Influence of electrode processing and electrolyte composition on multiwall carbon nanotube negative electrodes for sodium ion batteries

Nanostructured one-dimensional multiwall-carbon nanotubes have a variety of advantageous properties including good electrical conductivity and mechanical strength, and thus have been widely investigated for use in lithium-ion battery electrodes as conductive and microstructural additives, though they also possess some electrochemical activity. Their application to sodium-ion batteries has been less extensively researched, and therefore a greater understanding of the electrochemical reaction with sodium, and effects of slurry composition and electrolyte formulation is warranted, especially as these are likely components in future Na-ion electrode formulations. Here, we report the fabrication of aqueous and organic multi-wall carbon nanotube (MWCNT) negative electrodes processed by ball milling. The binder of choice is noted to greatly affect the electrochemical performance, both in terms of capacity retention and rate capability over a range of current densities from 25 to 500 mA g(-1). Switching from a carbonate- to diglyme-based electrolyte considerably improves initial coulombic efficiencies (similar to 10%-60%), attributed to less extensive formation of solid electrolyte interphase, and enables a reversible mechanism with capacities up to 150 mAh g(-1) over 100 cycles depending upon the binder used. Ex-situ characterization of the discharged and cycled carbon nanotubes by powder x-ray diffraction, transmission electron microscopy and Raman spectroscopy provide an insight into how MWCNTs undergo sodiation and demonstrate a partially reversible structural transformation during cycling when using the diglyme-based electrolyte. This work lays the foundation for a better understanding of these versatile materials, especially when used in the most promising alternative energy storage technology to lithium ion.