Effect of water-based sol gel method on structural, thermal and conductivity properties of LiNO3–Al2O3 composite solid electrolytes

M. Sulaiman; A.A. Rahman; N.S. Mohamed

doi:10.1016/j.arabjc.2015.04.031

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Original article

10 (

); 1147-1152

doi:

10.1016/j.arabjc.2015.04.031

Effect of water-based sol gel method on structural, thermal and conductivity properties of LiNO₃–Al₂O₃ composite solid electrolytes

M. Sulaiman^⁎, A.A. Rahman, N.S. Mohamed

Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

⁎Corresponding author. Tel.: +603 79675918. mazdidas@um.edu.my (M. Sulaiman)

Received: 2013-7-19, Accepted: 2015-4-23, Published: 2015-12-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Composite solid electrolytes in the system (1−x)LiNO₃–xAl₂O₃, with x = 0.0–0.5 were prepared by sol gel method. No organic solvents but water was used as medium for the sol preparation. The obtained materials were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, Fourier transform infrared spectroscopy and impedance spectroscopy. Structural analysis evidenced amorphous phase of LiNO₃ in all composite samples. Traces of crystalline LiAl₅O₈ and LiAlO₂ were also present in the composite samples with x = 0.3–0.5, as indicated by the X-ray diffractograms. The thermal analysis showed that the samples were in a stable phase between ∼48 °C – ∼100 °C and ∼120 °C – ∼220 °C. The conductivity values of the composites were in the range of 10⁻³ S cm⁻¹ at room temperature to 180 °C. Our study demonstrates that water is suitable for the preparation of the composite solid electrolytes.

Keywords

Composite material

Conductivity

Lithium nitrate

Alumina composite

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1 Introduction

The fast development and extensive application of electronic products, especially in automotive use and portable electronics require safe solid-state secondary batteries with high energy (Tarascon and Armand, 2001; Minami et al., 2005). For instance, lithium ion solid-state batteries are current power sources of choice due to their light weight, high voltage and high value of energy content and have contributed to the manufacture of countless electronic products (Scrosati and Garche, 2010). Solid-state batteries have both solid electrodes and electrolytes with good ion conductors for high power density. Solid electrolytes can be crystalline–polycrystalline, glassy-amorphous, composite and polymeric (Agrawal and Gupta, 1999). Composite solid electrolytes started to draw interest since the observation of enhanced ionic conductivity by Liang, 1973. Since then, both physical and chemical properties of composite solid electrolytes have been studied and the presence of number of surfaces and defect formation at free surfaces and interfaces in composites are main factor of conductivity enhancement (Uvarov and Vanek, 2000; Yaroslavtsev, 2009; Sulaiman et al., 2011, 2012 ). Uvarov observed that their structure could be either epitaxial crystalline or amorphous (Uvarov and Vanek, 2000). In 1979, Wagner et al. (Jow and Wagner, 1979) reported that conductivity enhancement in a binary system MX-Al₂O₃ was associated with high concentration of defects in the ionic salts, MX containing mobile ions, M⁺. Strong surface interaction of ionic salts with oxide leads to the spreading of MX over the surface of oxide and at adequately high oxide dispersity and concentration, the whole volume of the ionic salt turns into either a surface disordered state or an amorphous phase (Uvarov, 2000, 2011). These phenomena have been ascertained in our previous work on composite solid electrolytes in the system (1−x)LiNO₃–xAl₂O₃ with x = 0.0–0.5 based on ethanol sol gel method (Sulaiman et al., 2012). In this method, higher ionic conductivity values in the order of 10⁻³ S cm⁻¹ at room temperature were obtained compared to the conductivity values around 10⁻⁵ S cm⁻¹ (at 70 °C) for the same composite system prepared using mechanical milling (Uvarov et al., 1996a; Sulaiman et al., 2012). Here, we report the preparation of composite solid electrolytes in the system (1−x)LiNO₃–xAl₂O₃ using water-based sol gel technique instead of ethanol based sol–gel and investigate their structural, thermal and conductivity properties. Structural characterizations were carried out by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The conductivity study was determined by alternating current (AC) impedance measurements. Our study demonstrates for the first time that deionized water which is both environmental friendly and economical, is suitable for the preparation of (1−x)LiNO₃–xAl₂O₃ with x = 0.0–0.5 composite solid electrolytes with high conductivity at room temperature.

2 Experimental

Composite solid electrolytes in the system (1−x)LiNO₃–xAl₂O₃ with x = 0.1–0.5 mol were synthesized by sol gel method. LiNO₃ (purity >99.0%, Fluka) and Al₂O₃ (purity >99.7%, Sigma Aldrich) were used as purchased without further purification. First, LiNO₃ was dissolved in deionized water and Al₂O₃ was then added to the solution. The solution was continuously stirred at 40 °C for 20 min. Consequently, to the solution was added citric acid and the mixture was heated at 80 °C on a hot plate with continuous stirring until the formation of a wet gel. The wet gel was then fully dried in oven at 220 °C. The final product obtained was fluffy and brown. Structural characterizations for XRD, FTIR and SEM were performed on a D8 Advanced-Bruker X-ray Diffractometer with Cu Kα radiation, a Perkin Elmer RX1 spectrometer and INCA Energy 200 (Oxford Ins.), respectively. The thermal properties of the samples were measured on a Mettler Toledo DSC 822 with continuous heating at a rate of 10 °C min⁻¹. For conductivity studies, pellets were made by pressing the composite powders at a pressure of 6–8 tones cm⁻². The diameters of the pellets were 13 mm and their thicknesses were about 1.1 mm. The pellets were then sandwiched between two electrodes of a sample holder. Conductivities were measured by impedance spectroscopy on Solatron 1260 impedance analyzer at room temperature to 200 °C. An ac amplitude of 100 mV in the frequency range of 10⁻¹–10⁷ Hz was used.

3 Results and discussion

3.1

3.1 XRD analysis

Fig. 1 shows XRD spectra for the pure LiNO₃ and composite samples of (1−x)LiNO₃–xAl₂O₃. The diffraction peaks at 2θ of 25.7°, 35.3°, 37.8°, 43.5°, 52.6°, 59.4°, 61.4°, 66.5°, 68.6° and 76.9° observed in the spectra of composite samples with x = 0.1–0.5 were attributed to Al₂O₃. Only one peak of very low intensity of LiNO₃ appeared at 2θ ≈ 25° for all composite samples, except in sample with x = 0.4. The absence of some LiNO₃ peaks in the XRD spectra corresponds to the existence of either crystalline and amorphous phases of LiNO₃ or a mesophase phase (Uvarov, 2011). At x = 0.4, the crystalline LiNO₃ was fully transformed into amorphous state. At the same time, weak reflection appeared at 2θ ≈ 22° in the spectra of the samples with x = 0.3–0.4 and was assigned to crystalline LiAl₅O₈. At x = 0.3–0.5, the diffraction peaks appeared at 2θ ≈ 29°, ≈ 32° and ≈ 34° and were ascribed to LiAlO₂. The XRD spectra for composite samples with x = 0.3–0.5 showed that chemical reactions took place between LiNO₃ and Al₂O₃ during the sol gel preparation to produce traces of crystalline LiAl₅O₈ and LiAlO₂ (lithium aluminates). The transformation of the interaction in the composite samples can be represented by the following route (Sulaiman et al., 2012; Oksuzomer et al., 2004; Valenzuela et al., 2001 ): $Lithium nitrate + alumina \to amorphous + {LiAl}_{5} O_{8} + {LiAlO}_{2}$

XRD patterns of (1−x) LiNO3–xAl2O3 composites.

In earlier work, the LiAl₅O₈ phase was not detected (Sulaiman et al., 2012). This can be explained by the high degree of solubility of LiNO₃ in water than that in ethanol.

3.2

3.2 DSC analysis

DSC curves of the composite (1−x) LiNO₃–xAl₂O₃ samples are shown Fig. 2. An endothermic peak was observed at 100 °C and was attributed to dehydration. A stable phase for the composite system (x = 0.1–0.5) appeared between ∼120 °C and ∼200 °C. An endothermic peak around 220 °C was attributed to the decomposition of citric acid. Another endothermic peak at 252 °C was attributed to the melting of LiNO₃ (T_m) (Ribeiro et al., 2001). There was no melting point of crystalline LiNO₃ found in the composite sample with x < 0.5. This phenomenon could be attributed to the phase change of crystalline LiNO₃ to amorphous and mesophase phases that occurred due to the application of heat closed to the melting point of pure LiNO₃ during the preparation of the samples (Uvarov, 2011). At this state, the long-range and rigid ordering of crystalline LiNO₃ breaks down to a less ordered state, thereby promoting the Li⁺ ion mobility in the composites. The exothermic peaks at ∼290 °C – ∼350 °C in all composite samples were related to the thermal decomposition of LiNO₃ and the formation of lithium aluminates (Chatterjee and Naskar, 2003; Oksuzomer et al., 2004). There was no exothermic curve of the formation of lithium aluminates found for composite sample with x = 0.1.

DSC scanning of (1−x)LiNO3–xAl2O3 composites.

3.3

3.3 FTIR analysis

FTIR spectra of LiNO₃ (x = 0.0) and composite samples with x = 0.1–0.5 in the wavenumber range 1500–500 cm⁻¹ are presented Fig. 3(a). The very strong absorption band in the spectra of pure LiNO₃ at 1384 cm⁻¹ and the medium bands at 1051 cm⁻¹ and 827 cm⁻¹ were ascribed to the asymmetric stretching mode, symmetric stretching mode and out of plane deformation mode of NO₃⁻ ions, respectively (Chatterjee et al., 1998, 2002; Kamiya et al., 1991; Nakamoto, 1978; Tamura et al., 2004; Gaphurov and Aliev, 1999 ). These characteristic bands of nitrate groups appeared at 1389–1402 and 825–827 cm⁻¹ in all spectra of the composite samples demonstrating that LiNO₃ phase was present in all of the composite samples (x = 0.1–0.5).

FTIR spectra of (1−x)LiNO3–xAl2O3 composites in (a) 500–1500 cm−1 and (b) 1100–1000 cm−1 regions.

As can be seen in Fig. 3(b), there was a small asymmetric distortion in the contour of symmetric stretching mode band of NO₃⁻ for the composite sample with x = 0.4. The bandwidth of the vibration band became broader and the position was slightly shifted to 1050 cm⁻¹. The contour band of the symmetric stretching mode was inconspicuous for composite samples with x = 0.1, 0.2, 0.3 and 0.5. This effect was due to the heating of the sample up to a temperature ∼30 °C below the melting point of pure LiNO₃ allowing the formation of fully amorphous phase of LiNO₃ (Uvarov, 2011). Gaphurov et al. (2002) reported that the distortion was related to the orientation disordered anions of NO₃⁻ anions in the LiNO₃ crystal lattice. A similar observation was reported in RbNO₃–Al₂O₃ nanocomposite, where the position of absorption peaks in a disordered state was observed at lower wavenumber and broadened (Uvarov et al., 1996b).

The complex structure of asymmetric stretching mode band at 1389 cm⁻¹ of the spectra of composite samples with x = 0.3–0.5 was attributed to interaction between nitrates and alumina particles. As discussed in Section 3.1, traces of crystalline LiAl₅O₈ and LiAlO₂ were formed in these composite samples. The vibration bands of Al–O bonds could be observed in the infrared region between 900 and 500 cm⁻¹ (Chatterjee and Naskar, 2003; Boumaza et al., 2009). The bands at 550 and 636 cm⁻¹ observed in Fig. 3(a) corresponded from AlO₆ stretching modes and were linked to α-Al₂O₃ (Boumaza et al., 2009). Bands at 642, 544 and 520 cm⁻¹ in composite samples with x = 0.3–0.5 referred to AlO₄–LiO₄ lattice of the LiAlO₂ and LiAl₅O₈ phases (Oksuzomer et al., 2004; Li et al., 2002). The bands appeared at 610 and 680 cm⁻¹ corresponding to LiAl₅O₈ phase could be clearly be seen in composite sample with x = 0.3 (Oksuzomer et al., 2004; Ribeiro et al., 2001). The changes in the spectral properties in the nitrates could be attributed to the rotary mobility of molecular ion due to substantial thermal activation and the increase in dynamic interaction between the ions that occurred during the sample preparation (Gaphurov et al., 2002).

3.4

3.4 SEM analysis

Fig. 4 shows SEM micrographs of composite sample with x = 0.4. As observed from Fig. 4(a), only alumina particles dominantly appeared in the composite sample. On the contrary, the LiNO₃ (dark region) was observed to be in the amorphous form and apparently spreading over the alumina particle surface. The spreading of lithium nitrate over the surface of the oxide was the main factor to the formation of the amorphous phase of lithium nitrate. The ionic salt was expected to spread only in a molten state which occurred at temperatures closed to T_m. The amorphous phase was formed as the result of the ionic melt and physical interaction of both Al₂O₃ and LiNO₃ crystalline phases (Uvarov et al., 1996b; Neiman et al., 2007). According to Uvarov and Vanek (2000), amorphous phase may be formed at the interface phase when composites obtained are in strong nonequilibrium conditions or if the surface structures of ionic salts and oxide are very different. Fig. 4(b) shows that alumina particles (white region) were uniformly dispersed in the composite mixture confirming thus interfacial interaction between both Al₂O₃ and LiNO₃ crystalline phases.

Morphologies of composite sample with x = 0.4 magnified at (a) 6000× and (b) 1500×.

3.5

3.5 Electrical conductivity

Complex impedance plots of composite samples with x = 0.1–0.3 and x = 0.4–0.5 are shown in Figs. 5 and 6, respectively. For composite samples with x–0.1–0.3, the plots show a slanted spike at low frequency and distorted semicircles at higher frequencies. The formation of spike represented interfacial effects between electrode and electrolyte (Dygas et al., 2005; Godinho et al., 2003). The high and low frequency semicircles with the interception at the Z’-axis were assigned to bulk resistance (R_b) and grain boundary resistance (R_gb), respectively. The appearance of these two semicircles could be associated with different conduction process in the composite samples. The ionic conduction could be occurring through the grain and along the grain boundaries of LiNO₃. However, the semicircle portion at higher frequency disappeared with increase in alumina composition, as shown in impedance plot for composite samples with x = 0.4–0.5 (Fig. 6). The disappearance of semicircle portion implied that only resistive components prevailed in the composite samples. In this case, alumina particles were the only resistive component, as clearly shown in the SEM micrographs in Fig. 4. In the present work, by knowing the value of bulk resistance (R_b), the conductivity has been calculated using the relationship as shown below:

(1)

σ = \frac{d}{R_{b} A}

where d is the sample thickness (cm), R_b is bulk resistance (Ω), and A is the area (cm²) of the sample.

Complex impedance plot of composite samples with x = 0.1, 0.2 and 0.3 at room temperature.

Complex impedance plot of composite samples with x = 0.4 and 0.5 at room temperature.

Fig. 7 shows the compositional dependence of conductivity at room temperature (25 °C) for pure LiNO₃ and (1−x) LiNO₃–xAl₂O₃ composite samples. The conductivity dependence plot had two maxima at x = 0.1 and x = 0.4. The conductivities were found to increase from x = 0.0 to 0.1 but decreased steadily with further increase in x. However, beyond the composite samples with x = 0.2, the conductivity slowly increased up to x = 0.4. From Fig. 7, it was observed that the conductivity decreased again after the maximum point of x = 0.4. The enhancement in ionic conductivity in the composite sample with x = 0.1 was due to the formation of the amorphous phase of LiNO₃, as indicated by the XRD and DSC results, in Figs. 1 and 2, respectively. The collapse of LiNO₃ crystal lattice initiated the orientational disordered anions of NO₃⁻ and consequently increased the number of Li⁺ ions. It suggests that the ionic conductivity proceeded via amorphous interface phase (Gaphurov et al., 2002; Uvarov et al., 1996a; Rao et al., 2005 ). Decrease in conductivity for composite samples with x > 0.1 could be explained by the increasing amount of alumina (dispersoid) grains in the composites which reduced the surface area of contact between the host matrix of LiNO₃ and the dispersoid grains. This led to the blocking effect by the alumina grains that hinder the migration of the Li⁺ ions. Beyond x = 0.2, however, the conductivity continued to increase due to the presence of new phases of LiAlO₂ and LiAl₅O₈ in the composite samples with x = 0.3–0.5. The highest conductivity value of 1.97 × 10⁻³ S cm⁻¹ occurred in the composite sample with x = 0.4. The high ionic conductivity in this composite sample was related to the presence of the fully amorphous phase of LiNO₃ as revealed by the XRD spectra and the spreading of the LiNO₃ over the alumina as shown in the SEM micrograph in Fig. 4. The high ionic conductivity around 10⁻³ S cm⁻¹ at room temperature has also been observed for the same composite solid electrolyte system prepared using ethanol (Sulaiman et al., 2012). It means that the water-based system of the prepared composite samples is able to produce materials with high ionic conductivity.

The ionic conductivity as a function of composition (x) for composites (1−x) LiNO3–xAl2O3 at room temperature.

Temperature dependencies of conductivity for (1−x)LiNO₃–xAl₂O₃ composites prepared are shown in Fig. 8. Conductivities values of the composite samples increased with temperature. Interestingly, the composite samples exhibited conductivities in the same range of 10⁻³ S cm⁻¹ at room temperature to 180 °C (except x = 0.2). As discussed earlier, the amorphous phase of the LiNO₃ played a major role in conductivity enhancement in composite samples. Beginning at a temperature of 110 °C, conductivities continued to be constant (except x = 0.2). This was due to stable state between 110 °C and 200 °C as shown in the DSC curves. The low ionic conductivity in composite sample with x = 0.2 could be related to the absence of crystalline LiAlO₂ and LiAl₅O₈, which on the other hand, enhanced the conductivity of the composite samples with x = 0.3–0.5. The activation energy was obtained by fitting the ionic conductivity data with Arrhenius equation:

(2)

σ = A \exp (\frac{- E_{a}}{kT})

where A is the pre-exponential factor, E_a is the activation energy for conduction and k is the Boltzmann’s constant which is 8.52 × 10⁻⁵ eV/kelvin, T is the temperature in Kelvin. The activation energy for all samples is shown in Fig. 8. The low value of activation energy indicates high mobility of ions in the sample.

Temperature dependence of the conductivity of (1−x) LiNO3–xAl2O3 composites.

4 Conclusions

Our study demonstrate for the first time that composite solid electrolytes in the system (1−x)LiNO₃–xAl₂O₃ with x = 0.0–0.5 prepared by sol–gel method using solely deionized water as medium for sol preparation exhibited high ionic conductivity at room temperature and especially at x = 0.4.

Acknowledgment

The authors would like to thank the University of Malaya for granting the Research Grant (RG021/09AFR) to support this work.

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Introduction

Experimental

Results and discussion

XRD analysis
DSC analysis
FTIR analysis
SEM analysis
Electrical conductivity

Conclusions

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[1] Agrawal R.C., Gupta R.K., . Review superionic solids: composite electrolyte phase-an overview. J. Mater. Sci.. 1999;34:1131-1162.
[Google Scholar]

[2] Boumaza A., Favaro L., Ledion J., Sattonnay G., Brubach J.B., Berthet P., Huntz A.M., Roy P., Tetot R., . Transition alumina phases induced by heat treatment of boehmite: an X-ray diffraction and infrared spectroscopy study. J. Solid State Chem.. 2009;182:1171-1178.
[Google Scholar]

[3] Chatterjee M., Naskar M.K., . Novel technique for the synthesis of lithium aluminate (LiAlO₂) powders from water-based sols. J. Mater. Sci. Lett.. 2003;22:1747-1749.
[Google Scholar]

[4] Chatterjee M., Enkhtuvshin D., Siladitya B., Ganguli D., . Hollow alumina microspheres from boehmite Sols. J. Mater. Sci.. 1998;33:4937-4942.
[Google Scholar]

[5] Chatterjee M., Naskar M.K., Lakshmi N.S., . Sol-emulsion-gel synthesis of hollow mullite microspheres. J. Mater. Sci.. 2002;37:343-348.
[Google Scholar]

[6] Dygas J.R., Malys M., Krok F., Wrobel W., Kozanecka A., Abrahams I., . Polycrystalline BIMGVOX.13 studied by impedance spectroscopy. Solid State Ionics. 2005;176:2085-2093.
[Google Scholar]

[7] Gaphurov M.M., Aliev A.R., . Raman and infrared spectroscopic studies of the platinum electrode/molten nitrate interface. Spectrochim. Acta Part A. 1999;55:1237-1241.
[Google Scholar]

[8] Gaphurov M.M., Aliev A.R., Akhmedov I.R., . Raman and infrared study of the crystals with molecular anions in the region of a solid–liquid phase transition. Spectrochim. Acta Part A. 2002;58:2683-2692.
[Google Scholar]

[9] Godinho M.J., Bueno P.R., Orlandi M.O., Leite E.R., Long E., . Ionic conductivity of Bi₄Ti_0.2V_1.8O_10.7 polycrystalline ceramics obtained by the polymeric precursor route. Mater. Lett.. 2003;57:2540-2544.
[Google Scholar]

[10] Jow T., Wagner J.B., . The effect of dispersed alumina particles on the electrical conductivity of cuprous chloride. J. Electrochem. Soc.. 1979;126:1963-1972.
[Google Scholar]

[11] Kamiya K., Takahashi K., Maeda T., Nasu H., Yoko T., . Sol–gel derived CaO- and CeO₂- stabilized ZrO₂ fibres conversion process of gel to oxide and tensile strength. J. Eur. Ceram. Soc.. 1991;7:295-305.
[Google Scholar]

[12] Li F., Hu K., Li J., Zhang D., Chen G., . Combustion synthesis of γ-lithium aluminate by using various fuels. J. Nucl. Mater.. 2002;300:82-88.
[Google Scholar]

[13] Liang C.C., . Conduction characteristics of the lithium iodide – aluminium oxide solid electrolytes. J. Electrochem. Soc.. 1973;120:1289-1292.
[Google Scholar]

[14] Minami T., Tatsumisago M., Wakihara M., Iwakura C., Kohjiya S., Tanaka I., . Solid State Ionics for Batteries. Springer; 2005.

[15] Nakamoto K., . Infrared and Raman spectra of inorganic and co-ordination compounds. New York: Wiley; 1978.

[16] Neiman A.Y., Uvarov N.F., Pestereva N.N., . Solid state surface and interface spreading: an experimental study. Solid State Ionics. 2007;177:3361-3369.
[Google Scholar]

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