In-Situ Polymerization of an Internal, Conductive, and Conformal Polymer Coating for Mesoporous Si
Authors: Yersak Thomas A. , Estrada Daniel , Shin Jae Wook , Sailor M. J. , Meng Shirley 
Published: February 2020

A wide variety of a nano-structured Si electrodes have been proposed in the recent years that avoid pulverization of both the Si active nanoparticles and the overall electrode structure. It has been shown that nano-Si particles (dia. < 150 nm) can accommodate the strains associated with full lithiation without fracture (1). Additionally, encapsulation of nano-Si in clever carbon structures provides the volume needed to accommodate Si’s expansion without bulk expansion of the overall electrode (2). Unfortunately, many of these nano-structured electrodes are not suitable for commercialization because the complicated fabrication processes are too expensive. Previous work has identified mesoporous Si (mp-Si) as a scalable, commercially viable nanostructured anode material for advanced Li-ion batteries. mp-Si combines ease of manufacturability with the advantages of a Si nanostructure. The internal porosity of mp-Si has been shown to largely accommodate the strain of Si lithiation in order to avoid pulverization of the electrode structure.
An electrochemically etched mp-Si material developed by Sailor is utilized in this study (Figure 1) (3, 4). Porous Si can be carbonized to prevent SEI formation and provide fast electronic transport. Carbonization is typically accomplished by the thermal decomposition of acetylene gas. The resulting carbon coating is strongly bonded to the Si surface via Si-C bonds, but it is brittle so it cannot provide mechanical resiliency nor protect mp-Si’s surface from electrolyte decomposition. The degree to which mp-Si’s internal structure pulverizes with cycling is also not known. With this in mind, a flexible coating is ideal because it can impart the porous structure with mechanical resiliency for improved cycling performance.
Stabilized polyacrylonitrile (PAN) was found to be a promising conductive binder for nano-Si (5). PAN coatings on nano-Si (dia. < 50 nm) are conformal, elastic, and < 10 nm thick. Cyclizing and dehydrogenating PAN introduces delocalized sp2 π bonding for good intrinsic electronic conductivity, but avoids full carbonization to maintain the material’s polymeric elasticity. This process cannot be applied to mp-Si because the macromolecules of PAN (Mw > 100,000 g mol-1) are too large to infiltrate the mesopores. In another work, nano-Si electrodes were prepared by the in-situ polymerization of polyaniline (PANi) (6). PANi is a popular electronically conductive polymer, but it has not been applied to the mp-Si electrode material. PANi also acts as both the binder and the conductive additive.
Here, we propose that either the in-situ polymerization of PANi or PAN are ideal for mp-Si electrodes. We have already demonstrated the feasibility of PAN:mp-Si electrode by the in-situ radical polymerization of PAN in DMF. Our PAN:mp-Si electrode delivers a specific capacity of 1500 mAh/g-Si. We will also discuss the feasibility of a PANi:mp-Si electrode and the appropriate functionalities needed for stable adhesion of the polymer to Si’s surface.
Figure 1: a) Etched mesoporous Si particle. b) SEM micrograph of mesoporous Si’s porous structure (3, 4).

References

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