Binder-Free, Vertically Aligned Carbon Nanotubes on Nickel Foam as a Freestanding 3D Li-O2 Battery Cathode

The Li-O₂ battery system is known for its tremendous theoretical metric for specific energy. Despite lower practically obtainable values, this system is still a competitive next generation electrochemical energy storage system. The capabilities of this technology are currently being limited by scientific challenges that must be addressed in each component of the device, but the positive electrode is particularly complicated by its role in OER/ORR, leading to strict requirements for architecture design and physicochemical stability for optimal performance. In an effort to satisfy these requirements, we present herein one of the first experimental realizations of a controlled macroscale 3D CNT architecture.

Consisting of densely packed vertically aligned carbon nanotubes (VACNT, see SEM images below) grown directly on a nickel foam current collector with an ultra-thin Al₂O₃ ALD interlayer, this specially tuned hierarchically porous structure can accommodate a substantial amount of reduction product without blocking the cathode pore orifices, thus facilitating oxygen and Li⁺ transport throughout the electrode, even for high depths of discharge. This architecture is motivated by modeling of dual-pore systems in which a mesoporous sub-morphology can accommodate reduction product storage while a globally macroporous scaffold facilitates oxygen transport to the inner regions of the oxygen electrode.¹

This self-supporting electrode is the first to feature micron-length VACNT directly connected to a 3D integrated current collector without requiring a binder or delamination of the CNT from a separate growth substrate, which can lead to changes in tortuosity. TEM and electron diffraction (shown below) confirm that the nanotubes are multi-walled (~5-10 walls).  The robust connection results in capacity even at high discharge voltages (~2.8V@20 μA/cm² vs. Li in 0.1M LiClO₄/DMSO).

The lack of standardization and the current trend of normalization to extremely small masses of carbon is an issue in the field, particularly with nanostructured carbon, so an emphasis of these architectures is the practical carbon loading amount of more than 2 mg/cm²_.  O₂ cathodes are tested in a custom cell (shown below) similar to those used/designed previously for Li-O₂ research.²

Atomic layer deposition offers unique capabilities for further enhancement of this structure including alternative interlayer chemistries, nm-thick ALD coatings that can act as artificial solid electrolyte interphase protection layers and using nucleation behavior to decorate substrates with metal and/or oxide particles for catalysis. Preliminary ALD results include the use of a different ALD interlayer chemistry (TiN, see performance results below), and parallel testing of an ALD-decorated CNT structure that has exhibited 12 stable cycles. 

(1)          Kraytsberg, A.; Ein-Eli, Y.  J. Power Sources 2011, 196, 886–893.

(2)          Lu, Y.-C.; Gasteiger, H. a.; Parent, M. C.; Chiloyan, V.; Shao-Horn, Y. Electrochem. Solid-State Lett. 2010, 13, A69.