# Cu&Si Core-Shell Nanowire Thin Film as High-Performance Anode Materials for Lithium Ion Batteries
## 1 Introduction  
Silicon (Si) has the highest known theoretical capacity of 4200 mAh/g, almost ten times that of the currently used graphite anode, and is therefore being considered as the most promising anode material for high-energy-density lithium ion batteries (LIBs) [1, 2]. Unfortunately, Si suffers from as high as 300% volume change upon full (de)lithiation, inducing pulverization of the silicon particles and therefore losing electrical connectivity from the current collector and the thickening of the solid electrolyte interface (SEI) layer upon cycling; these eventually hinder the practical application of Si-based anode materials in LIBs [3, 4].  
To tackle these issues, enormous attention has been directed towards the development of Si nanostructured materials [5], of which Si nanowires (SiNWs) have attracted considerable interest owing to its small lithium diffusion length and facile strain relaxation during (de)lithiation [6, 7]. However, the complete lithiation of Si nanowire impedes charge transport in the longitudinal direction, limiting its rate performance [8]. [PERSON] et al. successfully synthesized a core-shell structured Si nanowire with a crystallite Si (c-Si) core and amorphous Si (a-Si) shell, in which the a-Si shell can be cycled alone as lithium ion storage whereas the c-Si core remains intact as mechanical starved support and efficient electron transport pathways by setting an appropriate cut-off potential owing to the higher lithiation potential of a-Si than c-Si (\(\sim\)220 mV vs. \(\sim\)120 mV, respectively), resulting in significant improvement in electrochemical performance over traditional Si nanowires, such as a high charge capacity of 1000 mAh/g and a capacity retention of 90% over 100 cycles [9]. On the heels of this work, a series of core-shell nanowires with Si as the core and electron conductive material as shell [10, 11], or with Si as the shell and other electron conductive materials as the core [12, 13], also have been fabricated, contributing to great achievement in Si nanowire-based anodes. However, they not only involved high temperature, complexsteps and the use of catalysts during preparation, but also have a low tap density when applied as an anode. Therefore, the Si core-shell nanowire-based anodes are relatively expensive to produce and hard to scale up.  
Alternatively, Si film anodes have also been studied since they are binder and conductive additive free. Ab-initio calculations suggested that the critical thickness of the silicon film to avoid crack is about 100 nm, beyond which the silicon film would fracture and lead to rapid capacity fading [14]. Such a thin Si-film is not able to provide sufficient Si loading density for practical application [15]. By using a Si-based multilayer thin film, with alternating layers of inactive materials as a buffer layer [16,17], its loading density can be increased but only to a very limited extent.  
In this study, to combine the advantage of both a Si thin film and core-shell Si nanowire, Cu@Si core-shell nanowire thin film (Cu@Si CSNWF) electrodes were fabricated through slurry casting, heat treatment, and magnetron sputtering. Subsequently, their electrochemical properties were investigated in detail. In comparison with other core materials, the Cu nanowire core has obvious advantage such as (1) higher electrical conductivity and fracture toughness [18]; (2) it is inactive to lithium and thus experiences no volume change during the lithiation/delithiation of Si; and (3) the Cu nanowires we used are commercially available and very cheap in price, and easy to be scale up in practical application.