Tailoring Cathode Nanostructures for Performance Improvement of Non-Aqueous Lithium-Oxygen Batteries

Abstract

Non-aqueous Li-O₂ battery has emerged as a promising energy storage technology. Unlike other intercalation-based methods, Li ions react with ambient oxygen directly, leading to a high specific energy density of up to 3600 W h kg⁻¹ (energy per the mass of Li₂O₂), which is several times higher than that of the state-of-the-art Li-ion batteries. However, the low round-trip efficiency and poor cycling stability of non-aqueous Li-O₂ batteries have hindered their commercial use. This thesis aims to develop novel cathode materials by tailoring their nanostructures to improve the performance of non-aqueous Li-O₂ batteries and gain an in-depth understanding of the battery electrochemistry. First, the relationship between the electrocatalyst structure and battery performance is explored through the investigation of NiCo₂O₄ (NCO) cathode materials as a model example. The {111} and {112} crystal planes exposed NCOs with identical morphology were developed via a hydrothermal method followed by calcination. The contribution of NCO nanostructures to their electrocatalytic activities was systematically evaluated through the investigation of crystal plane effects, surface areas and bulk compositions of cathodic NCOs. The {112} crystal planes are more active than {111} due to its availability to abundant dangling bonds and active octahedral Co³⁺ and Ni³⁺ sites. Ni³⁺ can improve oxygen evolution reaction (OER) activity and conductance, promoting the electrocatalytic performance of NCO. The NCO nanoplates with exposed {112} crystal planes, high surface areas and good conductance are identified as an ideal cathode material. Next, macroporous nanocomposites of reduced graphene oxide aerogels (GA) and NCO nanoplates were developed via a one-pot self-assembly approach and used as freestanding cathodes. In these cathodes, NCO nanoplates can fully cover the walls of the macropores, which provides active sites toward OER and protects GA support from corrosion. Moreover, the macroporous GA support can facilitate the mass and electron transportation as well as act as accommodation sites for Li₂O₂ discharge product. After systematic optimization, the nanocomposite cathode with a GA: NCO weight ratio of 1: 4 displays superior battery performance due to its optimal conductance and NCO coverage on the walls of macropores. Last, freestanding macroporous NCO@carbon nanotubes (CNT) cathodes were fabricated through a vacuum filtration-assisted self-assembly method followed by template removal to generate macropores. The surfaces of NCO@CNT are found to promote the formation of amorphous Li₂O₂ with improved conductivity. As a result, the layer can grow up to 50 nm before the cathode is fully passivated, which boosts Li₂O₂ production and discharge capacity of the batteries. During charge, the improved conductivity of amorphous Li₂O₂ promotes OER kinetics, leading to excellent cycling performance. The amorphous Li₂O₂ layer can be a viable alternative to crystalline Li₂O₂ toroid as the discharge product of high-performance Li-O₂ batteries. Through systematic investigation of the structures of electrocatalysts and their supports on the electrocatalytic activities and discharge product properties, the correlations between cathode nanostructures and the battery performance have been established. These results provide insights into the underlying mechanisms of the electrochemistry and pave paths for the future rational designs of novel cathode materials in high-performance non-aqueous Li-O₂ batteries.