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 ₀ = 10 ^-10 cm ² /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. A major advantage of the helical nanostructures is that the compressive force generated due to lithiation-induced expansion is an order of magnitude smaller than in straight nanocolumns that permanently buckle during lithiation. Thus, GLAD-fabricated films composed of dense periodic microscale helices with properly designed coil geometries are highly suitable for robust, high-capacity Li ⁺ anodes.