Nanofibrillar Si Helices for Low-Stress, High-Capacity Li Anodes with Large Affine Deformations.

We report on the chemical lithiation of long microscale helices composed of densely packed amorphous silicon (aSi) nanofibrils, fabricated by glancing angle deposition (GLAD) through e-beam evaporation. In situ electron microscopy and companion finite element modeling demonstrate that the nanofibrillar structure of the aSi helices allows for 2 orders of magnitude faster effective rates for Li diffusion ( D = 10 cm/s) compared to solid aSi nanowires, while also averting fragmentation during lithiation. More importantly, it is shown that specific helical geometries can accommodate large, lithium-induced, volumetric expansions without shape distortion.