The researchers found that sodium storage capacity of paper electrodes depends on the distance between the individual layers that can be tuned by heating it in argon or ammonia gas. For example, reduced graphene oxide sheets, or rGO, produced at high temperature have near zero sodium capacity, while reduced graphene oxide sheets produced at 500 degrees C have the maximum capacity.
Gurpreet Singh, assistant professor of mechanical and nuclear engineering, and Lamuel David, doctoral student in mechanical engineering, India, have published their findings in the Journal of Physical Chemistry in an article entitled 'Reduced graphene oxide paper electrode: Opposing effect of thermal annealing on Li and Na cyclability'.
Graphene oxide is an insulating and defective version of graphene that can be converted to a conductor or a semiconductor when it is heated. Singh and his team studied graphene oxide sheets as flexible paper electrodes for sodium- and lithium-ion batteries.
"The observation is important because graphite, which is a precursor for making graphene oxide, has negligible capacity for sodium and has long been ruled out as viable electrode for sodium-batteries," Singh said. "Graphite is the material of choice in current lithium-ion batteries because the interlayer spacing is just right for the smaller size lithium ions to diffuse in and out."
The researchers are the first to show that a flexible paper composed entirely of graphene oxide sheets can charge and discharge with sodium-ions for more than 1,000 cycles. Sodium perchlorate salt dissolved in ethylene carbonate served as the electrolyte in their cells.
"Most lithium electrode materials for sodium batteries cannot even last for more than a few tens of charge and discharge cycles because sodium is much larger than lithium and causes enormous volume changes and damage to the host material," explained Singh. "This design is unique because the distance between individual graphene layers is large enough to allow fast insertion and extraction of the sodium ions, thanks to the oxygen