Innovative three-dimensional approaches to sodium-ion batteries

Energy is becoming an increasingly important issue in recent centuries, when fossil fuels are rapidly depleting. In addition, with the increasing environmental pollution caused by fossil fuel consumption, the demand for efficient use of energy and the search for renewable energy sources that can be used instead of fossil fuels is increasing. There are many energy sources such as wind, sun, wave, but all of these sources are intermittent. For this reason, energy storage systems are needed to benefit from these energy resources in the best and uninterrupted way. Lithium-ion batteries are among the most commonly used secondary batteries in large-scale energy storage due to their features such as long-cycle stability and high energy efficiency. However, lithium resources are depleting and this requires finding alternative materials. Sodium has properties similar to lithium, such as a low negative redox potential, small electrochemical equivalent, and a specific energy density only approximately 17% lower than lithium-ion batteries. For this reason, it can be shown as an alternative to lithium. In addition, one of its biggest advantages is that it is much more abundant than lithium on earth. However, it has disadvantages such as having a larger ionic radius and molar mass than lithium. These properties cause slower Na+ diffusion, larger volume expansion and structure deterioration during the electrochemical process. The result of this situation is low cycle and speed performance and low capacity. These features greatly limit the development of sodium ion batteries, and therefore it is very important to develop high-performance electrodes with improved Na+ diffusion, long cycle numbers and speed capacity. In recent years, focus has been placed on sodium superionic conductor (NASICON), which has a covalent 3D structure that creates large gaps where Na+ can diffuse as the cathode. These structures, which have a 3-dimensional structure, create a larger interstitial space for sodium ion movement and improve the electrochemical properties and structural stability of the material. Among cathode materials with NASICON structure, Na3V2(PO4)3 is promising as it has better thermal stability, longer cycle life, and high performance thanks to its stable 3D main frame. As anode, metal oxides such as CuO and SnO2 attract attention. Although they are promising electrode materials, they have a significant disadvantage such as low electrical conductivity and therefore cannot fully reach the theoretical capacity even at low speeds. There are various methods used to overcome this problem. Using secondary phases such as graphene is one of these methods. Graphene is a carbon allotrope and is used in energy storage/conversion systems due to its high surface area and very high electronic, optical, thermal and mechanical properties. However, it also has features that limit its use, such as having limited electron transport channels and fewer active sites. Additionally, these 2-dimensional graphene layers generally tend to restack during the drying process due to van der Walls bonds between the layers. As a result, the accessible surface area decreases and causes insufficient electron and ion transport, which limits the high-speed performance of the electrodes. Therefore, lightweight, high surface area and 3D porous layers can be produced from graphene sheets. These 3D graphene aerogels with very low density and high porosity not only prevent sheets from restacking but also preserve the properties of graphene sheets such as high conductivity and large specific surface area. These properties enable graphene aerogel to have a wider range of uses compared to graphene, but their limited electron transport capabilities and weak surface polarities cause low electrochemical performance. This problem can be solved by preparing graphene aerogel-based composites. In this study, firstly, CuO and SnO2 metal oxides were produced as anode and Na3V2(PO4)3 as cathode. CuO nanostructures were produced using the precipitation method, SnO2 nanostructures in aqueous solution development method and Na3V2(PO4)3 solvent evaporation method. These structures were later used to produce doped graphene aerogel with graphene oxide. L-ascorbic acid was used to reduce graphene oxide. After the mixing and reduction process was completed, doped hydrogel structures were obtained by heating in a closed container. Then, doped aerogel structures were obtained using the freeze-drying method. The produced materials were mixed separately with the conductive additive, binder and solvent to obtain a mud form, and the resulting mud was plastered homogeneously on the foil. The mud dried in a vacuum oven was cut into appropriate dimensions for the half cell and the electrode was made ready. Finally, the resulting electrode, separator and electrolyte were combined in a glove box to obtain half cells. Characterization of the crystal structures of the produced anode and cathode structures was carried out using X-ray diffraction (XRD). Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques were used for the characterization of the samples. Field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) were used to examine the morphologies and chemical composition of the structures. Transmission electron microscopy (TEM) was used to analyze the microstructure in the material in more detail. Thermal analysis (TGA) was applied to analyze the thermal properties of the powders and the amount of carbon in the structure. To test the performance of the resulting half cells, cyclic voltammetry tests (CV), electrochemical cycling performance test, electro-chemical impedance spectroscopy (EIS), rate capacitance test were applied. The structural characterization analyzes show that CuO, GA-CuO, SnO2, GA-SnO2, Na3V2(PO4)3, GA-Na3V2(PO4)3 structures were successfully synthesized. According to the electrochemical analysis results of half-cell tests, doped electrodes increase the wettability of the electrode within the structure, thanks to the porous and three-dimensional structure of garaphene aerogel, and allow easier diffusion of sodium ion, thus achieving better cycle stability at increasing cycle numbers and higher capacity at increasing speeds. It is observed that it creates a more advantageous electrode material for batteries. To examine the post-cycle changes in the electrodes, XRD, FE-SEM and Raman analyzes were performed at the end of the 50th, 100th and 250th cycles. Post-cycle analysis shows that the structure is slightly degraded, and as the number of cycles increases, this deterioration increases in small amounts. This increase can be attributed to the fact that sodium ions deteriorate the structure slightly as a result of repeated intercalation and deintercalation, increasing the defects in the structure and increasing the distance between graphene layers. The reason for the small amount of deterioration in the structure is that the porous structure of graphene aerogel facilitates the movement of sodium ions.