Mesoporus Silicon through Magnesium Reduction of Polymer Templated Silica for High Power Li-Ion Batteries

This work describes the synthesis and electrochemical characterization of mesoporous silicon for high power Li-ion battery applications. Silicon is an attractive Li-ion battery anode material for its high energy density, high elemental abundance and non-toxicity. However, full lithiation of silicon is accompanied by a 300% volume expansion which pulverizes the particles leading to rapid material failure. We have addressed these issues with silicon by developing a highly tunable synthesis method to produce mesoporous silicon from ordered mesoporous silica. This synthesis method leads to micron sized grains consisting of a 3-D interconnected silicon network that should accommodate the electrochemical induced volume expansion while the nanocystalline pore walls provide short ion diffusion lengths for high rate applications.

The ordered mesoporous silica starting material is synthesized by evaporation induced self-assembly of sol-gel type precursors and a di-block copolymer. The mesoporous silicon is produced through reduction of the silica precursor with magnesium metal at 650 °C followed by selective chemical etching. The particle morphology and the pore size of both the silica and the silicon can be tuned over a broad range, and have been characterized by small angle scattering, high angle X-ray diffraction, nitrogen porosimetry, and SEM imaging. This synthesis technique produces 1-10 µm silica particles with a cubic lattice of 15 – 20 nm pores and 10 - 15 nm thick pore walls. After magnesium reduction, the porous polycrystalline silicon retains the microscale morphology and the porous nanostructure of the parent material. The silicon pore walls consist of ~30 nm crystallites which are optimal for high power density electrochemical applications.

Magnesium reduction of silica is an effective method to reduce silica while preserving the nanostructured architecture, but little is understood about reaction mechanism. In an effort to control the nanoscale architecture of silicon we have investigated the reaction mechanism. We have performed in-situ X-ray diffraction during the reduction process combined with Raman spectroscopy to reveal a multi-step reaction pathway which leads to the mesoporous crystalline silicon final product. This study demonstrates that the reaction of mesoporous silica and magnesium metal first forms amorphous silicon that further reacts to form crystalline magnesium silicide. The remaining mesoporous silica is reduced by both the magnesium metal and the magnesium silicide to produce mesoporous crystalline silicon.

The electrochemical properties of mesoporous silicon have been characterized by galvanostatic cycling and cyclic voltammetry.  Galvanostatic measurements show capacities on the order of 1700 mAh/g while the nanocrystalline silicon pore walls and interconnected 3-D framework led to an extremely high rate capability. Over 40% of the capacity, or 720 mAh/g, can be accessed in 150 seconds. In light of the high capacity and highest reported power density for mesoporous silicon, this material is an exciting candidate for the next generation of high performance Li-ion batteries.