Binary-pore anodic aluminum oxide template based fabrication and advanced microscopic characterization of three-dimensional sodium-ion micro-batteries

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The heavy reliance on lithium-ion batteries has caused global concern due to the rising cost and uneven global distribution of lithium reserves. Owing to the abundance of sodium sources and electrochemical similarities between sodium and lithium, sodium-ion batteries are regarded as sustainable and price-competitive battery technology. Especially, sodium-ion micro-batteries (SIMBs) are the desirable complementation to lithium-ion MBs to satisfy the increasing demand for micro power sources toward matching the rapid progress of microelectronics. Yet, traditional two-dimensional thin-film MBs face a compromise between energy and power. A three-dimensional (3D) MBs design has been proposed to effectively decouple the energy-power compromise by taking advantage of the third dimension of height. A key challenge for obtaining a 3D MBs is to delicately integrate anode, cathode, electrolyte, separator, and current collector in a limited space and on a single substrate without deteriorating the attainable energy and power. In this project, we propose to realize the first fully operational 3D SIMBs using binary-pore anodic aluminum oxide (AAO) templates. Two separate sets of nanopores in binary-pore AAO templates allow independently deposit anode (SnO2) and cathode (NaxCoO2, NaxMnO2 and NaxVO2) materials by atomic layer deposition to obtain interdigitated nanopillar arrays of anode and cathode within a small volume, overcoming the challenge of constructing full cells on a single substrate. Solid-state 3D SIMBs will be obtained after infiltrating free space between alternated anode and cathode nanopillars with solid-state electrolytes. Such 3D interpenetrating-electrode internal architecture of anode and cathode will provide short electron/ion transport pathways in electrodes and electrolytes (yielding high-power density) while maintaining a high volume of electrode materials (yielding high-energy density). Meanwhile, the proposed 3D SIMBs are ideal for studying the (de)sodiation behaviors of electrodes in a confined small space, on which so far there is little knowledge. We will employ scanning electron microscopy, focused ion beam tomography and in-situ transmission electron microscopy to get insights into the structural and surface evolution of electrodes occurring in (de)sodiation process and reveal its influence on the kinetics and thermodynamics of charge storage as well as on formation of solid electrolyte interphase layer. By understanding the underlying electrochemical mechanism up to the atomic scale, a geometry-performance relationship for 3D SIMBs will be established to provide a guideline to improve the battery performance. Finally, we aim to realize solid-state 3D SIMBs with an energy density of more than 10 mWh cm-3, a power density of above 150 mW cm-3, and a cycle life of up to 5000 cycles. The accomplishment of this project shall contribute to the fundamental battery research and promote the advance of future generation microelectronics.