Sn-Cu Alloy Microtubes for Li-Ion Battery Anode Prepared By Electroless Plating

Abstract

Graphite is generally used for the anode of Li-ion batteries. One of the disadvantages of graphite anode is its small specific charge of 372 mAh/g for lithiation and delithiation. Sn-based materials have been studied as the anode materials alternative to graphite, because Sn has relatively large specific charge of 994 mAh/g. However, the significant volume change (300% expansion) during lithiation is a serious disadvantage of Sn anode causing the pulverization of the anode. To overcome the limitation of Sn-based anode due to the large volume changes, one approach is reducing the particle size of the anode materials. In this regard, varieties of nanostructures and micro 3D architectures of Sn-based materials have been proposed to accommodate the expansion. Sn alloys with the inactive elements such as Cu have also been studied because the alloying elements are expected to buffer the volume change. We propose here the novel and facile processes to prepare the micro architectures of Sn-Cu alloys for the high performance Li-ion battery anode. Cu 6 Sn 5 microtubes are prepared by electroless deposition of Cu and Sn on a polyethylene terephthalate (PET) nonwoven fabric, and their performances as the Li-ion battery anode are evaluated. Preparation of Sn-Cu alloy microtubes PET nonwoven fabrics were hydrophilized by dipping in an O 3 saturated 1M HNO 3 . Pd catalysts were adsorbed on every fiber of 12 micrometers in diameter, and Cu was subsequently deposited from a conventional electroless plating bath. Sn was deposited on the Cu-coated fibers by the contact immersion processes (1). In the contact immersion processes, the Cu-coated fabric was dipped in a Sn 2+ –citrate solution, and was electrically contacted to metallic Sn dipped in the same solution. Sn was deposited on Cu at underpotentials, and diffused into the Cu substrate to form the intermetallic phase, Cu 6 Sn 5 , without annealing. We obtained the Cu (1500-nm thick) / Cu 6 Sn5 (200-nm thick) bilayer microtubes which completely coat the PET fibers as shown in Fig. 1. Charge-discharge Cycling Charge-discharge characteristics of the Sn-Cu microtube fabric anodes were evaluated in a two electrode flat type cell using a Li foil as the counter and reference electrode and a 1M LiPF 6 solution in 1:1 EC/DEC as the electrolyte. Measurements were carried out at a constant current corresponding to the C-rate of 0.5 over the range of 0.001-2.0 V vs. Li/Li + . Charge-discharge capacity vs. cycle number plots showed that the capacities of the microtube anode were stable at around 850 mAh/g-Sn during the first 50 cycles. This is as high as 85% of the specific charge of Sn. The coulombic efficiencies were also high, exceeding 95%, during the 50cycles. Reference (1)Y. Fujiwara, Thin Solid Films , 425 , 121 (2003). Figure 1