Nanostructured Thin Film Silicon Anodes for Li-Ion Microbatteries

Thin film microbatteries require electrode materials with high areal specific capacities and good cyclability. Use of vapor-deposited silicon thin films as anodes in Li-ion microbatteries offers the advantage of high capacity as well as compatibility with other processes used for microsystem fabrication. Unfortunately, monolithic silicon films greater than 200 nm in thickness pulverize during lithiation and delithiation. We have used metal-assisted-chemical-etching of sputter-deposited amorphous silicon films to make nanoporous silicon layers and arrays of silicon nanopillars as a means of achieving anodes with high areal capacity and good cyclability. We have compared the performance of these nanostructured layers with the performance of monolithic silicon films in Li half-cells. A reduced first cycle coulombic efficiency was observed in all cases and was attributed to the irreversible formation of Li2O due to the presence of oxygen in the sputter-deposited silicon films. This was controlled through modifications of the sputtering conditions. As expected, monolithic films thicker than 200 nm showed poor cycling performance due to pulverization of the film. Nanoporous silicon showed good initial cycling performance but the performance degraded due to porosity collapse and delamination. Arrays of silicon nanopillars made from 750 nm silicon films exhibited good cycling, rate performance and an areal capacity (0.20 mA h cm(-2)) 1.6 times higher than what could be obtained with monolithic Si films with similar cyclability.