Processes to fabricate porous silicon and its use as feedstock for secondary battery electrodes

Abstract

Provided are processes to form microporous silicon useful as mi active material in an electrode of an electrochemical cell the processes including subjecting a mixture of silicon oxide and a metal reducing agent, optionally aluminum, to mechanical milling to form mechanically activated silicon oxide / aluminium, thermally treating the silicon oxide / aluminium to reduce the silicon oxide and form Si / Al2O3 and removing at least a portion of the alumina from the Si to form a microporous silicon. The resulting electrochemically active microporous silicon is also provided with residual alumina present at 15% by weight or less that demonstrates excellent cycle life and safety.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application depends from and claims priority to U.S. Provisional Application No. 62 / 189,826 filed Jul. 8, 2015, the entire consents of which are incorporated herein by reference.FIELD[0002]The invention relates generally to methods of producing anode active materials. More specifically, methods are provided for the production of microporous silicon that can be used as an anode active material in secondary batteries.BACKGROUND[0003]Microporous silicon has been widely produced in the semiconducting industry for over 50 years. Electrochemical etching has been the most investigated approach for the formation of microporous silicon for chip-based applications and has bees used to create highly directional mesoporosity and macroporosity. A number of techniques such as anodization, vapor etching, glancing angle deposition, lithographic etching, and photoetching are also suitable for Si wafer-based processing. Other methods have been used wi...

AI Technical Summary

Benefits of technology

The present patent provides a process for making electrochemically active silicon material using materials and processes with lower environmental impact than prior methods. The process involves subjecting a combination of powdered silicon oxide and aluminum to mechanical milling to form a mechanically activated silicon oxide / aluminum composite, thermally treating the composite by exposure to heat at a temperature of 500 degrees Celsius to 700 degrees Celsius under an inert or reducing atmosphere to form Si / Al2O3, and removing at least a portion of alumina from the Si / Al2O3 by exposing it to an etchant to form microporous silicon. The process is carried out at relatively lower temperatures and uses etchants that do not include fluorine. The resulting material has lower environmental impact and can be used in electrochemical cells. The process also includes a step of thermally treating the silicon oxide / aluminum powder by exposure to heat at a temperature of 500 degrees Celsius to 700 degrees Celsius under an inert or reducing atmosphere to form Si / Al2O3. The resulting material has a high cycle life and low initial capacity loss. The alumina in the material can be alpha-alumina, gamma-alumina, or both. The process can be carried out using the same materials and equipment as prior methods, making it suitable for industrial production.

Problems solved by technology

A great disadvantage to the etching approaches is the usage of the highly dangerous HF solution to create the pores on the silicon surfaces.

Producing highly porous structures at high volumes via etching techniques typically generates large quantities of unreacted or under-reacted solid silicon substrate as a waste product unless it is recycled.

In the case of the magnesiothermic reduction the major challenge is that the highly exothermic reaction would make it difficult to avoid sintering and collapse of the desired pore structure, particularly as the reaction is scaled up.

A second challenge regards applications where porous silicon is required with minimum oxide content.

Residual oxides may increase irreversible loss or initial capacity loss (ICL) of lithium in the battery.

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