Electrochemical Performance of Sn/SnO2 Nanoparticles Encapsulated in Carbon Matrix Derived from Plant Polysaccharides
The anode materials for Li-ion batteries based on graphite and related carbon materials do not already have appropriate, required operating parameters, due to their low capacity, which is about 370 mAh/g . Tin has been shown to be one of the possible solution for replacing existing anodes in view of its high capacity (994 mAh/g). However, the intercalation process of lithium ions into the tin structure is accompanied by changes in elementary cell volume, reaching up to 300% . Consequently, this leads to loss of electrical contact between active material and current collector. Pulverization of the material can be avoided by downsizing of the material particles, nevertheless, the cycling stability of nano-Sn particles remains insufficient . It was found that carbon materials such as nanofibres or nanotubes can be used as a stress-accommodating intermediate phase, however, preparation of such composites is mostly complicated (multi-step) and expensive [4,5].
The goal of the present work was development of carbon-tin nanocomposite anode material with tailored morphology and electrochemical properties. The nanocomposite, obtained in a simple and inexpensive process, consisted of tin-based nanograins encapsulated in a flexible carbon buffer matrix derived from plant polysaccharides.
Carbon-tin nanomaterials were prepared using a modified reverse microemulsion method (w/o), which allowed controlling size and shape of the obtained tin oxide(IV) particles . Then, precursor of active material was coated by a plant polysaccharide in gelatinization process. The carbon-tin nanocomposites were obtained in one step pyrolysis and carboreduction process, providing formation of tin-based nanograins encapsulated in carbon buffer matrix. Optimal conditions of the process were determined by thermal analysis methods (EGA-TGA). The resulting materials with different carbon loading (20-60 wt.%) were investigated by X-ray diffraction (XRD) and by transmission electron microscopy (TEM) as well. Comprehensive electrochemical characterization of obtained nanocomposites including the electrical conductivity (EC), cyclic voltammetry (CV) and impedance spectroscopy (IS) was carried out. Discharge - charge tests were performed in R2032-type coin cells within 0.02–1.5 V potential range.
Carbon-tin based anode materials were successfully prepared in one step pyrolysis and carboreduction process of nanometric tin oxide(IV). As a source of carbon, the plant polysaccharide was successfully used. The obtained composites revealed good columbic efficiency and capacity retention in discharge/charge tests. TEM images indicate that morphology of obtained carbon matrix as well as electrochemical properties of the composites can be easy controlled by carbon precursor composition and loading.
The authors acknowledge a financial support from the National Science Center of Poland under research grant No. 2012/07/N/ST8/03725 and from the European Institute of Innovation and Technology under the KIC InnoEnergy NewMat project.
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