Study of Molecular Layer Deposition Coating for Silicon-Based Lithium-Ion Anodes
Authors: Ban Chunmei , Molina Piper Daniela , Travis Jonathan J. , Lee Younghee , Son Seoung-Bum , George Steven M. , Lee Sehee 
Published: February 2020

Silicon has attracted much attention as a promising Li-ion anode material, due to its high theoretical capacity and natural abundance. However, progress towards a commercially viable Si anode has been impeded by the rapid capacity fade of silicon caused by large volumetric expansion and unstable interface. Surface modifications, which chemically or physically change the surface of electrode components, have been applied to improve the interfacial chemistry, conductivity, and mechanical integration in Si-based electrodes.[1-3]   Unlike other electrode materials, Si particles are covered by an insulating oxide layer, but also suffer from the morphological changes during Li cycling. Therefore, besides the chemical stability, a functional coating also requires seamless coverage with the control of thickness and elasticity to address the volumetric changes of Si anodes.

This paper will focus on the development of conformal, ultrathin coatings with desirable elastic properties and good conductivity by using molecular layer deposition (MLD). Based on the sequential, self-limiting surface reactions, MLD method allows for the integrating the organic fragments into metal oxide matrix, leading to the formation of hybrid organic-inorganic materials. The thin, conformal, and flexible MLD coating is able to penetrate the electrode's porous structure and covalently bind to available surfaces. This creates a strong, flexible network within the electrode that binds the materials and ensures sufficient contact area throughout cycling. Progress towards synthesis of elastic and conductive coating for Si anodes has been achieved by using MLD reactions between trimethylaluminum (TMA), glycerol.[4] The improvements in the electrochemical performance have been demonstrated for the coated Si anode with the polymeric aluminum alkoxide (alucone) coatings.[4] The chemical and physical properties of this surface coating are studied by using X-ray absorption spectroscopy and nanoindentation. In-situ characterization was applied to understand the impact of coating on structure, morphology and surface chemistry of electrode materials.[5] Due to its unique mechanical properties, the MLD alucone coating proves to be robust and resilient enough to accommodate the extreme volumetric changes of the Si nanocomposite electrodes, helping maintain an intimately linked conductive network and allowing for faster ionic and electronic conduction.

This work elucidates the significance of elastic, conductive, ultrathin, and conformal coatings for battery materials with large volume changes, while providing a platform for the development of advanced battery materials.

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