Fabrication of Tin-Plated Three-Dimensional Copper Nanostructure Using Electroless Plating and Its Anode Performance in Lithium-Ion Battery

Abstract

Introduction Next-generation lithium-ion batteries with higher energy density are urgently needed for hybrid and electric vehicle applications, among others. Currently, graphite is typically used as the anode active material in lithium-ion batteries, but it is necessary to consider alternative, higher capacity materials. For example, tin has a larger theoretical capacity (991 mAh g-1) than that of graphite (372 mAh g-1). However, the volume of tin changes significantly during charging and discharging, causing it to slip down from the copper current collector, which deteriorates the cycling characteristics. Three-dimensional copper structures are effective at preventing such slippage.1) However, in general, the methods for fabricating such structures are very complicated. We have reported that a three-dimensional copper nanostructure (3DC-1) can be fabricated easily by one-step electrodeposition.2) In this study, a tin layer was formed on the 3DC-1 by electroless deposition, and its anode performance in a lithium-ion battery was evaluated. Experimental An acidic copper sulfate bath1) containing 0.85 M CuSO4∙5H2O + 0.55 M H2SO4 + 2.5×10-4 M polyacrylic acid (MW=5000) was used in the fabrication of the 3DC-1. Electrodeposition was conducted under galvanostatic conditions (1.2 A dm-2) at 25 °C without agitation. A bright Watts nickel plating bath (1 M NiSO4∙6H2O + 0.2 M NiCl2∙6H2O + 0.5 M H3BO3 + 0.01 M saccharin sodium dehydrate + 0.0025 M 1,4-butynediol) was prepared, and electrodeposition was conducted under galvanostatic conditions (3.0 A dm-2) at 25 °C without agitation. Tin was electrolessly plated onto the 3DC-1 at 80 °C for 15 s without agitation. An immersion plating bath containing 0.5 M K4P2O7 + 0.15 M Sn2P2O7+ 3 M thiourea was prepared. The pH was adjusted to 5 with hydrochloric acid. The microstructure of the tin anode was examined using field-emission scanning electron microscopy (FE-SEM). The phase structure of the deposits was analyzed by X-ray diffraction (XRD). A chemical composition analysis was carried out using X-ray fluorescence spectrometry (XRF) and energy-dispersive X-ray spectroscopy (EDX). Electrochemical studies of the tin anode were carried out with coin cells that were assembled in an Ar-filled glove box. Each coin cell consisted of a lithium foil as a counter electrode and a reference electrode. The plating film functioned as the working electrode. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%). Cycling tests were performed in the range of 0.02–1.5 V (vs. Li/Li+) at a constant temperature of 25 °C. Results and Discussion Fig. 1 shows a surface SEM image of the tin anode, which reveals that the tin film was plated homogeneously onto the three-dimensional copper nanostructure. Fig. 2 shows a cross-sectional SEM image of the tin anode and corresponding EDX mapping results for copper, nickel, and tin. The tin was distributed homogeneously on the 3DC-1. Fig. 3 compares the cycling characteristics of tin anodes fabricated using 3DC-1 (a) and a flat copper foil (b). For the former, the discharge capacity was about 900 mAh g-1 after the first cycle. Even after 100 cycles, the discharge capacity remained over 700 mAh g-1. In contrast, for the latter, the discharge capacity was 650 mAh g-1 after the first cycle and 250 mAh g-1after 100 cycles. References 1) Shichao Zhang, Yalan Xing, Tao Jiang, Zhijia Du, Feng Li, Lei He, Wenbo Liu; Journal of Power Sources,196, 6915-6919 (2011) 2) S.Arai and T.Kitamura, ECS Electrochemistry Letters, 3 (5), D7-D9 (2014) Figure 1