Creating a Two-Dimensional Heterointerface in Layered Oxide Electrodes for Advanced Electrochemical Energy Storage
Authors:   , Andris Ryan , Pomerantseva Ekaterina 
Published: November 2023

Secondary batteries are an important area of research to help create grid-scale energy storage solutions, improve the performance of small electronic devices, and expand electric transportation. Specifically, lithium-ion batteries are the dominant form of rechargeable energy storage due to its high energy density, high power density, and long-term stability over many cycles. A battery with high energy density allows for more compact devices with an extended battery life. In addition, high-power densities can lead to faster charging times as well as more efficient and reliable power delivery to high-performance machines such as laptops and electric vehicles. Therefore, cost effective and efficient energy storage devices are fundamental for founding more sustainable communities. Since the advent of Li-ion battery commercialization, layered oxide materials have dominated as intercalation type cathode materials. However, these common cathodes have limited capacities, low electronic conductivity, and relatively dense crystal structures that can negatively impact ionic diffusion and the general rate performance of the cell. Therefore, next-generation cathode materials require transition metals with high oxidation states that can undergo multiple reduction steps, improved electronic conductivity, and open ion diffusion channels. Various transition metal oxides have been studied extensively in these electrochemical systems due to their high theoretical capacity, low toxicity, and high natural abundance. Vanadium oxide is of particular interest due to vanadium's high redox activity in reversible charge storage reactions and its wide range of morphologies and structures. However, metal oxides are limited by their inherent low conductivity that can negatively impact rate performance and long-term stability. Therefore, this dissertation generates new knowledge needed to synthesize stacked 2D heterostructures combining bilayered [delta]-V₂O₅·nH₂O and conductive carbon-based materials using the 'bottom-up', 'hybrid', and 'top-down' approaches to enhance bilayered vanadium oxide's (BVO) electrochemical performance in energy storage systems. The 'bottom-up' approach uses small organic molecules to intercalate into the interlayer region of BVO that are subsequently carbonized using hydrothermal treatment. The 'hybrid' strategy integrates small organic molecules into a BVO xerogel using a post sol-gel diffusion process. Again, these molecules are subsequently carbonized using heat treatment processing. Finally, stacked 2D heterostructures are synthesized using 'top-down' cation-driven assembly of exfoliated BVO and graphene oxide nanoflakes. The combined phases with increased electronic conductivity can lead to improved charge storage capability, faster charging, and longer lifetimes. Relationships between the heterostructure synthesis, the final structure, properties, and the electrochemical performance of each material are examined to use this knowledge to create improved electrodes for energy storage devices. The 'bottom-up' chemical preintercalation of dopamine hydrochloride into BVO was the first report to demonstrate a 2D oxide-carbon heterointerface using these materials. While carbon layers were only found intermittently among the vanadium bilayers, this material exhibited higher electronic conductivity and improved capacity retention, rate performance, and charge-transfer resistance when tested in Li-ion batteries compared to the reference material. The 'hybrid' diffusion synthesis method controllably intercalated small organic molecules into BVO resulting in a very repeatable heterostructure synthesis procedure with promising electrochemical performance in Li-ion cells. Next, the 'top-down' cation-driven assembly created stacked 2D heterostructures of LVO and reduced graphene oxide nanoflakes (rGO). These heterostructures demonstrated superior electrochemical stability, attributed to improved structural stability originating from bonds formed between rGO and LVO nanoflakes that preserved lamellar order of the layers in the LVO structure and a rGO encapsulation effect preventing significant dissolution of LVO nanoflakes in the electrolyte. Further, the improved electron transport of the heterostructures with enhanced rGO content was supported by both the rate capability study and decreased charge transfer resistance. Finally, we found that the 'top-down' cation-driven heterostructure assembly approach can be used to define the interlayer spacing of the bilayered vanadium oxide phase by changing the nature of the assembling cation. In turn, the CV curves and galvanostatic cycling highlight the benefit of using an active material with a large interlayer spacing for improved initial capacities. Moreover, the cations used to assemble the heterostructures can define intercalation sites for charge carrying ions and improve ion diffusion kinetics when the assembling cation and charge carrying ion are identical.