AEROSOL SYNTHESIS OF CATHODE MATERIALS FOR NA-ION AND LI-ION BATTERIES
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Energy production and storage are important issues that play a key role in our daily lives. There is a need for high energy and high power systems for portable electronic devices and zero-emission vehicles. Lithium-ion batteries are crucial in addressing these needs. However, for the smart electric grid and renewable energy storage where cost is critical but weight and footprint requirement is less important, the sodium-ion battery is the most suitable power sources. To achieve both high power density and high energy density, nanostructured sphere particles with controlled porosity and high tapping density are desired for both Li-ion and Na-ion batteries. The versatile and facile ultrasonic spray pyrolysis method allows for the synthesis of a variety of electrode materials with sphere morphology. Work has been done to develop electrode materials through an aerosol method that can be readily applied to industry. Two classes of high energy cathodes suitable for lithium-ion batteries were studied. These include the 5V spinels and lithium-rich materials. The 5V spinels are a promising class of electrodes for secondary lithium batteries. This class of material has the highest intrinsic rate capability of the intercalation cathodes with high safety, low toxicity, and low cost making it ideal for high-power applications such as electric vehicles, while the lithium-rich compounds exhibit high capacity and reasonable cycle stability. Two classes of stable cathodes suitable for sodium-ion batteries were studied. The first was carbon coated porous hollow Na2FePO4F spheres with 500 nm diameter and 80 nm wall thickness synthesized by a one-step template-free ultrasonic spray pyrolysis process using sucrose as the carbon source. Nano-sized porous hollow Na2FePO4F spheres allow electrolyte to penetrate into the hollow structure, and thus the electrochemical reaction can take place on both the outside and inside surface and in the pores. Also, the carbon coating on Na2FePO4F hollow spheres enhances the electronic conductivity and charge transfer reaction kinetics. The exceptional performance of hollow Na2FePO4F spheres combined with mature aerosol spray synthesis technology make these carbon coated porous hollow Na2FePO4F spheres very promising as cathode materials for practical applications in Na-ion batteries. Finally, P2-type earth abundant layered oxides with high energy density and long cycling stability were also developed and studied. These layered materials were investigated due to their high theoretical capacity. A novel ultrasonic spray pyrolysis system has been developed to effectively coat any cathode, including layered oxides, with a thin layer of carbon to improve the kinetics and increase the electronic conductivity. The residence time in air is sufficiently short to allow the decomposition of the carbon source (sucrose) without further reduction of the cathode material. A vertical configuration allows the solid particles to reach the filter for collection with high efficiency. As a test sample, lithium-rich cathodes have been successfully carbon coated and compared with the bare material.