Improving Si anode performance by forming copper capped copper-silicon thin film anodes for rechargeable lithium ion batteries

Abstract In this work, we deposit a bare and a Cu capped CuSi films (that contains 10 %at. Cu 3 μm thicknesses) by magnetron sputtering. The samples are galvanostatically tested with C/12 rate: the Cu capped CuSi film delivers 850 mAhg^-1 after 30th cycles, and it retains stable up to 100 cycle with 99% coulombic efficiency, whilst the bare CuSi film performs a gradual decrease in capacity over 100 cycles. To understand the electrochemical process kinetics, influence the cycling performance, the impedance is measured by using impedance spectroscopy in equilibrium conditions at various states of charges. The results show Cu atoms being ductile, form a network to prevent the electronic isolation of Si particles or delamination of the film. Plus, they buffer the mechanical stress generated in the electrode following the volumetric changes during cycling. In the presence of the Cu top layer the surface reactivity of the CuSi electrode, and its interaction with the electrolyte, is also changed leading to a stable passive film formation and longer cycle life. Therefore, the top layer does not only exert remarkable favorable effects on the capacity, but also improves the coulombic efficiency and the rate capability of the electrode.