Development and Characterization of Nanostructure Tin alloys as Anodes in Lithium - Ion Batteries
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E. Peled, A. Ulus and Y. Rosenberga School of Chemistry and (a) Wolfson Applied Materials Research Center Tel-Aviv University, Tel Aviv, Israel 69978 ABSTRACT Several tin-antimony and tin-zinc nanostructure alloys were electroplated from an acid bath, on a copper foil, at current densities higher by an order of magnitude than the limiting current density. They have been characterized as potential high-capacity anodes for lithium-ion battery applications. SEM micrographs of the tin-based alloys reveal nanosize particles, which aggregate into larger agglomerates of fractal shapes. On the nanoscale, the zinc-tin alloys have house-of-cards or honeycomb morphology. The composition of one series of tin based alloys was Sn:Sb (atomic ratio) 1.4:1 to 9:1; another alloy consisted of Sn:Sb:Cu in the ratio 34:10:4. All contained about 5% carbon and about 20% oxygen. The zinc-rich tin alloys contain at least 80 atomic percent zinc (their electrochemical characterization will be reported elsewhere). Tin-based alloys with low antimony content, have high reversible capacity (up to 700mAh/g), low irreversible capacity (about 24%), a better rate capability, and a lower average working potential vs. Li. On the other hand, alloys rich in antimony have a longer cycle life, but poor rate capability and a high average working potential vs. Li. The addition of copper to the tin-based alloys improved cycle life and slightly reduced irreversible capacity. INTRODUCTION Lithium-ion batteries are the most advanced rechargeable batteries on the market. Their anode is either disordered carbon or graphite. In order to increase their energy density, efforts are being made to develop anodes and cathodes with higher capacity density. Lithium alloys have two major advantages: 1) high volumetric and gravimetric capacity density; 2) the melting point of several lithium-rich phases (like Li1i. 4Sn) is higher than 4000C. A high melting point reduces the probability of the anode melting during hazardous situations; this is of special importance for large cells. These advantages have attracted the attention of many research groups, many of which have investigated tin-based alloys [1-8]. High capacity density is associated with large volume changes. These in tum lead to cracking of the alloy anodes during cycling and short cycle life [3]. Several approaches have been taken in order to solve this problem. Fuji's approach was [1,2] to use tin-based glasses as anode material. In this approach a very large irreversible capacity is obtained as a result of the need to reduce the oxides [3]. Besenhard et.al, have studied tin-based alloys, and recently showed that the mechanical strain can be controlled by using smaller particles. The combination of small particles and multiphase hosts have shown the best cycle life [3]. These alloys were made by either electroplating from basic solutions [4] or by chemical reduction [9]. Dahn et al. recently showed that the use of active-inactive phases, as in the cases of glasses or tin-iron alloys, is associated with longer cyc
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