Unravelling graphene oxide’s role in the exfoliation of mesocrystals
12 Dec 2014 NUS physicists have found a new way to overcome a problem faced by conversion materials, namely the swelling and shrinking during lithiation-delithiation (pulverisation), which causes damage to the structure and leads to capacity fading over time.
A team led by Prof SOW Chorng Haur, Prof BVR Chowdari and Prof Andrivo RUSYDI from the Department of Physics in NUS has found a new way to tackle this problem – by protecting highly crystalline ultrathin Co(OH)2 with reduced graphene oxide. This was done by an unconventional route of mesocrystal exfoliation: Co(OH)2 flower-like mesocrystals were first grown using dimethoxyaniline as additive; graphene oxide, which possess abundant chemical functional groups is mixed with as-grown Co(OH)2 flower-like mesocrystals in water and treated hydrothermally. Under high pressure and temperature, Graphene oxide interacts with the organic additive that holds the flower-like mesocrystal together and overcomes the van der Waal’s forces holding together the individual petals. The micro-sized mesocrystals were shed into ulthrathin hexagonal layered composites interleaved with the reduced graphene oxide (rGO) sheets. Cross-sectional Transmission Electron Microscopy reveals a thickness of ~5.8nm, much thinner than reported Co(OH)2 nanoplates.
When tested as an anode material, the hybrid Co(OH)2-rGO heterostructures showed an ability to maintain a capacity of 860 mAg-1 for as many as 30 cycles and 690 mAg-1 after 60 cycles. This value exceeds the theoretical capacity (576 mAhg-1) and the cycling profile is much more stable than commercial Co(OH)2 powder.
The team proposed that the good order within the hybrid nanostructure allows for enhanced surface areas, facile lithium ion diffusion and electron transport, while the flexible graphene bound to Co(OH)2 helps buffer large volume variation and reduce aggregation of electroactive materials during cycling processes. To verify this, advanced characterization techniques like Raman mapping, X-ray Absorption Spectroscopy (SSLS) and Fourier Transform Infrared Spectroscopy helps unveil the interactions and arrangement at the nanoscale. Hence the unique amphiphilic nature of GO not only promotes the exfoliation of mesocrystal by appropriate tailoring of intersheet interactions, it also helps to stabilize the exfoliation nanosheets to improve its electrochemical performance as an anode material for Lithium ion battery.
The exfoliation of mesocrystal developed in their work is a versatile strategy towards designing novel graphene-based nanocomposite architectures to combine the best of both worlds – high surface area and electrochemical activity of the nanostructures, coupled to enhanced conductivity and flexibility of graphene that helps to prevent pulverization and improves conductivity (see Figure). Tackling these issues is critical in the pursuit of revolutionary, high-performance batteries needed for the electrical grid, electric cars, and other types of energy storage.
This work was published in Chemistry A European Journal and selected for the Inside Cover page. The link can be found over here: http://onlinelibrary.wiley.com/doi/10.1002/chem.v20.39/issuetoc
This figure shows the ß-Co(OH)2 mesocrystals were disassembled by using Graphene Oxide as an exfoliant. [Image credit: DENG Suzi]
Deng S. “Ultrathin Hexagonal Hybrid Nanosheets Synthesized by Graphene Oxide-Assisted Exfoliation of β-Co(OH)2 Mesocrystals.” Chemistry - A European Journal 20 (2014) 12326.
The author of this paper, Dr Deng Suzi is currently a Research Scientist at Johnson Matthey Singapore; Dr Christie Thomas Cherian is a Research Fellow in Graphene Research Centre, National University of Singapore.