Collaborative Research: High-performance nanowire cathodes with stabilized microporous tunnels for Na-ion batteries

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Rechargeable batteries help to enable a sustainable energy future through the storage and demand-matched delivery of electricity generated from fluctuating renewable sources such as wind or sun. Currently, lithium-ion batteries offer the best performance in terms of capacity, power delivery, and longevity. However, lithium is a limited resource with respect to both its total supply in the Earth's crust and its geographic availability. Rechargeable batteries which use sodium ions instead of lithium ions for storing charge address the possibility of lithium scarcity in the future, because sodium is a very abundant element. However, sodium ions are much larger ion than lithium ions, and this large size creates a variety of operational problems within a rechargeable battery, particularly swelling of the battery electrode during charging and discharging. Through nanotechnology-based approach, this project will develop new cathodes for sodium-ion batteries using hollow nanowires which confine the sodium ions within a tunnel structure. The nanowires will be made of magnesium oxide with tunnel diameters of about 2 nanometers. The key innovation is that by confining the sodium ions, within the tunnel, the swelling can be controlled and storage capacity can be increased simultaneously. The scientific outcomes of this project can be used to help research with other future metal ion battery systems based on magnesium, aluminum and potassium, where similar issues might occur. The educational activities associated with this project include outreach to school-age children from under-represented groups in engineering through the Richmond Area Program for Minorities in Engineering, the Philadelphia Science Festival, and the Annual Young Women's conference.The goal of this research is to improve the mechanical stability and electrochemical energy storage capacity of sodium-ion batteries by tuning of the pore geometry and ionic content of microporous, sodium-ion intercalation electrodes. The key hypothesis is that both swelling and storage capacity can be controlled by carrying out the sodium-ion intercalation process within tunnel-structured, sodium-stabilized manganese oxide nanowires. These nanowires contain one-dimensional microporous tunnels forming defined diffusion paths that facilitate reversible sodium ion intercalation. The research plan has three objectives. The first objective is to synthesize magnesium oxide nanowire cathodes that contain one-dimensional, microporous tunnels with defined sodium ion diffusion channel dimensions, and then evaluate their electrochemical performance and mechanical strength. These measurements will be realized through bulk-scale electrochemical characterization and single nanowire-based nanoelectrochemical probing and mechanical degradation testing. The second objective is to improve the specific capacity within the nanowire channels by increasing the sodium content within the microporous tunnels through chemical routes. Through the first and second objectives, the top performing materials will be identified that maximize capacity, mechanical stability, and life cycle. The third objective is to improve electrochemical storage capacity and mechanical stability during repeated charge/discharge cycles through dopant-induced crystal stabilization of the nanowire electrode materials.