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Tin anodes deposited electrochemically on a copper foil current collector are studied to develop a next-generation lithium-ion battery with higher energy density. Better cycle performance through ten initial cycles under full charge and discharge conditions was attained by annealing tin electrodeposited on a rough surface copper foil. The annealing process was found to change the main active material from Sn to Cu6Sn5 with some minor compounds. Furthermore, a microcolumnar structure of the active material portion was found to be self-organized in accordance with the surface profile of the foil during the first charge-discharge cycle. Advantages of these structural features are discussed in terms of the initial charge and discharge performance, including specific capacity and coulombic efficiency measured by using a three-electrode cell. © 2003 The Electrochemical Society. All rights reserved.

An amorphous Sn–Co alloy film electro-codeposited on rough Cu foil was found in our previous works to self-organize a microisland structure, which is a crucial factor to improve Sn alloy anodes for cyclability. This work focused on the electrochemical properties, film morphology, and phase structure, and the behavior of Co atoms was analyzed by extended X-ray absorption fine structure and magnetic susceptibility measurements. Cycle performance up to 20cycles , in which the capacity anomalously fluctuated, is discussed based on these results, where the capacity change is explained by the morphological and phase structural changes. The stabilization process of the phase is clarified in connection with formation of ferromagnetic face-centered cubic Co particles.

In our previous work, an amorphous Sn-Co alloy film prepared by electrocodeposition on Cu foil with rough surface was found to transform to a new phase structure during charge-discharge cycle to show more stable reaction with lithium. In this work, we prepared Sn-Co film on copper current collector which originally included the new phase structure, and examined its electrochemical and structural properties by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, extended X-ray absorption fine structure, and magnetic susceptibility measurement. Influence of the phase transformation on the cyclability is discussed in terms of adhesion between the film and current collector.We have proposed that a Sn-Co alloy is a candidate for the alternative anode material, which can be formed into film by electrocodeposition and offer a larger electrochemical capacity as an anode of lithium ion batteries. 1 In this previous work, the 79.8Sn-20.2Co alloy in mass % shows well-balanced mechanical property between ductility and brittleness, and its alloy film on a Cu foil as a current collector with rough surface self-organizes the microisland structure, where the film cracks at the first charge-discharge to divide into islands of a few micrometers in width. This structure prevents the film from pulverizing or peeling off the foil by relaxing the stress from which the film and foil suffer during charge-discharge cycles. As a result, the film keeps more than 487 mAh/g of discharge capacity in the first 20 cycles.The phase structure of the film transforms from the amorphous Sn-Co to the other one during the cycling, and subsequently the film shows more stable reaction with lithium. 2 This result indicates that there should be room to improve the Sn-Co film in cyclability by optimizing the phase structure as prepared. We also have found that a 92.1Sn-7.9Co film in mass % prepared by electrocodeposition shows the same X-ray diffraction ͑XRD͒ pattern as the phasetransformed 79.8Sn-20.2Co film. In this work we examine electrochemical and structural properties of the 92.1Sn-7.9Co film prepared on the rough Cu foil by electrocodeposition, and then analyze stability of the film in charge-discharge cycles. We also examine the cycle performance of the 92.1Sn-7.9Co and 79.8Sn-20.2Co films up to 40 cycles and observe adhesion between the films and Cu foil after the cycle test to discuss the influence of the phase transformation on the cyclability. ExperimentalThe 92.1Sn-7.9Co alloy film was prepared to be 1.2-1.3 m of thickness by electrocodeposition on rough Cu foil. The process was performed in the same manner as we previously reported. 2 The electrodeposition bath contained 50 g/L of stannous pyrophosphate, 9 g/L of cobalt chloride, 250 g/L of potassium pyrophosphate, and 110 mL/L of an additive for forming a Co complex. To remove water, the test electrode was dried at 105°C for 2.5 h before electrochemical measurements.The electrochemical measurements employed a three-electrode test cell with the test electrode of...