Sol-gel Synthesis and Characterization of MgCO3 – Al2O3 Composite Solid Electrolytes
M. Sulaiman* | M. F. Z. Kadir | A. N. Che Mat
© 2021 IIETA. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
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Composite solid electrolytes in the system (1-x)MgCO3 – xAl2O3 with x = 0.1 - 0.9 were synthesized by a sol-gel method and analyzed by X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy, energy dispersive X-ray, Fourier transform infrared spectroscopy and alternating current impedance spectroscopy for the determination of the phases and crystallinity, thermal stability, surface morphology, elemental composition, chemical bonding and conductivity, respectively. The composites show that, the crystallinity of the composites decreases as the amount of Al2O3 increase which could lead to higher conductivity. The thermal decomposition studies also indicate that the melting and/or decomposition of the composites occur at lower temperature than the pure MgCO3 which normally take place at 350 oC. The composite with the ratio of x = 0.9 give the highest conductivity value in order of 10-4 S.cm-1 as compared to other ratio due to effective transfer of charge carriers in addition to the formation of MgO-Mg2+ species at the interface.
High energy demand and climate change due to extensively burning fossil fuel had forced the researchers around the globe towards green energy generation mostly focusing on electrochemical energy conversions and storage. Research in multivalent battery such as magnesium (Mg) ion battery is indeed an utmost imperative in fuelling electric vehicles and most of the portable devices although it is still at the commencement stage [1]. However, investigation in battery is not as easy as projected. Numerous criterions need to be taken into account such as naturally abundance, cheap, less toxic towards the environment, possesses high energy and power density, long life cycle and enable to work in a wide range of temperature.
According to the recent report, investigation on Mg ion battery contribute about 80% of the literature from 1985 – 2015 [2]. The awareness for Mg battery lies on the fact that Mg has low reduction potential which is about -2.37 V versus normal hydrogen electrode (NHE) as well as high volumetric capacity (3866 mA cm-3) which is almost double the value of Li ion battery (2046 mA cm-3) [3, 4]. The research interest in our group is mainly on the development of solid composite electrolyte for Mg ion battery due to its better performance against the liquid electrolyte in terms of operating temperature, battery durability and safety [5]. There is no doubt that the first developed and the benchmark of solid composite electrolyte was LixPOyN2 in 1992 [6, 7] due to excellent electrochemical performance in terms of broad electrochemical window ranging from 0 – 5.5 V vs Li+/Li. However, low ionic conductivity (10-6 S cm-1) and mechanical strength are the major apprehension apropos this type of solid electrolyte. Moreover, in terms of commercialization, magnesium ion battery is still at the beginning stage. Slow magnesium intercalation and/or diffusion into cathode as well as irreconcilability between anode and electrolyte due to high overpotential (polarizing ability) of Mg2+ are some major issues that should be taken into consideration for practical purposes [8, 9].
Optimization of the ionic conductivity is undeniably a vital essential for solid composite electrolyte which deals closely with the preparation of the materials. Precipitation reaction [10], solid-state reaction [11], exchange reaction [12], sol-gel reaction [13], to name a few are among synthetic methods that have been explored to fabricate the solid composite electrolyte. Through a cautious control on the experimental conditions, the formation of porous nanoparticles with large specific surface area is the most desirable where the interconnection and/or the interfaces between the insulator and ionic conductor are prominently heightened which could enhanced the conductivity of solid composite electrolyte [14]. This phenomenon has been recounted hitherto by several authors who demonstrated that sol-gel technique could be able to deliver the homogeneous dispersion of the insulator into the ionic host conductor which promote better ionic conductivity of the electrolyte [15, 16].
To the best of the author’s knowledge, there is no report in the literature regarding the preparation of (1-x)MgCO3 – xAl2O3 with x = 0.1 - 0.6 based solid composite electrolyte. The existence of CO32- anion is expected to enhance the ionic conductivity through disorientation of the anion as discussed from our previous study [5]. The as-prepared material is characterized by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), thermogravimetric analysis (TGA) and differential scanning calorimtey (DSC), Field Emission Scanning Electron Microscope-Energy Dispersive X-ray (FESEM-EDX) and electrochemical impedence spectroscopy for the determination of crystallinity and phases, chemical bonding, thermal stability, surface morphology and ionic conductivity, correspondingly.
2.1 Sample Preparation
Composite solid electrolytes in the system (1-x) MgCO3-xAl2O3 with x = 0.1 – 0.9 mole were synthesized by a sol-gel method. MgCO3 (purity > 99.0%, Fluka) and Al2O3 (purity > 99.7%, Sigma Aldrich) were used as purchased without further purification. First, a desired mole of MgCO3 and Al2O3 were mixed in ethanol under magnetic stirring at room temperature. An equivalent amount of citric acid to the mass of Al2O3 was introduced to the solution after 10 min of stirring. Next, the solution was refluxed under magnetic stirring at 80 °C for several hours, followed by a drying process in a controlled oven at 120 °C for overnight. The powder was then ground in an agate mortar into a fine powder. In the final step, the powder was calcined at 900 °C in a furnace for 2 h. The sample was stored in a desiccator until conductivity measurement.
2.2 Characterization techniques
Structural characterizations for XRD and FTIR were performed on a PANanalytical EMPYREAN diffractometer with CuKα radiation, λ=1.5406 Å and a Perkin Elmer Frontier spectrometer with 4 cm-1 resolution, respectively. The XRD data was obtained using X’Pert HighScorePlus software. The morphology and chemical content of the samples (pellets) were analyzed using FESEM-EDX SU8220 Hitachi. The thermal properties of the samples were measured on a Perkin-Elmer Pyris Diamond TG/DTA thermal analyzer in nitrogen atmosphere at a constant heating rate 5 °C min-1 at temperatures ranging from 30 to 900 °C. The DSC was obtained from the temperature range of 35–300 oC by Perkin-Elmer DSC 6 with a scan rate of 10 oC min-1. For conductivity studies, pellets were made by pressing the composite powders at a pressure of 9-10 tones cm-2 at room temperature. The diameter of the pellet was 13 mm and its thickness was about 1.1 mm. Conductivities were measured on a PGSTAT30 Autolab potentiostat/galvanostat (Ecochemie, Netherlands) impedance analyzer at frequencies ranging from 10-1 – 107 Hz with an a.c amplitude of 100 mV. The conductivity measurements were conducted by sandwiching composite pellet between two stainless steel electrodes at temperatures ranging from room temperature to 150 °C.
3.1 XRD analysis
The crystallinity of the materials was investigated by XRD as shown in Fig.1 below. The XRD pattern for almost all ratios demonstrated about similar pattern where the formation of MgO can be observed at 2θ = 36.5o, 42.9o and 62.5o which are in good agreement with the previous study [5, 17]. Meanwhile, Al2O3 was positioned at 2θ of 32.3o, 39.5o, 45.2o, 60.3o, 66.8o as indicated recently [18-20]. The absence of MgCO3 from XRD analysis indicative of complete conversion to MgO during the calcination process and the occurrence of the crystalline and amorphous phase of the prepared materials. This observation is in agreement with our previous experience in fabrication LiNO3-Al2O3 composite where there was an absence of LiNO3 peak can be detected from XRD analysis [21, 22]. From Figure 1, the XRD analysis of the materials showed that the crystallinity increases as the amount of Al2O3 increased. The increments of the amorphousness of the composites give rise to the higher conductivity of the material. Furthermore, nanometer size of dispersoid which act as filler, where in the current work the filler is Al2O3, prevent agglomeration thus enhancing the conductivity of the composite [23]. This finding is in parallel with Joge et al. [24] where they found that the conductivity reduced significantly as the crystallinity of the polymer electrolyte increased when the amount of Al2O3 increased to maximum of 6 wt%.
Figure 1. XRD patterns of Al2O3 and (1-x)MgCO3–xAl2O3 composites.
3.2 DSC and TGA analyses
The thermal stability of the as-prepared materials were investigated by using DSC and TGA as demonstrated in Figure 2 and Figure 3, correspondingly. Typically, from the DSC analysis (Figure 2), the decomposition of adsorbed water ensue around 100 oC followed by the degradation of citric acid about the temperature of 140 oC until 170 oC [22]. However, as the temperature increased up to more than 250 oC, another endothermic peak due to the decomposition of MgCO3 [25]. The melting point of the composites are lower than the melting point of MgCO3 which normally occur at 350 oC indicative of local structural change phenomenon through disorder rearrangement when alumina was added into the composites which is supported by the previous studies where the decomposition temperature or the melting point of PEG-Mg(CH3COO)2-CeO2 composites are much lower than pure PEG after the introduction of cerium oxide [26]. To support the DSC analysis, TG analysis has been conducted and almost all samples underwent three decomposition phase or region namely the dehydration and decomposition of citric acid, decomposition of MgCO3 and the formation of MgO as validated in Figure 3. The weight loss of the as-prepared materials in region I from TG analysis was due to the dehydration process of the adsorbed water followed by the degradation of citric acid where the reaction took place up to the temperature of less than 200 oC. Nevertheless, as the temperature increased to more than 200 oC, MgCO3 started to decompose (Region II) until it became stable at the temperature beyond 425 oC (Region III) where the formation of MgO can be obtained. It is noteworthy to mentioned that Region II from TG analysis where the application of temperature is more than 200 oC is the most crucial part in order to break the ionic bonding of MgCO3 to produce mobile Mg2+ ion for the enhancement of ionic conductivity of the composite solid electrolyte. This argument is in agreement with the previously reported study from the previous investigations where the Li+ ion mobility could be promoted and enhanced at temperature of 252 oC whenever the long range and rigid order of crystalline LiNO3 has been broke down [22].
Figure 2. DSC curves of (1-x)MgCO3– xAl2O3 composites
where x of (a) 0.2; (b) 0.7; (c) 0.6; (d) 0.5; (e) 0.4; (f) 0.9.
Figure 3. TGA curves of (1-x)MgCO3– xAl2O3 composites.
3.3 FTIR analysis
The chemical bonding of the as-prepared materials after calcination were determined by using FTIR as indicated in Figure 4. The chemical bonding of interest in the present investigations are the occurrence of MgCO3, Mg-O, CO32- and Al-O bonds which can be divided into several distinct regions. The combination band appeared with a weaker strength at 2020 cm-1 in accordance to previously reported by Colomban [27]. Whereas, the formation of asymmetric stretching vibration of carbonate anion from MgCO3 can be observed between 1300 - 1500 cm-1 which is closed to Hu et al. [28]. Furthermore, the existence of Al-O-Al bond in the as-prepared composite can be obtained at 1100 - 1150 cm-1 which is in parallel with other studies by Khazaei et al. [19] where they reported that the presence of Al-O-Al bond took place in the wide region of 300 – 1000 cm-1. Another report by Potdar et al. [29] stating that the occurance of Al-O-Al bond could be obtained at 1073 cm-1 and a shoulder at 1163 cm-1. Meanwhile, Mg-O-Mg bond of MgO can be obtained from a weak intensity around 870 cm-1 [30, 31]. On the other hand, the formation of Al-O stretching vibration also can be obtained at around 550 cm-1 which in agreement with Tong et al who reported that the Al-O bond can be deduced between 900 – 500 cm-1 [32]. Additionally, the existence of MgCO3 can be observed at wavenumber of around 420 cm-1 which is in accordance with BrusenTsova et al. [33] which is appeared in between 380 – 439 cm-1.
Figure 4. FTIR of MgCO3 and (1-x)MgCO3– xAl2O3 composites.
3.4 FESEM-EDX analyses
Structural information of the as-prepared composites can be investigated by FESEM micrographs as indicated in Figure 5 below. The surface structure of the composite seems to form intense with spherical-like agglomeration and a lot of visible pores either on the surface or at the cross-section. The formation of agglomerated structure might provide better charge/discharge capacity due to excellent trap density that shortens the diffusion of Mg2+ ion [34, 35]. Moreover, calcination of the composite at such a high temperature would be able to reduce the electrical resistivity, induce Joule heating, producing more compact and dense structure of composite leading to possibility of higher conductivity [36]. In addition, the formation of all the expected elements namely Mg, Al and O can be found in good agreement between each other as indicated by EDS analysis from two different spots in Figure 6 which further confirm the uniform dispersion of MgO particles.
Figure 5. FESEM micrographs of (1-x)MgCO3– xAl2O3 composites with x = 0.9. On the pellet surface (a – d); pellet cross-section (e – f).
Figure 6. EDX spectra for (1-x)MgCO3– xAl2O3 composites with x = 0.9.
3.5 Onductivity measurements
The electronic conductivity of the prepared composite was investigated through chronoamperometry method at 50 mV by investigating the current density as a function of time as indicated in Figure 7. The prepared material seems to be stable after the measurement although it underwent a sudden drop at the start of the measurement. The sudden decrease in current density at the beginning of the measurement (t = 0s) from 2.75 μA.cm-2 to 2.41 μA.cm-2 at t = 2000s is due to over potential or polarization of the material involving the rearrangement of Mg2+ ions which is in agreement with recently reported studies by Mohammad et al. [37] and Nowak et al. [38]. Furthermore, the decrement in current to the steady state values also can be assigned owing to the growth of the passivation layer at the electrode and the accomplishment of salt concentration gradient in the prepared composites [39]. The electronic conductivity (σel) obtained for these materials for all ratios as indicated in Table 1 are in order of 10-6 S.cm-1 which is lower than the σel of metals and semiconductors, suggestive the electronic insulating behavior of these newly prepared composites [40, 41]. The electronic conductivity of the material can be obtained using the formula of σel = j.d/U where σel is electronic conductivity, j is current density, d is diameter and U is the applied voltage [37]. Additionally, cationic transference number (tion) can be obtained from the formula of tion = (IT – IR)/IT where IT and IR is defined as total and residual current, respectively and it is decreased as the value of x increased or as the amount of alumina increased in the composites (Figure 8). From the author’s point of view, as the amount of MgCO3 increased in the composites, more ionic species of Mg2+ and CO32- ions are produced from the dissociation reaction of MgCO3 which could lead to agglomeration or forming new ionic cluster that could be able to block the transport of ions thus could lower the conductivity of the composites. Furthermore, instead of Mg2+ cations, CO32- anions also play a significant role in governing the total conductivity of the prepared composites as previously reported by Zainuddin et al. [42]. Moreover, the involvement of anions towards the conductivity of the solid electrolyte can be further acquainted when the group of Nicotera reported that TFSI- anions are more transportable than Li+ ion from self-diffusion coefficient studies [43].
Typically, several factors contribute to the ionic conductivity of the electrolyte such as ionic mobility, charge carrier concentrations and ionic charges in which the conductivity will be upsurge as the mobility of ions increases [44, 45]. From Figure 9, the ionic conductivity increased as the amount of alumina decreased or x values increased which give the highest conductivity value of 1.8 x 10-4 S.cm-2 when x = 0.9. The decrement in conductivity of the composites as the amount of MgO increased might be due to blocking of conducting pathway of the ions when ionic dissociation processes is more dominant process than ionic association process [46]. Furthermore, addition of more MgO in the composites leads to the agglomeration of the ionic species producing new ionic clusters that could retard the mobility of the ions, thus contribute to the decreasing conductivity [47, 48]. In order to enhance the conductivity if the solid composite electrolyte, the concentration of defect in the space charge layer formed between MgO and Al2O3 should be taken into account. Taking the ratio of x = 0.9 as an example with the highest conductivity among other ratios, from the author’s knowledge, the enhancement of conductivity followed the Maier’s space charge model due to the presence of high defect concentration in the space charge region at the interface as a consequence of surface interaction between two different phases of MgO and Al2O3. Based on our previous publication [49], due to slightly electronegative behavior of MgO naturally, strong chemical adsorption of ions namely Mg2+ take place at the surface of MgO forming MgO-Mg2+ species [49, 50]. This process is a reversible process and the formation of MgO-Mg2+ species might be also due to the lattice distortion associated with local asymmetric electric field at the surface [49]. Meanwhile, the filler (dispersoid) which act as insulating oxide or support is regarded as hydroxyl group which is referred as nucleophile. The cationic species namely Mg2+ on the surface of MgO from MgO-Mg2+ species, will be attracted to this hydroxyl group at the interface of MgO-Al2O3. As a consequence, the interface of MgO-Al2O3 will be enriched by positively charged layer of Mg2+ and a layer of cationic vacancies which describe the surface disordering lead to the enhancement of defect concentration at the interface of MgO-Al2O3 which further increase the movement of Mg2+ ion and the conductivity of the composite [51]. The ionic conductivity (σion) can be determined using the formula of σion = t/RbA where t is referred to the thickness of the pellet, Rb is bulk resistance (Ω) and A is the area of the pellet (cm2).
Figure 7. Chronoamperometry of (1-x)MgCO3– xAl2O3 composites with at 30 oC (J versus time (s)) with x = 0.9.
Figure 8. Transference number (tion) of (1-x)MgCO3– xAl2O3 at room temperature with x = 0.1 – 0.9
Figure 9. The ionic conductivity as a function of composition (x) for (1-x)MgCO3– xAl2O3 at room temperature with x = 0.1 – 0.9.
Furthermore, Figure 10 showed an example of Nyquist impedence plot at 30 oC for the determination of ionic conductivity (σion) of the material with x = 0.9. The Nyquist plots for other ratios show the same patterns. The impedence plot of composite which comprises a broad semicircle can be assigned as bulk properties of the material. The intercept of the semicircle at low frequency (right) with the real axis or Z’-axis give the information on the grain interior or bulk resistance (Rb) of the material [52]. Moreover, it is generally understood that the electrical resistivity will be decreased as the temperature increased. From Figure 11, the Rb values for example for x = 0.7, 0.8 and 0.9 ratio increased as the temperature increased up to certain temperature before going down as the temperature rises up. Other solid composite electrolytes have been reported to undergo the same phenomenon in lowering the electrical resistivity as the temperature increase due to increasing Joule heating, thus forming high dense pellet which could probably lead to higher conductivity [53, 54]. The temperature dependence of conductivity can be obtained by Arrhenius equation of σ = A exp (Ea/RT) where A is the pre-exponential factor, Ea is the activation energy (eV) while k is the Boltzmann’s constant with the value of 8.52 × 10-5 eV/K and T is the temperature (K). From Figure 12, the trends in conductivity for all ratios are the same where the conductivity started to decrease as the temperature more than 90 oC.
Figure 10. Complex impedance spectroscopy of (1-x)MgCO3– xAl2O3 composites with x = 0.9.
Figure 11. Rb values of (1-x)MgCO3– xAl2O3 composites versus temperature with x = 0.7, 0.8 and 0.9.
Figure 12. The ionic conductivity of (1-x)MgCO3– xAl2O3 composites versus inverse of temperature with x = 0.1 – 0.9.
To conclude, for the first time the composite solid electrolyte of (1-x)MgCO3– xAl2O3 with x = 0.1 – 0.9 by sol-gel method show the highest conductivity in order of 10-4 S.cm-1. The highest conductivity of the composites in the present studies account for several factors such as, increasing amorphousness of the materials as the amount of Al2O3 in the composites increased, better interfacial contact between MgCO3 and Al2O3 and formation of MgO-Mg2+ species which govern the charge carriers transfer.
The authors would like to thank the University of Malaya for granting the Research Grant (GPH005H2018) to support this work.
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