ethanol and stirred for 1 h, and 0.94 g of TEOS was mixed with 2 mL of ethanol and was also stirred for 1 h at room temperature. The resulting solution was stirred for 5 h and then kept for 3 days in an open Petri dish, then subjected to heat treatment at 673 K for 90 min. 2.2. Characterization. To delineate the pore size distribution in SBA-15, N2 adsorption measurements were carried out using a Quantachrome Autosorb-1C at 77 K. The samples were pretreated at 423 K for 3 h under high vacuum. Surface area was calculated using the BET (Brunauer− Emmett−Teller) equation.13 Pore size distribution was estimated by employing the cylindrical pore NLDFT equilibrium model.14,15All the samples were characterized by X-ray diffraction patterns in a Bruker D8 XRD SWAX diffractometer using Cu Kα radiation. All the X-ray patterns confirmed the amorphous nature of the samples. This was further corroborated by the transmission electron micrographs and selected area diffraction patterns taken in a JEM 2010 transmission electron microscope operated at 200 kV. Electrical measurements at different frequencies were carried out using an Agilent E4980A precision LCR meter on compacted powder pellets after painting silver paste electrodes (supplied by Acheson Colloiden B.V., Netherlands) on both surfaces. XPS measurements were done using a spectrometer supplied by Omicron Nanotechnology with the serial no. 0571 using an Al−Kα radiation source under 15 kV voltage and 5 mA current.

Figure 1. X-ray diffractogram obtained from mesoporous silica SBA-15 powder.

Figure 2. (a) Adsorption−desorption isotherms of nitrogen for mesoporous silica SBA-15. (b) Pore size distribution as extracted from (a).

3. RESULTS AND DISCUSSION Figure 1 is the X-ray diffractogram obtained from mesoporous silica SBA-15 powder. The small peak at 2θ = 1.1° confirms the presence of ordered nanochannels within the silica glass.12 The first peak represents the reflection from (100) planes of a hexagonal lattice formed by the nanopores within the mesoporous silica matrix. The interplanar spacing calculated from this angle is 75.7 Å. The two other peaks arise from reflections contributed by the planes (110) and (200), respectively. The interplanar spacings as calculated from the angles of diffraction are 47.3 and 42.0 Å, respectively. All of these are in the same range as reported earlier by Zhao et al.12 Figure 2a shows the adsorption−desorption isotherms of N2 for

the mesoporous silica SBA-15 synthesized in the present work. The pore size distribution was extracted using the NLDFT procedure, and Figure 2b shows the resultant curve. It is evident that the pore diameter in the SBA-15 template is 5.5 nm. The transmission electron micrograph of the specimen is shown in Figure 3a. Figure 3b shows the electron diffraction pattern obtained from Figure 3a, indicating the amorphous nature of SBA-15. X-ray photoelectron spectroscopy was carried out on the mesoporous silica synthesized. This gave strong evidence of the 432

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Figure 3. (a) Transmission electron micrograph of mesoporous silica SBA-15. (b) Selected area electron diffraction pattern obtained from (a).

Figure 5. (a) dc resistivity variation as a function of inverse temperature for SBA-15 sample. (b) ac conductivity variation as a function of frequency at temperature 423 K.

presence of Si2+ species in the sample. Figure 4 shows a typical spectrum. The experimental curve was deconvoluted and two

σ (f ) ∝ f n

(1)

where f is frequency and n is a constant. In the present case, it was found to be ∼0.45. This implies that electron hopping occurs between localized states.19 Using the expression for ac conductivity as derived by Pollak,20 viz. σ(ω) =

π3 2 e kT[N (E F)]2 a5ω[ln(ν0/ω)]4 96

where N(EF) is the density of states at the Fermi level, ν0 is the optical phonon frequency with a value ∼ 1012 s−1, and a is the radius of the localized wave function assumed to have a value 1.5 Å. We calculated the value of N(EF) to be 1.3 × 1019 cm−3 eV−1 at 423 K. Such results were reported earlier in silicate glasses containing variable valence ions like Sb3+ and Sb5+.21 It must be pointed out at this stage that the electrical conductivity arising out of electron hopping between the states represented by Si2+ and Si4+ is much smaller than that arising due to Li+ ion migration, as discussed below. The dc resistivity variation as a function of temperature for the SBA-15 sample shows a resistivity value of the order of 1016 ohm·cm (extrapolated from the straight line in Figure 5a). This is at least 12 orders of magnitude higher than that measured in the case of SBA-15− nanoglass composite (see Figure 7c). Also, further measurements on the transport number of Li+ ions as discussed below provide unmistakable evidence of the fact that, in our nanocomposite system, the ionic conductivity determines solely the transport properties of the samples. Figure 6a,b shows the transmission electron micrograph for the nanoglass−SBA-15 composite and the electron diffraction pattern of the same, respectively. It can be seen that, in both of

Figure 4. XPS spectrum obtained from mesoporous silica SBA-15.

peaks, viz., binding energies 103.3 and 102.6 eV, respectively, were observed. These correspond to Si4+ and Si2+, respectively. Many valence states of Si were observed earlier.16 The presence of Si2+ in the surface of amorphous silica film was reported.17,18 In Figure 5a is shown the dc resistivity variation as a function of inverse temperature for the SBA-15 sample. Figure 5b gives the ac conductivity variation as a function of frequency at temperature 423 K. From this figure, it should be evident that the change of ac conductivity σ( f) with frequency can be described by 433

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to be 0.95 or more. This substantiates the fact that the material we have prepared has ionic conductivity. To obtain the true values of dc resistance contributed by the lithium ion movement, therefore, we carried out ac impedance measurements on the samples at different temperatures, and the resistance was found by the point where the high frequency arc intersected the real axis.9 Figure 7b shows a typical one obtained from the data collected at a temperature of 318 K. From Figure 6a, it should be evident that the areal fraction occupied by the lithium silicate nanoglass phase is less than 1. This value was estimated from the fraction of the dark regions in the micrograph by using SPIP software with the evaluation license supplied by Image Metrology, which is typical for the nanocomposite sample. The value was found to be 0.14. The latter was used in calculating the dc resistivities of the nanoglass sample at different temperatures. In Figure 7c is shown the variation of the logarithm of dc resistivity as a function of inverse temperature. In the same figure are shown the resistivity data for the nanoglass concerned and the corresponding bulk glass. It can be seen that the room temperature conductivity of the nanoglass of composition 35Li2O·65SiO2 is around 10−4 S· cm−1. The latter is about 5 orders of magnitude higher than that of the bulk glass of the same composition. The activation energy of lithium ion migration was calculated from the slope of the plot to be 0.1 eV. This is also an order of magnitude lower than that of the bulk glass (0.77 eV). The model for activation energy of alkali ion migration in silica-based glasses was first given by Anderson and Stuart.22 Basically, it is explained as arising due to both strain as well as electrostatic energy involved in such movement. A detailed discussion regarding this property can be found elsewhere.22,23 In the present nanocomposite system, the effect of strain energy can be ruled out because of an increase in free volume in the nanoglass phase9 due to interactions at the interface between the latter and the SBA-15 mesoporous silica. We believe that the electrostatic energy in our case also has been reduced drastically because of the oxygen ion vacancies introduced into the system due to the presence of Si2+ species in the mesoporous silica glass. These defects, which are positively charged, would reduce the electrostatic energy of attraction for the moving lithium ions, causing a reduction of the activation energy for electrical resistivity.Figure 7d shows the variation of the logarithm electrical conductivity σ of the nanoglass phase as a function of the logarithm frequency at temperature 308 K. It should be evident that, at high frequencies, the variation is linear with a slope of 0.45. The latter indicates ionic conduction in two-dimensional space,24,25 confining movement of lithium ions along the interface of SBA-15 and the nanosilicate glass phase. We would like to mention here that the PNP (Poisson− Nernst−Planck) model26 developed in recent years will be helpful in delineating the relaxation processes in nanodimensional glasses discussed in this paper as compared to those in the corresponding bulk glasses. Such investigations would throw much light on the glass structure in its nanodimensional form. We hope to take up such an investigation shortly.

Figure 6. (a) Transmission electron micrograph for the composite of mesoporous silica SBA-15 and nanoglass of composition 35Li2O· 65SiO2. The nanopores occupied by the glass phase are shown in the inset. (b) Selected area electron diffraction pattern obtained from (a).

these samples, no diffraction spots could be detected for this sample. This confirms that, in this study, we are dealing with purely noncrystalline phases, viz., SBA-15 (Figure 3) and 35Li2O·65SiO2 (Figure 6), respectively. The nanopore size was estimated from the dark regions (indicated by the arrows in Figure 6a) of the micrograph in Figure 6a, which represented the nanoglass filled pores. By comparing the diameter of the pores with the diameter estimated from the pore size distribution, as shown in Figure 2b, we estimate the error in pore size calculation to be of the order of ±0.03 nm. Electrical resistivity measurements were carried out on the compressed pellets of the nanocomposite powder with silver paint electrodes applied on the opposite faces of the sample. Electrical conductivity due to lithium ion movement was confirmed from the transport number measurement.9 For measuring the transport number accurately, we took a nanocomposite sample and reduced the silver electrode area suitably so that the current flowing through it after the application of a dc voltage was reduced. This way, the variation of current as a function of time could be recorded conveniently. Figure 7a shows the variation of current as a function of time after a voltage of 0.5 V was applied across the sample. The current decreased at a rapid rate during the initial period of time due to electrode polarization. A very low value of 0.088 μA was observed after 240 min of application of 0.5 V across the specimen (see inset in Figure 7a). It can be seen that the current was still going down. We have taken the values at this stage and the ratio of (1.6−0.088) × 10−6 A and 1.6 × 10−6 A gives a value of 0.95, implying the transfer number for Li+ ions

4. CONCLUSION Mesoporous silica SBA-15 was exploited to synthesize a nanocomposite in which a nanoglass of composition 35Li2O· 65SiO2 could be incorporated within the nanochannels of diameter 5.5 nm of the template. The nanoglass showed an electrical conductivity of ∼10−4 S·cm−1 at room temperature. 434

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Figure 7. (a) Variation of current as a function of time after applying 0.5 V across the nanocomposite. The inset shows the magnified view of the current−time curve 60 min after application of the dc voltage. (b) Typical plot of real part (Z′) vs imaginary part (Z″) of impedance of the nanocomposite at temperature 318 K. (c) Variation of log resistivity as a function of inverse temperature for the SBA-15−nanoglass composite, for the nanoglass concerned, and that of the corresponding bulk glass. (d) ac conductivity variation as a function of frequency for the SBA-15−nanoglass composite at 308 K.

Honorary Scientist position. S.K.S. acknowledges the Department of Science and Technology, New Delhi, for financial support through Project No. SR/NM/NS-1089/2011.

The activation energy of lithium ion migration was found to be 0.1 eV. This was ascribed to oxygen ion vacancies at the interface caused by the presence of Si2+ and Si4+ species, respectively, in the mesoporous silica at its interface with the nanoglass. These nanocomposites are expected to have applications in lithium-ion batteries for storage of renewable energy.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-33-2473 4971, ext. 1580. Fax: +91 33 2473 2805. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.C. acknowledges the award of the INSPIRE Fellowship by the Department of Science and Technology, New Delhi. R.M. thanks the Council of Scientific and Industrial Research, New Delhi, for the award of a Research Associateship. D.C. thanks the Department of Science and Technology, New Delhi, for the award of the SERB Distinguished Fellowship and the Indian National Science Academy, New Delhi, for giving him an 435

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