Synthesis and characterization of lisicon structured solid electrolytes for potential application in solid state electrochemical cells / Syed Bahari Ramadzan bin Syed Adnan

The aim of this study is to obtain LISICON structured Li4SiO4 based solid electrolyte with adequate conductivity for application in electrochemical cells. The parent and modified Li4SiO4 compounds were synthesized by sol gel method. The modified compounds were prepared by partial substitution using divalent ion (Li4-2xZnxSiO4), trivalent ion (Li4-3xCrxSiO4), tetravalent ion (Li4SnxSi1-xO4 and Li4ZrxSi1-xO4) and double partial substitution using trivalent and tetravalent ions (Li4-3xCrxZrySi1-yO4). The prepared samples were characterized using various techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscope, energy dispersive X-ray, laser particle analyser, differential scanning calorimetry, impedance spectroscopic, lithium transference number, linear sweep voltammetry and charge-discharge study. The XRD results showed that the Li4SiO4 system can be indexed to monoclinic structure in space group P21/m. The highest bulk, grain boundary and total conductivity of this compound at ambient temperature were 3.36 × 10-6, 1.58 × 10-6 and 1.51 × 10-6 S cm-1 respectively. The frequency dependence of conductivity followed Jonscher's universal power law, σac (ω) = σo + Aωs. The plot of pre-exponent, s versus temperature suggests that the conduction mechanism in the system can be described using correlated barrier hopping model. The increase in dielectric constant and dielectric loss and peak shift of tan to higher frequencies with temperature indicated that the increase in conductivity with temperature was due to the increase in number and hopping rate of charge carriers with temperature. The effects of partial substitution to this parent compound have also been investigated. The impedance spectroscopic analysis shows that the conductivity of the parent compound increases with different doping ions. The substitutions using divalent ions Zn2+, (Li3.88Zn0.06SiO4) showed increment of conductivity at ambient temperature. The value of bulk, grain boundary and total conductivity were 3.20 × 10-5 , 2.79 × 10-5 and 1.51 × 10-5 S cm-1 respectively. This effect was due to increase in number of vacant sites in the crystal lattice. Meanwhile, differential scanning calorimetry analysis showed increases in phase transition and melting temperature in this compound indicating the enhancement in thermal stability of the Li4SiO4 compound upon substitutions of Zn2+ to 2Li+ ions. The transference number corresponding to Li+ ion transport value was 0.82 which indicated that the majority of charge carrier in the compound was Li+ ions. The substitutions using Cr3+ into Li4SiO4 structure was confirmed by X-ray diffraction and Fourier transform infrared studies. This partial substitution also showed enhancement of conductivity at ambient temperature. The highest bulk, grain boundary and total conductivity were 7.93 × 10-5, 3.68 × 10-5 and 2.51 × 10-5 S cm-1 respectively for Li3.94Cr0.02SiO4 compound. The scanning electron microscope and particle size analysis showed that particle size decreases with Cr3+ ion doping. The average grain size decreases from 11.8 μm in Li4SiO4 sample to 0.59 μm in Li3.94Cr0.02SiO4 sample. Ionic transference number of Li+ ion determined by means of Bruce and Vincent technique was 0.79 for Li4SiO4 compound is increased to 0.95 for Li3.94Cr0.02SiO4 compound. Linear sweep voltammetry results showed that the doping by Cr3+ ion improved the limit of electrolyte decomposition from 2.73 V in Li4SiO4 to 4.51 V in Li3.94Cr0.02SiO4 versus Li/Li+ reference electrode. The partial substitutions on silicon ion by cation of larger size (Sn4+ and Zr4+) also enhanced the conductivity of Li4SiO4 compound by one order of magnitude. The highest bulk, grain boundary and total conductivity were 1.00 × 10-4, 4.42 × 10-5 and 3.07 × 10-5 S cm-1 respectively for Li4Sn0.02Si0.98O4 compound. Meanwhile, Li4Zr0.06Si0.94O4 compound showed maximum bulk, grain boundary and total conductivity values of 1.19 × 10-4, 4.75 × 10-5 and 3.41 × 10-5 S cm-1 respectively. The size of Sn4+ (0.069 nm) and Zr4+ (0.080 nm), are larger than that of Si4+ (0.041 nm) increased the size of Li+ migration channels which led to increase of ion mobility. The charge carrier concentration was found to be constant over the temperature range from 303 to 773 K while mobility of ion increased with temperature. This was due to increase in ion mobility. Linear sweep voltammetry results showed that Li4Sn0.02Si0.98O4 sample is electrochemically stable in the voltage range of -5.3 to 5.3 V versus Li/Li+ reference electrode. The average particle size for Li4Zr0.06Si0.98O4 was 1.198 μm, is smaller compared to the parent compound. Energy dispersive x-ray analysis also showed that the chemical compositions of the prepared Li4Zr0.06Si0.94O4 samples were very close to the desired compositions. The value of lithium transference number for Li4Zr0.06Si0.94O4 compound was 0.92. This value was higher compared to that of the parent compound. A more significant enhancement occurred on double partial substitutions on Li+ and Si4+ sites by Cr3+ and Zr4+. The DC conductivity at ambient temperature rose to 1.88 × 10-4 S cm-1 in Li3.94Cr0.02Zr0.06Si0.94O4 compound with a maximum value in the order of 10-3 S cm-1 at 500oC. Apart from created vacant sites by Cr3+ doping, the doping by Zr4+ was believed to enlarge the Li+ migration channels of the sample. The lithium transference number also increased to 0.97 in this double substituted compound. Li3.94Cr0.02Zr0.06Si0.94O4 has a maximum discharge capacity of 103 mA h g-1 at constant current of 5 mA g-1 (0.05 C) between 3.0 and 4.2 V. This indicated that this ceramic electrolyte can be used in lithium cells