Electrochemical Degradation Mechanism of Nano-Sized Sn-C Composite Negative Electrodes for Libs

Lithium-ion batteries (LIBs) are an important power source for both portable devices and electric vehicles. In order for electrical vehicles with LIBs to be embraced more fully, the specific energy of LIBs needs to be improved without sacrificing safety, material availability, and cost. Tin (Sn), as a negative electrode material with various morphologies or nanostructures, is one possible solution because tin not only provides high specific energy (994 mAhg^-1) compared to graphite (~374 mAh g^-1), but is also inexpensive and abundant [1]. Moreover, tin can be used for the negative electrode of a sodium ion battery, which have recently received much attention as an alternative of LiBs due to much lower cost due to the natural abundance of Na [2].

  In spite of the aforementioned advantages of tin as negative electrode materials, tin based electrodes are still difficult to commercialize due to their severe capacity fading during battery cycling. The fade has been associated with the large volume expansion during lithiation, but the mechanism is not yet fully understood. In order to use tin materials in practical devices, the first step is to elucidate the cause of the degradation behavior, and next is to improve the cyclability of tin based electrodes through design of material, coating, and electrolyte additive, etc.

   Herein, we investigated the degradation behavior of the Sn-C composite negative electrodes, using electrochemical and micro-structural methods.

The Sn material used in this study was synthesized by a chemical reducing method [3].  The Sn is spherical-shaped with diameter of 80 ~120 nm, as shown in Fig. a. To fabricate negative electrodes using the prepared nano-sized Sn particles, carbon black as conducting agent and PVdF binder were added and mixed well together in NMP solvent. The ink was applied to a copper foil, dried in a vacuum oven, and calendered. Fig. b shows the XRD pattern of the prepared Sn-C electrode, where Sn peak was mainly detected.

 Fig.c-d shows the lithiation and delithiation curves and capacity fade of the half cell employing the Sn-C negative electrode. The cell was lithiated and delithiated at 40 mAg^-1 (20 mAg^-1 for the 1^st cycle) between 0.01 and 1.5 V_Li/Li+. As shown in Fig. c, the first irreversible capacity loss (ICL) is large (~60%) presumably due to formation of a thick SEI layer, thus reducing the cyclable Li ions and active material during the 1^st lithiation. During subsequent cycles (Fig.d), the capacity decreased gradually by about 1.2 % cycle^-1 from 2^nd to 50^thcycles probably due to loss of conductivity and active materials.

   To reveal more clearly this degradation behavior, we analyze the initial cycle and subsequent cycles separately. Specific methods used in each cycle include: differential capacity (dC/dV) curve, electrochemical impedance spectroscopy (EIS), Focus Ion Beam (FIB)-Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS).

References

[1] B. Wang, B. Luo, X. Li, L. Zhi., Mater Today, 2012, 15, 544-552

[2] Y. Xu , Y. Zhu , Y. Liu , C. Wang, Adv. Energy Mater, 2013, 3, 128-133

[3] M.Noh, Y. Kwon, H. Lee, J. Cho, Y.Kim, M. Kim, Chem Mater, 2005, 17, 1926-1929

Figure  (a) HR-SEM surface morphology of as-prepared nano-sized Sn particles (the agglomerate of Sn particles), (b) XRD patterns of Sn-C composite electrode coated on a Cu foil. (c) Lithiation/delithiation curves and (d) a capacity fade of the Li-ion half cell employing the nano-sized Sn and carbon composite negative electrode, which was cycled at 40 mAg^-1 between 0.01 and 1.5 V.