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 2.27g/cm3 (film porosity ~3%) down to 1.64g/cm3 (~30% porosity) were fabricated by magnetron sputtering at argon pressures varying from 0.2Pa to 2.6Pa, respectively. High density Si thin film anodes of 2.27g/cm3 suffered from an unstable cycling behavior during charging/discharging depicted by a continuous reduction in specific down to ~830mAh/g at the 100th cycle. Electrochemical properties of lower density films with 1.99g/cm3 (~15% porosity) and 1.77g/cm3 (~24% porosity) got worse resulting in only ~100mAh/g capacity at 100th cycle. On the other hand, as the density of anode was further reduced down to about 1.64g/cm3 (~30% porosity), cycling stability and capacity retention significantly improved resulting in specific capacity values ~650mAh/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.