Breathing Parylene-Based Nanothin Artificial SEI for Highly-Stable Long Life Three-Dimensional Silicon Lithium-Ion Batteries

Abstract

Here, we present on the application and characterization of parylene layers of nanometric thickness as a highly efficient artificial elastic SEI layer for three-dimensional nano-silicon anodes. The simple deposition process of the parylene layers creates an extremely conformal layer, even on the most complex three-dimensional substrate geometries and can be easily controlled down to thicknesses of below 30nm with excellent uniformity. The parylene nanothin layers are shown to be extremely stable chemically, electrochemically and mechanically, while notably allowing the free diffusion of Li-ions into the Si underlying active 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, while partially preventing and slowing the formation of a secondary SEI layer. Combined with our unique, novel substrate which demonstrates a simple catalyst-free growth of highly electrically conductive Si-stainless steel composite anodes with high loadings of silicon nanostructures and binder-free nature, remarkable electrochemical results can be achieved. Electrochemical characterization reveals that the Parylene-F VT-4 nanolayer is superior to other parylene types, exhibiting more than 450 cycles with 75% capacitiy retention vs Li metal anode. Importantly, the parylene layer does not allow delamination and cracking of active Si material, as evident from the 95.5% capacity retention after 400 cycles at low C-rate. The unique combination of the parylene artificial SEI layer and highly conductive substrates allow for high C-rates (>2C) to be achieved with significant capacity retention. The parylene layer is shown to dramatically improve the anode cycle life compared to bare silicon-based anodes, while not limiting the active material loading degree achievable. Most importantly, parylene is shown to fulfill all the required properties of artificial SEI, including impeccable chemical stability in organic solvents, elasticity and mechanical stability, high adhesion to the silicon under-layer, lowering the SEI formation, ionically conductive and electrically insulating and cost-effective, scalable and simple fabrication. 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. Figure 1