Study of alkali metal oxides doped magnesium vanadate glasses and glass ceramics

Abstract

The present thesis is related to the study of (75) V2O5-(25-x) MgO-xA2O (A= Li, Na and K) (x =0-12) systems. These samples are synthesized by the quench technique followed by heat treatments to the selected samples. The physical, structural, thermal, and conducting properties of as prepared samples are studied using various experimental and testing techniques to determine their suitability as cathode material in battery and electrochemical fuel cells. The research work is presented in different chapters. Chapter 1: This chapter describes the background of renewable and nonrenewable energy sources. Different energy sources are being explored that can meet the demand without harming the environment. Fuel cell and batteries could be a new sustainable and renewable source of energy. The battery has shown great potential to become an efficient and reliable energy source associated with its high efficiency, simple processing and environment friendly nature. The role of different components of the battery has also been discussed in this chapter. Since the present work is related to the development of cathode materials, therefore, the main focus has been given to the particularly cathode as described in context to specific properties such as thermal stability at battery operating temperature in oxidizing and reducing medium and their electrical conductivity. Chapter 2: This chapter deals with the literature review on vanadium based glasses/glass ceramics/ceramics as cathode material for batteries, fuel cells and supercapacitors. The physical, structural, optical, thermal and conducting properties of the cathode materials are discussed in light of various processing parameters, dopants and their concentration. Based on the literature review the motivation of the present study along with the objectives of the present research is also given in the last of this chapter. xviii Chapter 3: This chapter gives details about the source of raw materials synthesis parameters and the experimental method employed for sample preparation and characterization to achieve the proposed research objectives. The technical details of the experiment are also given. It also explains the experimental conditions in which structural, optical, thermal and conducting analyses have been done. For phase determination and structural analysis XRD, FTIR, Raman spectroscopy was used. The thermal properties of as prepared samples were investigated using DTA and dilatometer. Microstructure and chemical analysis were done by FESEM and EDS analysis. The conductivity measurement was done on the impedance analyzer with AC biasing ±1V. Chapter 4: This chapter is related to the results and discussion of the prepared sample as well as heat treated samples. In this chapter, interpretations of data obtained from various characterizations have been discussed. Two composition of (75)V2O5-(25)MgO (MV-25) and (60)V2O5-(40)MgO (MV-40) was prepared using the melt-quench technique. Differential thermal analysis (DTA) is applied to calculate the activation energy of crystallization along with other parameters using two different theoretical models. Higher activation energy is observed for partially crystallized MV-40 glass. A negative thermal expansion coefficient (NTEC) is observed in both glasses. However, MV-25 glass shows three distinct regions of NETC and these regions diminish as MgO content increases in the composition. These glasses may have applications in photonics where NTEC is required. The present finding would help design glasses and glass ceramics as cathodes for battery applications. Based on the findings MV-25 glasses were further investigated with alkali different dopants. Thus, the composition of 75V2O5-(25-x)MgO-(x)Li2O (x= 0, 1.5, 3.0, 4.5, 6.0, 9.0, 12) is synthesized by the melt quench method. Effect of Li2O on devitrification physical, thermal, xix structural, and conducting properties of as-quenched samples are analyzed utilizing various experimental techniques. X-ray diffraction and DTA confirmed the formation of phase-separated glasses up to 3.0 mol% of Li2O. Above this concentration of Li2O, the samples are glass ceramic. With the increase in the concentration of Li2O, the density increases in all the samples. Raman spectra demonstrate that as the concentration of Li2O increases, there is a transition from VO5 units into different structural units such as VO4, VO3, and VO2 of vanadium oxide. The highest conductivity of x = 4.5 is observed i.e. 10-4 S/cm at 250ºC. The activation energy indicated that the present samples could be mixed conductors in nature. These samples may be used as cathode materials in energy storage devices due to their mixed conduction with an appropriate conductivity at 250ºC. The composition of 75V2O5-(25-x) MgO-(x)Na2O with x=0, 1.5, 3.0, 4.5, 6.0, 9.0, and 12 are synthesized by melt quench technique. The prepared samples were characterized by the XRD method. The addition of Na2O converted glass into glass ceramic/ceramics. The density shows no trend with Na2O content in the glass compositions. DTA confirmed the glassy nature of the x=0 sample. For all samples, the conductivity variation as a function of the temperature follows an Arrhenius relationship. The highest conductivity is found for the x=12 sample i.e. 10-2 S/cm at 250°C with better thermal stability. The developed samples can find applications as cathode materials in Mg or Na-based batteries due to good conductivity with better thermal and structural stability. Glasses are synthesized by melt quench technique using the composition of 75V2O5-(25-x) MgO-(x)K2O with (x= 0, 6.0, 9.0, 12, and 15). Synthesized samples were characterized by the XRD method. The concentration of K2O leads to phases separated glass formation as confirmed by differential scanning calorimeter (DSC) and FESEM. Broad bands in Raman also confirm the xx glass formation. For all the compositions the conductivity variation as the function of the temperature follows the Arrhenius equation. Further activation energy of conduction is also calculated using an Arrhenius relationship. The conductivity of phase-separated glasses increases with temperature. It was around 10-3 Scm-1 at 250°C which is in the required range for cathode material. Based on the results, it can be concluded that these materials are appropriate for batteries and electrochemical fuel cells, thermo electrical materials, switching devices, sensors, etc. Further three samples viz. MVL-12, MVN-12, and MVK-12 are heat treated and investigated for conductivity and electrochemical properties. Heat treated MVL-12 and MVN-12 samples exhibit excellent results and could be used as supercapacitors. Chapter 5: The summary of the results is given in this chapter. The best conductivity is observed for the MVN-12 sample at 250°C~ 5.31×10-2 Scm-1. Sample MVN-12 shows excellent specific capacitance Cs ~ 469.26 (F/g) at a current density of 1 A/g. However, MVL-12 heat treated at 500 °C shows high capacitance retention of about 128.9% with > 140% columbic efficiency even after 2000 cycles. At the end of this chapter, a future scope of the current study is also provided.