18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS A SOLVENT-FREE COMPOSITE SOLID ELECTROLYTES OF Li 2 CO 3 – Al 2 O 3 SYSTEM PREPARED VIA WATER BASED SOL GEL METHOD M. Sulaiman 1, *, A.A. Rahman 1 , N.S. Mohamed 1 1 Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia * M. Sulaiman ( mazdidas@um.edu.my ) Keywords : interphase; solid electrolyte; sol gel; conductivity; scanning electron microscopy Abstract temperature dependence of ionic conductivity were Composite solid electrolytes in the system (1- studied. The structural and thermal properties of the x )Li 2 CO 3 - x Al 2 O 3 were produced via a water based composites were also studied in order to understand sol-gel process. The yielded gels were subsequently their conductivities behavior. heated at 80 o C and crushed in an agate mortar. The 2 Experimental composites were identified by X-ray diffraction Powder compositions of the sample prepared are (XRD), differential scanning calorimetry (DSC), described by the general formula of (1- x )Li 2 CO 3 - scanning electron microscopy (SEM) and Fourier x Al 2 O 3 with x = 0.0 – 0.7 (mol percentage). Li 2 CO 3 transform infrared spectroscopy (FTIR). The ionic (high purity grade) and Al 2 O 3 (high purity grade) conductivity was also carried out by impedance powders were employed as starting materials with spectroscopy. Ionic conductivity studies showed that amorphous phase of Li 2 CO 3 and intermediate deionised water as solvent. In this sol-gel method, Li 2 CO 3 was dissolved in water at room temperature. crystalline phase formed at the interface of both Subsequently, Al 2 O 3 was added to this solution at Li 2 CO 3 and alumina influence the ionic transport in the composite solid electrolytes. The maximum constant stirring. After 45 minutes, the solution was values of 10 -3 S cm -1 were obtained at 130-180 o C slowly added to citric acid powder. The solution was then continuously stirred for 20 minutes at 60 o C and for composite samples having composition x = 0.4- left to stand at room temperature for four weeks. The 0.5. solution was then dried at 80 o C in an oven until it 1 Introduction yielded a powder. The powder was ground in an agate mortar until a fine powder of the composite Composites are usually obtained by doping ionic was obtained. conductors with insulating oxides such as ZrO 2 , Al 2 O 3 , SiO 2 , TiO 2 etc. [1-4]. This heterogeneous Structural characterizations of the composite were doping has been widely used to improve physical performed using a D8 Advanced-Bruker X-ray properties and conductivity of the ionic conductors. Diff ractometer with Cu Kα radiation for XRD and Metal carbonates are one of the groups of Perkin Elmer RX1 spectrometer for FTIR. The compounds that are widely used in various fields [5] morphology was analyzed by SEM using INCA and have also been used as solid electrolytes [6]. Energy 200 (Oxford Ins.). The thermal properties Among the alkali and alkaline-earth carbonates, only were measured on a Mettler Toledo DSC 822 with lithium carbonate, Li 2 CO 3 shows a fairly good continuous heating at a rate of 10 o C min -1 . The conductivity and good stability against the humidity conductivity measurements were carried out by in the air [6]. However, poor ionic conductivity at impedance spectroscopy technique on a Solatron low temperature is obtained when Li 2 CO 3 salt is 1260 impedance analyzer. An AC amplitude of 100 used as a composite material where alumina, Al 2 O 3 mV in the frequency range 10 -1 – 10 7 Hz was used. is employed as dispersoid. The conductivity of Li 2 CO 3 – Al 2 O 3 composite was found to be in the 3 Results and Discussion order of 10 -8 S cm -1 at 250 o C [7]. The aim of this 3.1 SEM study was to improve the ionic conductivity of Li 2 CO 3 – Al 2 O 3 composite solid electrolyte. In this The microstructures of the (1- x ) Li 2 CO 3 - x Al 2 O 3 work, Li 2 CO 3 – Al 2 O 3 composite samples were composites ( x = 0.0, 0.3, 0.4 and 0.6) studied by synthesized via a simple sol-gel process without the using SEM are shown in Fig. 1. Surface morphology use of organic solvent. Instead, deionised water was of the pure ionic salt of Li 2 CO 3 , x = 0.0 appeared to used as the solvent. The compositional and be agglomerated with crystalline features. However,
the alumina particles were homogenously dispersed components. However, the diffraction patterns in the composites mixture of x = 0.3 and 0.6. These suggested, the growth of crystal to be blocked by the types of distribution strongly indicate interfacial presence of amorphous phase. There is no peak contact between the Al 2 O 3 grains and the Li 2 CO 3 corresponding to these crystalline phases for the phase. Fig. 1(b) shows the surface of composite composite sample with x = 0.6 as shown in Fig. 2(b). sample with x = 0.3 with the ionic salt of Li 2 CO 3 3.3 DSC dominating the surface. Large grains of alumina seemed to dominate the whole surface when x Fig. 4 shows DSC curves of the (1- x ) Li 2 CO 3 - x Al 2 O 3 composite samples. The temperature of glass increases. The change can be clearly seen in the transition was observed at ~67 o C for x = 0.1 - 0.6. It micrograph of the sample with x = 0.6 (Fig. 1(c)). showed that the composite samples prepared by this sol-gel method displayed glassy state at this The microstructure in Fig. 2(a) show that the temperature. For composite samples with x = 0.1- composite with x = 0.3 consists of alumina particles (white) which are littered in an amorphous phase of 0.3, endothermic peaks of crystallization were observed at ~110 o C (T c ) and ~130 o C (T c’ ) which Li 2 CO 3 (dark area). A small amount of intermediate may correspond to incongruent melting of the crystalline phase was also observed in the composite due to chemical reaction between both the Li 2 CO 3 intermediate phases discussed earlier. Similar behavior was also obtained for x = 0.5, but the and Al 2 O 3 crystalline phases. The amorphous feature crystallization temperatures were shifted to the of Li 2 CO 3 can be seen in the composite sample with lower values of ~99 o C and ~122 o C due to the high x = 0.6 as shown in Fig. 2(b). This sample is amorphicity of the composite sample. dominated by the alumina phase as depicted in Fig. 1 (c). The heat flow of the crystallization of samples at 3.2 XRD ~110 o C decreased for x = 0.1 to x = 0.3. Much less heat at ~99 o C was determined for the crystallization Fig. 2 shows the XRD patterns of the prepared (1- x )Li 2 CO 3 - x Al 2 O 3 composites. All the composite of sample x = 0.5. In contrast, the heat flow at ~130 o C was observed to increase for x = 0.1 to x = 0.3. samples ( x = 0.1-0.6) consisted of a mixture of crystalline Li 2 CO 3 , Al 2 O 3 and an amorphous phase. From table 1, we can see that much heat was The peaks at 2θ 21°, 23.8°, 29.7°, 30.5°, 31.6°, absorbed for crystalline growth at ~122 o C for the composite sample with x = 0.5 in order to overcome 34°, 36°, 37°, 39.6°, 42.7° and 48.5° in the spectra are attributed to crystalline Li 2 CO 3 . amorphous phase formed. There is no phase transition observed in the DSC The intensities of Li 2 CO 3 peaks were observed to curve for sample with x = 0.6. The samples were decrease when x increases. The XRD spectra found to be stable in the temperature range of 140 - revealed that the peak intensities of Li 2 CO 3 250 o C. Table 1 shows the thermal properties of the especially at ~23.8°, 29.7°, 36 - 39.6° in the (1- x ) Li 2 CO 3 – x Al 2 O 3 composite samples and their composite samples were too low compared to those conductivities at higher temperatures. These will be of the pure Li 2 CO 3 ( x = 0.0). This indicated that a discussed later. great amount of Li 2 CO 3 had undergone a chemical interaction with alumina in the samples. Some peaks 3.4 FTIR of Li 2 CO 3 at 2θ ~42.7 ° and 48.5° disappeared with FTIR spectra obtained for (1- x )Li 2 CO 3 - x Al 2 O 3 increasing x indicating that a high concentration of composite samples are shown in Fig. 5. The FTIR the amorphous phase was formed. This phenomenon results confirmed that all samples with x = 0.1 - 0.5, is indicated by some peak broadening of the have stretching and bending of Al – O bonds at 450 remaining Li 2 CO 3 in the 2θ regions of ~29 -30.5° and – 800 cm -1 [8]. Bands at this range are also attributed ~36-37°. to stretching and bending of AlO 6 atomic group [8] which indicates the aluminum cations are residing in A small amount of crystal phases of α -LiAlO 2 , γ - the octahedral sites in the alumina phase. The bands LiAlO 2 and LiAl 5 O 8 were also detected in the at 810, 700, 627, 550, 500 and 450 cm -1 are composite samples with x = 0.1 - 0.4 as indicated by associated with γ – LiAlO 2 and LiAl 5 O 8 phases [9]. additional reflections at 2θ ~10°, ~15-29° and ~33°. These confirmed their presence in the composite This is due to the crystallization of this phase as a result of chemical reactions between the composite
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