STRUCTURAL, MORPHOLOGICAL, AND ELECTROCHEMICAL STUDIES OF LAYERED CATHODE MATERIALS FOR NA-ION BATTERIES

Abstract

Batteries are among the essential technologies required to enable the world to move beyond fossil fuels towards a more efficient and environmentally friendly society based on electricity from renewable sources. Unfortunately, the rapidly increasing number and size of batteries that the world needs to perform this paradigm shift is putting an enormous strain on the supply of traditional raw materials for batteries, such as lithium and cobalt. Batteries built using only earth-abundant elements could guarantee that the supply of energy storage will be available to everyone at reasonable prices. Sodium-ion batteries (SIBs) are among the best possible alternative to achieve battery systems that can provide performance close to or on par with lithium-ion batteries at a lower cost and environmental impact. Although sodium-ion and lithium-ion batteries share many properties, there is much research required before sodium-ion batteries can compete with highly optimized lithium-ion batteries. Cathode materials play a significant role in improving the energy density of the SIBs, and there have been numerous cathode materials reported in broad range, mainly transition metals oxides, polyanions and Prussian blue analog. Among them, sodium transition metal oxide such as NaxMO2 (where 0 ≤ x ≤ 1; M= Fe, Cr, Co, Mn, etc.) have enticed much attention as sodium intercalation cathodes. Hnce, the research wirk reported in this thesis mainly focuses on the synthesis, characterization, and investigation of layered transition metal oxide cathodes for SIBs, as well as their applications in practical electrochemical cells to achieve high performance, environmental friendly, low-cost, safe cathode for SIBs. The research conducted within the thesis framework demonstrates that sodium transition metal oxides are an attractive contender to implement them as cathode materials in SIBs. The optimization and designing of the proposed materials' structural properties, combined with the investigation of temperature-dependent electrochemical studies, led to the identification of highly stable cathode materials with superior electrochemical performance. The investigated material can replace the commonly employed lead acid batteries, and LIBs. These can be defined as lower cost, extended cycle life, safer energy storage technologies for large-scale stationary applications. The results of the current research have been divided into seven chapters with the following chapter-by-chapter brief details as; Chapter 1 contains an overview and literature review to the battery technology with an emphasis on different types of developed cathode materials for rechargeable sodium ion batteries. Among those, layered types materials has been identified as the potential cathode material for the further investigation in this study. Chapter 2 describe the synthesis and characterization techniques used for the synthesis of the pristine NaFeO2, NaCrO2, C2H2 treated NaFeO2 and NaCrO2, AlPO4 coated NaCrO2 and Ni doped NaCrO2 samples. Synthesis of the pure phase sampleshas been attempted via solid-state route. This chapter includes details of experimental conditions and parameters used for different physicochemical characterization techniques such as XRD, SEM, TEM, FTIR, TGA, and AC/DC conductivity. This chapter also includes the details regarding electrode preparation and coin cell fabrication along with parameters used for electrochemical characterization such as EIS, CV, and GCD. Chapter 3 includes a comprehensive physicochemical and electrochemical analysis of pristine NaFeO2, NaCrO2 samples and C2H2 exposed samples. It has been found that among all the synthesized samples, the NaCrO2 sample with an exposure of C2H2 for 10 minutes (NC10) shows the most enhanced electrochemical properties. NC10 delivers a high discharge capacity of 126.5 (±5) mAhg-1 at 0.5C and it retain capacity of about 89% after 40 cycles at 1C rate. The low polarization and highest power density of 16,172 WKg-1 for 8 seconds of discharge time has been noticed for NC 10 sample. Ex-situ SEM micrographs of dismantled cells has depicted the reduction in formation of dendrites and more uniform distribution for C2H2 core-shell type coated sample NC10. Chapter 4 deals with the effects on physicochemical and electrochemical properties of AlPO4 coating on the surface of NaCrO2 sample. Electrical conductivity measurements showed that AlPO4 coated NaCrO2 has lesser resistivity, hence-forth, more conductive in nature. Electrochemical analysis such as CV and GCD of pristine NaCrO2 and AlPO4 coated NaCrO2 samples has revealed that the presence of AlPO4 results in a decrease in polarization and, consequently, improvement in sodium-ion kinetics. Ragone plot shows AlPO4 coated NaCrO2 shows high power density of ~ 3308 Wkg-1 . Moreover, the EX-situ SEM images of post-mortem coin cells after 100 cycles have shown retention of structure as before cycling, and no feature of dendrite formation in the AlPO4 coated NaCrO2. Chapter 5 deals with the Ni-doped NaCrO2 as cathode material. In order to understand the limitation of the as-prepared samples, physiochemical, dielectric and electrochemical properties were studied. XRD reveals that the synthesized materials with proper phase with minor impurity. Increasing frequency results in the increase in the dielectric properties such as dielectric constant and dielectric loss, as an usual behaviour of dielectrics. Conductivity measurements with respect to the changing temperature indicate the NTCR property exhibited by Ni-doped NaCrO2. The frequency exponent (S parameter) shows the NCO sample evident large polaron tunneling, whereas the NCN sample follows the correlated barrier hopping model. Electrochemical studies indicate that Ni doping improves the material's conductivity, resulting in improved stability of the cathode material and higher capacity. Chapter 6 deals with further investigation of variable amount of Ni doping in NaCrO2 as NaCr1-xNixO2; 0.1 ≤ x ≤ 0.3. The temperature dependent (low to high) physiochemical and electrochemical studies were carried out for all the Ni doped composition. Impedance measurements of the bulk material in temperatures ranging from -150 0C to 150 0C reveal the bulk conductivity's direct dependency on the decreasing temperature. Temperature-dependent GCD curve shows that the minimum dependency of the electrochemical behavior on the temperature ranges between 25 0C to 100 0C. However, all the samples performed best at room temperature. The Galvanostatic Intermittent Titration Technique (GITT) analysis reveals the sodium-ion insertion, and extraction from the cathode material during the charge-discharge cycle as a function of temperature. Ni-doping results in the reduction of lattice parameters, thus contraction of the c-lattice results in the increase of sodium migration barriers, consequentially reducing the sodium diffusion coefficient. Chapter 7 contains the conclusions of the results obtained in the present research work. The results related to the electrochemical performance of the optimized samples at various temperature are outlined. This section also includes the outline of the future scope of the present investigation.