TRANSPORT AND STORAGE PROPERTIES OF SODIUM SILICATES CATHODE FOR SODIUM-ION BATTERIES

Abstract

Currently, lithium-ion batteries (LIBs) are the most popular power sources for portable electronic devices because of their high voltage, high energy density, and long cycle life. However, the limitation of lithium supplies is a major challenge to fulfill the demands of large-scale applications. Since the need for energy storage is expanding quickly, therefore battery technologies must be diversified to meet the demands of various applications. In this regard, sodium-ion battery (SIB) is considered to be the most viable alternative to lithium-ion battery because the raw materials required are abundant and low cost which in turn will provide the energy storage to a large scale at a low price. The key success in the development of advanced SIBs for large-scale storage is the cathode material in particular (being a cathode-limited cell). It is thus necessary to develop and employ cost-effective and environmentally benign cathode materials of high capacity, high energy density, long cycling life, and excellent rate capability. Although, several materials have been explored in the past, but continuous research and development of different cathode materials are still needed. A promising class of cathode materials for SIBs is polyanionic-based compounds, which have already demonstrated their viability in lithium-ion batteries. These compounds exhibit high thermal stabilities, improved oxidative stability at high voltages and flat voltage response. In particular, the ortho-silicate (Na2MSiO4, M = Co, Mn, Fe, etc.) received considerable research attention due to their high reversible capacity using exchange of two electrons during redox activity. Among these silicates, Na2(Fe/Mn)SiO4 attracts most of the attention due to the non-toxicity and natural abundance of Fe and Mn. Despite the merits mentioned above, polyanion-based cathode materials exhibit poor electronic conductivity in particular. To deal with this problem, in the past, various strategies have been attempted to improve the electrochemical performance of these electrode compounds, including nano sizing, carbon coating, and other cation doping. However, no one has yet reported the intrinsic Na-ion transport behavior of different crystal structure frameworks of these materials and their correlation with the Na-storage properties. Further, Sodium manganese silicate has not been prepared in pure phase too. This thesis mainly focuses on the preparation of Na2(Fe/Mn)SiO4 and characterization and then investigates the transport properties and their correlation with the electrochemical behaviour as cathode material for SIBs. Furthermore, the interfacial Na-ion kinetics has also been investigated in this thesis. Monoclinic (Pn)-Na2MnSiO4 is successfully synthesized using quick (10 minutes), simple, and economical combustion technique for the first time. Precursor amounts of Na, Mn, and Si are optimized to get the elemental stoichiometry of the final product. A study of the transport behavior of NMS compounds is also conducted to determine correlations between the structural and morphological properties and the electrical properties. The results surprisingly show a high value of electrical conductivity of 9.6 (±0.01) × 10-7 Scm-1 as compared with its Li counterpart, i.e., 4.0 × 10-8 Scm-1. Combustion-derived monoclinic-NMS electrode shows an initial charge capacity of 148 mAh g−1 within the potential window of 1.5-4.5V at a current rate of 0.1C (13.7 mAg−1). An irreversible capacity loss (ICL) of about 92% occurred during the initial cycle of the electrode material, leading to structural instability or amorphization of NMS sample, which is primarily caused by Jahn-Teller distortion and manganese solubility. On the other hand, Na2FeSiO4 (NFS) is synthesized using the combustion technique for the first time. A suitable fuel and precursor amount is optimized to achieve the final product's elemental stoichiometry. A correlation between the structural and morphological properties of orthorhombic-NFS compounds and their electrical properties was studied. The electrical conductivity of orthorhombic-NFS is found to be 4.5 (±0.01) × 10−8 Scm−1 at room temperature, and the activation energy of the sample is calculated to be 2.64 eV. Orthorhombic- NFS electrode shows an initial charge capacity of 168(±5) mAhg-1, while a discharge capacity of 115(±5) mAhg-1, showing irreversible capacity loss (ICL) of ~53 mAhg-1 (about 31.5% loss of capacity) in the first cycle. Further, the PVA-assisted synthesis technique has also been utilized to develop the monoclinic (Pn)-NFS for the first time with improved morphological characteristics (such as porosity and nanosizing, etc.). A correlation between the structural and morphological characteristics of monoclinic-NFS compounds and their electrical properties is studied. The as-synthesized monoclinic NFS exhibits an impressive electrical conductivity of 2(±0.01) × 10−6 Scm−1, which is found to be two orders higher than the combustion-derived orthorhombic-NFS, i.e., 4.5(±0.01) × 10−8 Scm−1. Furthermore, the activation energy obtained for the monoclinic-NFS, i.e., 1.57 eV, is comparatively lower than the orthorhombic-NFS, i.e., 2.64 eV. The structural arrangement with the existence of a larger volume of pores/voids and nanosizing particles causes improved transport behaviour in PVA-assisted NFS. The initial charge capacity is found to be 225 mAhg-1 at a current rate of 0.1C (13.5 mAg-1) in the voltage range of 1.5-4.2V, which is closer to the theoretical capacity (~276 mAhg-1) of NFS. Better conducting properties along with a larger volume of pores/voids in the NFS sample, appear to be improved for extracting ~1.63 moles of Na ions from the host NFS. But, the cyclability of monoclinic-NFS was not found to be encouraging even if it had all the beneficial structural and morphological features.