Synthesis and characterization of Li2O/K2O/CuS modified sodium-phosphate glasses

Abstract

Lots of research has been carried out to develop oxide/sulfide glasses as solid electrolytes for batteries applications. The ionic conductivity of the glasses is generally higher than that of the corresponding crystalline counter part because of their open structure. Sulfide based glasses show an increase in ionic conductivity more than three orders of magnitude as compared to the oxide glasses of the similar composition. However, sulfide based glasses are hygroscopic in nature. So, reaction with environmental moisture leads to generation of H2S gas. Addition of oxides in the sulfide glasses can increase their chemical stability dramatically with marginal decrease in ionic conductivity. Mixed anions effect in either oxide or in sulfide based glasses, enhances the ionic conductivity and thermal stability simultaneously. Based on the literature review, different series of glasses were synthesized and studied in detail. Moreover, for binary sodium-phosphate glass-ceramics, mechanical properties and their relationship with structural changes are discussed. The effect of modifiers (Li2O/K2O/CuS) on thermal stability and on crystallization kinetics in the sodium-phosphate glasses is also presented in this thesis. The entire work of the thesis is presented in five chapters as follows: First chapter deals with the general introduction of the glasses and their synthesis by different routes. Different theories for glass formation and different class of fast ion-conducting glasses as solid electrolytes for batteries applications are also described. Mixed anions effect on stability and on overall conductivity is discussed in detail. Second chapter is related to the literature review of sulfide and oxide glasses, their synthesis and properties. Synthesis and characterization of oxy-sulfide based glasses for better stability in air are discussed. Mixed modifiers effect on thermal stability and crystallization are also described in this chapter. xiii Increase in ionic conductivity because of sulfur ion is also discussed in this chapter. Based on some gaps in the study, the aims and objectives of the present research work have been planned. Third chapter describes about the experimental procedure in the present work. Detailed procedure of synthesis of glasses/glass-ceramics and their characterizations is given in this chapter. Different characterization techniques such as X-ray diffraction (XRD), Differential thermal analyzer (DTA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Dilatometric measurements, Scanning electron microscopy (SEM), Vickers micro-hardness and Impedance spectroscopy are also discussed. Fourth chapter deals with the results and discussion part of the present work. This chapter is divided into five sections; the results of oxysulfide glasses are discussed in the first section. The H2S generation and stability of Na2S–P2S5 glasses after exposure to air at different time intervals are investigated. With the help of Raman spectroscopy and FTIR, structural changes in glasses are discussed detail. Two stable samples of the compositions 45Na2S–55P2S5 and 55Na2S– 45P2S5 (mol %) are selected for further study. Crystallization kinetics of these glasses and their ionic conductivity to check their applicability for solid electrolyte are described in detail in this section. Second section describes the effect of CuS on 55Na2S–45P2S5, particularly when exposed to air. Best composition 55Na2S–45P2S5 was selected from above study for further investigation. The addition of CuS increases the overall conductivity of the system and durability of glasses. CuS is varied from 5 to 10 mol% on the expense of Na2S and investigated their structural, thermal and electrical properties. The effect of Na2O on structural and mechanical properties in the Na2O-P2O5 glass-ceramic system is given in the third section. In the present study, longitudinal and shear velocities of the xiv prepared glass–ceramics are investigated using ultrasonic echo pulse system. These parameters are used to calculate the mechanical properties of the glass–ceramics. The structural changes in glass–ceramics have been observed with the help of FTIR spectroscopy. The obtained mechanical properties and their relationship with structural changes are also discussed in this section. In the fourth section, the effect of mixed modifiers on crystallization and thermal stability has been described. Based on the results, the most stable glass having 15 mol% Li2O has been selected for further study. In the last section, effect on crystallization and thermal stability of the composition 55P2O5-(45- x)Na2O-xK2O; (5 ≤ x ≤ 25) has been given. Stable samples having 10 and 20 mol% K2O samples were selected for further conductivity study to check their suitability as solid electrolyte. The conclusion and future scope of the present work is given in the fifth chapter. The present work shows that the sodium-phosphate glasses of compositions 45Na2S–55P2S5 and 55Na2S– 45P2S5 are more stable in air. Addition of CuS in the 55Na2S–45P2S5 compositions increases the thermal stability and overall conductivity of the glasses, which may also be used as cathode materials in battery applications. The maximum hardness is observed for 55 mol% Na2O glass– ceramic sample. The SEM study revealed that interlocked crystals dispersed in a glassy matrix and increases the density. In mixed modifier glasses, 15 mol % Li2O, 10 and 20 mol % K2O glasses shows higher thermodynamic and kinetic stability. These glasses can be used as solid electrolyte for battery application.