Effect of Polymer Capping Layers on Initial Fracture Mechanism of Silicon Anodes

Abstract

Using silicon nanomaterials is a widely accepted strategy for mitigating the extreme volume expansion associated with the lithiation of silicon and the accompanying capacity fade. However, the high surface area of silicon nanomaterials provides more opportunity for electrolyte decomposition which leads to high irreversible capacity loss and low Coulombic efficiency. Silicon microparticles area available in large scale at a low cost and offer an alternative with lower surface area that have been shown to have high initial Coulombic efficiency. However, to make silicon microparticles viable, the drastic volume change and high stress of these larger particles must be mitigated. In order to develop a more thorough understanding of the initial lithiation mechanics of the silicon crystalline microparticles, this work studies the initial lithiation mechanism through model crystalline electrodes. We have discovered that the thickness of the crystalline silicon wafer itself affected the initial crack formation mechanism. The next step for this study was to modify the mechanical properties of the top surface layer of the crystalline silicon substrates by adding a polymer capping layer and observe how these polymer layers affect the electrochemically induced fractures in crystalline silicon anodes through TEM, SEM and ellipsometry analysis. Polymer films of typical binder materials such as PVDF, PAA and CMC as well as novel modified PAA polymer binders were deposited on crystalline silicon substrates and lithiated to the same extent as an uncoated silicon anode to directly compare crack formation under the same level of silicon lithiation. The results of this work provide important analysis targeted at the initial crack formation of silicon and how polymer capping layers affect the lithiation depth and cracking formation of crystalline silicon. These findings may be applied to create a viable capping layer to reduce cracking in larger, more economical, silicon particles. Figure 1