Introduction Electrospun polymer derived carbon nanofibers have received attention in recent years as an anode material for energy storage devices due to their enhanced surface area, single step synthesis and shorter diffusion length.[1] Due to these enhanced surface properties, carbon nanofibers showed promising performance even at high C-rate applications.[2] More often, carbon derived from Polyacrylonitrile and its composites are explored as anode material for energy storage devices due to high carbon yield.[1,3,4] However, some other polymeric carbon precursors like polypyrrole, polyvinyl alcohol, and more recently, SU-8 negative photoresist were also explored as an electrode material for a rechargeable Lithium-ion battery. Use of SU-8 facilitates fabrication of three-dimensional electrodes as it can be patterned using photolithography, but it is an expensive source for carbon. The present study is the first report, where cellulose acetate derived carbon fibers have been used as an anode for lithium-ion battery. Cellulose acetate is an inexpensive polymeric carbon precursor. Moreover, our group has recently developed template-assisted electrospinning technique to produce patterned three-dimensional cellulose acetate nanofibrous structures [5] which can be explored further for higher surface area and enhanced performance. Experimental Cellulose acetate polymer (Mw: 29,000 g/mol; Sigma Aldrich, India) was mixed with a binary solvent (acetone and dimethylamide) and the 16 wt.% solution was electrospun to form the nanofibers. As-prepared electrospun cellulose acetate nanofibers mat was regenerated to cellulose nanofabric by deacetylation in 0.05 M NaOH in ethanol, as direct carbonization of cellulose acetate fibers results in melting. After deacetylation cellulose fiber mat was stabilized at 240 °C prior to carbonization at 900 °C in inert atmosphere. Cellulose acetate derived carbon nanofabric was then directly used as a free-standing electrode without any further modification. Electrochemical performance of mat was investigated using Swagelok cell, where lithium metal was used as counter electrode. Results and discussions: Surface morphology of the electrospun fibers was investigated with scanning electron microscope (Figure 1(a)) and deacetylation of cellulose acetate fibers was confirmed by Raman spectroscopy (Figure 1 (b)). A band near 1095 cm -1 can be clearly observed only for regenerated cellulose fiber in figure 1 (b) which corresponds to C-O ring stretching mode, and confirms the conversion of cellulose acetate nanofibers to cellulose nanofibers. Electrochemical performance of cellulose acetate derived carbon fibers was studied at 37.2 mA/g current density. Figure 1(c) shows the cyclic performance of cellulose acetate derived carbon fibers for 100 cycles of charge/discharge. The reversible capacity of these carbon nanofibers was observed to be similar to polyacrylonitrile derived carbon fibers. [3,4] Specific reversible capacity after 100 cycles was found to be ~290 mAh/g. Initial lower columbic efficiency can be attributed to faradic reactions and SEI formation, which is characteristic of carbon materials. Due to the enhanced surface area of free standing carbon fiber mat and entangled three-dimensional porous structure, initial fade was significant. This may be attributed to irreversible surface adsorption of lithium ion. However, after initial 10 cycles, more than 90% of columbic efficiency was retained. Electrochemical performance at higher current densities and detailed structural analysis will be presented in conference presentation.
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Jun 25, 2018
Huagen Li + 4
Open Access