Sodium layered-oxide cathodes for lithium-free and cobalt-free batteries
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Sodium-ion batteries (SIBs) are gaining attention as alternatives to lithium-ion batteries (LIBs), particularly in the field of building-scale and grid-scale energy storage systems. The natural abundance and affordability of sodium relative to lithium makes the SIB a good contender for large-scale storage applications where battery cost is a more critical parameter than battery size and weight. At the cathode side of the battery, sodium layered-oxide materials are of great interest due to their similarities with the standard lithium layered oxide, and their high theoretical capacity of 240 mA h g ⁻¹. However, in practice, a high-energy sodium-ion cell is difficult to achieve, and their poor surface stability leads to rapid degradation in contact with both air and electrolyte. This dissertation begins with a comprehensive study to evaluate the various degradation routes of a sodium layered oxide. This evaluation is used to guide the future modification projects which aim to stabilize the material. A sodium phosphate coating applied post-calcination is found to greatly stabilize the cathode surface in both air and during cycling. The coating acts as a stable, artificial surface layer during cycling and protects the material from degradation on exposure to air due to moisture and CO₂. An alternative approach to surface stabilization is examined with the molten-salt synthesis method. In the molten salt, the particles preferentially grow parallel to the sodium diffusion channels, forming a micron-scale plate-like morphology. The edge planes, where reactivity and degradation are most severe, are highly narrow. The small surface area of the edge planes helps minimize surface reactions and subsequent capacity loss. Furthermore, the molten-salt synthesis method has benefits over the industrial coprecipitation technique and may be a practical method for large-scale synthesis of layered-oxide materials. Finally, the primary source of capacity fade during cycling is addressed at the electrolyte. The standard carbonate electrolytes degrade into an unstable, organic surface layer on the cathode surface. The electrolyte also causes transition-metal dissolution and its migration and attack on the anode. Two new classes of electrolytes are found to form a stable surface layer with minimal degradation of the bulk layered oxide. Together, the methods presented herein provide practical solutions to stabilizing layered-oxide cathode materials that can be applied either alone or in combination.