Fabrication of 3D Core–Shell Multiwalled Carbon Nanotube@RuO₂ Lithium-Ion Battery Electrodes through a RuO₂ Atomic Layer Deposition Process

Pushing lithium-ion battery (LIB) technology forward to its fundamental scaling limits requires the ability to create designer heterostructured materials and architectures. Atomic layer deposition (ALD) has recently been applied to advanced nanostructured energy storage devices due to the wide range of available materials, angstrom thickness control, and extreme conformality over high aspect ratio nanostructures. A class of materials referred to as conversion electrodes has recently been proposed as high capacity electrodes. RuO₂ is considered an ideal conversion material due to its high combined electronic and ionic conductivity and high gravimetric capacity, and as such is an excellent material to explore the behavior of conversion electrodes at nanoscale thicknesses. We report here a fully characterized atomic layer deposition process for RuO₂, electrochemical cycling data for ALD RuO₂, and the application of the RuO₂ to a composite carbon nanotube electrode scaffold with nucleation-controlled RuO₂ growth. A growth rate of 0.4 Å/cycle is found between ∼210–240 °C. In a planar configuration, the resulting RuO₂ films show high first cycle electrochemical capacities of ∼1400 mAh/g, but the capacity rapidly degrades with charge/discharge cycling. We also fabricated core/shell MWCNT/RuO₂ heterostructured 3D electrodes, which show a 50× increase in the areal capacity over their planar counterparts, with an areal lithium capacity of 1.6 mAh/cm<sup>2