Graphene wrapper boosts lithium-sulfur battery performance
Lithium-sulfur batteries provide theoretical specific energy densities greater than those of lithium ion batteries.
To create the sulfur cathode, the research team led by Dr. Vasant Kumar at the University of Cambridge and Professor Renjie Chen at the Beijing Institute of Technology used Metal organic frameworks (MOFs) ‘as a template’ to produce a conductive porous carbon cage – in which sulfur acts as the host and each sulfur-carbon nanoparticle acts as energy storage units where electrochemical reactions occur.
"Our carbon scaffold acts as a physical barrier to confine the active materials within its porous structure," explained Kai Xi, a research scientist at Cambridge. "This leads to improved cycling stability and high efficiency." They also discovered that by further wrapping the sulfur-carbon energy storage unit within a thin sheet of flexible graphene speeds the transport of electrons and ions.
Fast charge-transfer kinetics are made possible by an interconnected graphene network with high electrical conductivity. The composite structure of a porous scaffold with conductive connections makes a promising electrode structure design for rechargeable batteries.
The work provides a "basic, but flexible, approach to both enhance the use of sulfur and improve the cycle stability of batteries," Xi said. "Modification of the unit or its framework by doping or polymer coating could take the performance to a whole new level."
Schematic of the preparation of a 3-D hierarchically structured graphene-sulfur/carbonZIF8-D composite. Credit: K.Xi/Cambridge
The battery design’s novel integration of energy storage with an ion/electron framework has opened the door for fabrication of high-performance non-topotactic (not involving a structural change to a crystalline solid) reactions-based energy storage systems.
"We’ll focus on fabricating hybrid free-standing sulfur cathode systems to achieve high-energy density batteries, which will involve tailoring novel electrolyte components and building lithium ‘protection layers’ to enhance the electrochemical performance of batteries," said Xi.
‘Graphene-wrapped sulfur/metal organic framework (MOF)-derived microporous carbon composite for lithium sulfur batteries’ is authored by Renjie Chen, Teng Zhao, Tian Tian, Shuai Cao, Paul R. Coxon, Kai Xi, David Fairen-Jimenez, R. Vasant Kumar and Anthony K. Cheetham. It will appear in the journal APL Materials on December 16, 2014. After that date, it can be accessed at: https://scitation.aip.org/content/aip/journal/aplmater/2/12/10.1063/1.4901751
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