Engineering the architecture and surface of 2D electrode materials is crucial for making good battery electrodes. An optimized nanostructure allows for fast ion transport, controlled energy storage, stable structural properties and high-rate battery cycling.

“Nanofluidic channels are an important nanostructure in this context,” explains team leader Guihua Yu of the University of Texas at Austin. “By controlling the surface charge and the spacing between these channels, we can achieve enhanced and selective ion transport through them when they are constructed with dimensions comparable to the so-called double-Debye length and opposite surface charges with respect to the transporting ions.”

High charge storage

The surfaces of the stacked ultrathin Co3O4 nanosheets made by the researchers are functionalized with various anion groups (OH, NO3– and CO32–) and the layers are separated by polyvinylpyrrolidone. The 2D nanofluidic channels present in this stacked structure offer extra sites on which lithium can be stored and through which lithium ions can rapidly propagate. They also allow for sufficient buffering space to allow for the natural volume change that occurs during lithium battery cycling (lithiation/delithiation). Finally, the surface functional groups also help minimize restacking of the ultrathin Co3O4 nanosheets and shorten lithium–ion diffusion distances.

The researchers tested their material as an anode and found that it delivers a high specific capacity of 1230 and 1011 mAh g–1 at current rates of 0.2 and 1 A g–1, respectively, excellent rate performance and long cycle capability of over 500 cycles at 5 A g–1.

Outstanding anode material

“It is a promising anode material for lithium-ion batteries,” Yu tells nanotechweb.org. “By further engineering its nano-architecture – for example by tuning the interlayer spacing – we may further push the material’s energy storage performance. The enhanced and selective ion transport that we observed in our designed 2D nanofluidic channels could benefit many energy-related applications in which fast ion transport kinetics are essential.”

The team says that it is now continuing to extend the concept of 2D nanofluidic channels to other electrode materials, with or without layered structures. “We are also taking this research direction even further by looking into transport and storage properties for energy storage systems based on larger charge-carrying ions, such as Na+ and Mg2+,” says Yu. “We are interested in investigating this structural engineering strategy for applications other than energy storage too – for example, catalysis.”

The work is detailed in Advanced Materials DOI: 10.1002/adma.201703909 and is featured on the cover of the Dec 13 issue of the journal.