The application of tin in commercially available Li-ion batteries is limited by the poor cyclic performance due to the pulverization caused by cyclic volume changes of elementary cell, during the insertion and extraction processes of lithium ions [1]. Consequently, it results in loss of electrical contact between active material and current collector. One of the ways to overcome this issue is development of tin-carbon nanocomposite, in which nanometric tin grains are encapsulated in a flexible and conductive carbon layers. Such nanocomposites are able to provide appropriate electrochemical properties, only if the formed carbon layer is flexible, has appropriate porosity which featuring local free space to compensate volume changes.
The goal of the presented studies is to examine the effects of carbonization temperature as well as origin and content of carbon on the electrochemical performance of carbon-tin nanocomposites.
Electrode materials consisted of tin-based nanograins encapsulated in carbon buffer matrix of different origins (starch or water soluble polymer) were obtained in a simple and inexpensive process. The tin precursor (SnO2) was prepared using a modified reverse nanoemulsion method (w/o) and then was coated by a various source of carbon (potato starch or modified poly(N-vinylformamide)) [2,3]. The carbon-tin precursors were pyrolyzed, providing formation of tin-based nanograins encapsulated in buffer matrix. The optimal condition for preparation process was studied using thermal analysis method (EGA-TGA/DTG/SDTA). The structure and the morphology of the samples were characterized by X-Ray diffraction analysis (XRD), low-temperature nitrogen adsorption method (N2-BET) and transmission electron microscopy (TEM) as well. The quality of carbon matrix in resulting composites was determined by Raman spectroscopy (RS) measurements. The working electrodes were prepared from a carbon-tin active material and polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) solvent on copper foil. R-2032 coin type cells were assembled using obtained electrodes combined with Li metal discs as a counter electrode. The electrolyte consisted of a solution of 1M LiPF6in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with EC/DEC molar ratio of 1:1. The lithium cells were characterized by cyclic voltammetry (CV), charge-discharge tests (CELL TEST) as well as electrochemical impedance spectroscopy (EIS).
The thermal analysis results enabled determination of the optimal carbonization temperature for obtaining carbon-tin anode materials. N2-BET data as well as TEM images indicated that samples, in which the potato starch was used as a carbon source, have high specific surface area and pore volume as well as reveal presence of the opened pores through the composites. The electrochemical tests showed that better encapsutation of tin nanograins in carbon buffer matrix resulted in higher gravimetric capacity and better cell performance. The C/Sn material in which carbon matrix was fabricated from modified polymer exhibited higher gravimetric capacity and better cell performance (589 mAh g-1 after 70 cycles).
The authors acknowledge a financial support from the National Science Centre of Poland under research grant No. 2012/07/N/ST8/03725.
[1] S. Ding, X.W. Lou, Nanoscale, 3 (2011) 3586
[2] M. Molenda, A. Chojnacka, M. Bakierska, R. Dziembaj, Mater. Technol., 29 (2014) A88
[3] A. Chojnacka, M. Molenda, M. Bakierska, R. Dziembaj, Procedia Eng., 98 (2014) 2