Low-density silicon thin films for lithium-ion battery anodes

Abstract

Density of sputter deposited silicon (Si) thin films was changed by a simple working gas pressure control process, and its effects on the cycling performance of Si films in Li-ion batteries as anodes was investigated. Higher gas pressure results in reduced film densities due to a shadowing effect originating from lower mean free path of sputter atoms, which leads to a wider angular distribution of the incoming flux and formation of a porous film microstructure. Si thin film anodes of different densities ranging from 2.27 g/cm(3) (film porosity similar to 3%) down to 1.64 g/cm(3) (similar to 30% porosity) were fabricated by magnetron sputtering at argon pressures varying from 0.2 Pa to 2.6 Pa, respectively. High density Si thin film anodes of 2.27 g/cm(3) suffered from an unstable cycling behavior during charging/discharging depicted by a continuous reduction in specific down to similar to 830 mAh/g at the 100th cycle. Electrochemical properties of lower density films with 1.99 g/cm(3) (similar to 15% porosity) and 1.77 g/cm(3) (similar to 24% porosity) got worse resulting in only similar to 100 mAh/g capacity at 100th cycle. On the other hand, as the density of anode was further reduced down to about 1.64 g/cm(3) (similar to 30% porosity), cycling stability and capacity retention significantly improved resulting in specific capacity values similar to 650 mAh/g at 100th cycle with coulombic efficiencies of >98%. Enhancement in our low density Si film anodes are believed to mainly originate from the availability of voids for volumetric expansion during lithiation and resulting compliant behavior that provides superior mechanical and electrochemical stability. (C) 2016 Elsevier B.V. All rights reserved.