Nanoindentation and nanowear study of pure Sn and Sn-Ni alloy films on copper

作者: J Chen , SJ Bull , S Roy , A Kapoor , H Mukaibo

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摘要: As potential high capacity anode materials for lithium ion batteries, Sn and Sn based intermetallic alloy films have been investigated by many electrochemical researchers. The Ni-Sn alloy containing 62 atom% Sn displays very good electrochemical cycling response ie good capacity and longer lifetime in service. However, its mechanical properties have not been extensively studied, despite the fact that such anode films may fail mechanically during service. Thus, in this study nanoindentation and nanowear tests have been performed. The hardness and Young’s modulus of electroplated Sn film and Ni-Sn alloy film were measured by nanoindentation. This reveals that the ability to carry the load dramatically reduces in a Sn coating after 1 charge-discharge cycle which makes the plastic strain accumulation in the copper substrate play a greater contribution to crack formation and propagation in repeated charge-discharge cycling. It also shows that the pores formed by lithiation/delithiation can easily collapse at low loads. In nanowear tests, the damage resistance of the Sn-Ni alloy film significantly improves after 1 charge-discharge cycle but it decreases in the pure Sn film after the same charge-discharge cycle; this explains why the degradation rate of the Ni-Sn alloy is slow after the first charge-discharge cycle and why the high capacity is maintained in further cycling. Thus, the mechanical response of the system provides a useful indicator for optimised design of the anode coating for lithium ion batteries. A detailed discussion of the links between the mechanical characterization and the degradation in charge-discharge cycling is given in this paper.

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