Breathing parylene-based nanothin artificial SEI for highly-stable long life three-dimensional silicon lithium-ion batteries

Here, we report on the application and characterization of parylene (poly p-xylylene) layers of nanometric thickness as a highly efficient artificial elastic SEI layer for three dimensional nanosilicon anodes. The simple deposition process creates an extremely conformal layer, even on complex 3D substrate geometries. The deposition process can be easily controlled down to thickness of 30 nm with excellent uniformity. The parylene nanothin layers were shown to be extremely stable, chemically, electrochemically and mechanically, while notably allowing the free diffusion of Li-ions into the Si underlying active layer permitted by the finite solvent diffusion through the polymeric layer. The elasticity of the organic parylene layer plays a key role in the applicability of parylene to act as an efficient artificial SEI structure, as it was shown to be capable to withstand the dramatic natural "breathing" effect of the underlying silicon nanostructures, without interruption to the active material volumetric changes, causing no additional stress on the silicon structures, while partially preventing and slowing the formation of a secondary SEI layer. Electrochemical characterization reveals that parylene VT-4 (parylene F) layer is superior to other parylene types, exhibiting >500 cycles with >75% capacity retention, dramatically improving the anode cycling life compared to bare silicon-based anodes, while not limiting the active material loading degree achievable. Importantly, the parylene layer efficacy in preserving the silicon active material is determined via 95.5% capacity retention at low c-rates. The simplicity, applicability and control of parylene deposition, along with its vast improvement of stability of bare silicon anodes, make it an excellent candidate for future applications as an efficient artificial SEI layer for next-generation silicon anodes and other active materials of interest.