Si-Fe-O-based Anodes for Lithium-ion Batteries

Abstract

Conventional lithium-ion batteries with graphite anodes are approaching their capacity limit. Silicon-based anode materials are expected to be incorporated into the next generation lithium ion batteries because of the high theoretical capacity of Si. However, issues regarding the huge volume expansion/contraction of Si upon lithiation/delithiation need to be alleviated. Nanostructured Si-M (M = transition metal) alloy negative electrode materials have received lots of attention since they show good cycling performance and suppressed volume expansion. However, Si-M alloys still suffer from side reactions with electrolyte during cycling. SiOx (Silicon oxide) is another focus for Li-ion anode research. SiOx has relatively low volume expansion, less side reactions with electrolyte, and high capacity retention during cycling. However, the first coulombic efficiency of SiOx is low because of the irreversible formation of lithium silicates. For practical application of Si-based alloys, anode material design and optimization efforts are required. In this thesis, the synthesis, microstructure and electrochemical properties of ball milled Si85Fe15Ox and SiFexOy alloys are investigated. Specifically, Si and Fe alloys are ball milled in air for different amounts of time to make Si-Fe-O alloys. The effects of oxygen and iron content on structure and electrochemistry were studied. These alloys also have high thermal stability, which makes them compatible with the chemical vapor deposition process, enabling the formation of composite materials with further enhanced performance. It was demonstrated that the SiFe0.20O0.39 alloys can be embedded into spherical natural graphite and CVD-coated to create high performance composite anode particles. The resulting carbon-coated graphite composite particles can cycle well even without the use of advanced binders or electrolyte additives.