(120d) Yolk-Shell Composite Nanostructured Electrodes for High Capacity Lithium-Ion Batteries

Advances in technologies for lithium-ion batteries (LIBs) have been so spectacular in recent years that they have become one of the most popular sources for portable computing and telecommunication equipment. In the last five years there has been considerable work on the electrochemical properties of electrode Materials for Lithium Ion Batteries (LIBs). In particular, a much work has been done on the electrochemical properties of anode materials which have higher specific capacity than graphite, the most common anode material in use today. Among the top candidates for high capacity anodes is silicon, which has the highest know capacity of any material.

However, there are two well-known issues with utilizing silicon in anodes, both related to the fact that silicon undergoes a high expansion ratio upon lithiation (during the cycling of a battery). As the battery is charged (lithiated) expansion occurs in the anode, and as the battery is discharge (delithiated) contraction occurs in the anode. The volumetric expansion is only 10% percent for graphite, but is approximately 300% for silicon. The high volumetric expansion and contraction in the silicon can induce mechanical stress and leads to reformation and breakage of the solid electrolyte interface (SEI) during cycling producing loss of capacity and stability. The SEI layer is a polymeric boundary which naturally forms from the electrolyte at the surface of the anode and is known to further protect the electrolyte from further reactions at the surface of the anode. However, if the expansion ratio of the anode materials is sufficiently high, it appears that the SEI layer is broken and reformed to some extent during each cycle, thus leading to loss of efficiency and cycling stability.

Although there has been considerable research into using silicon, these works suffer from several drawbacks associated with the breakage of the Solid Electrolyte Interface (SEI) during repeated cycling. In this work we developed a new method that avoids breakage of the SEI at the silicon surface. We first created novel core shell/hollow nano-carriers made with graphene oxide and then incorporated silicon nanoparticles as the high capacity electrochemically active material which has the potential to greatly increase the capacity of these materials. By incorporating using yolk-shell particles, which have a void space between the silicon nanoparticles and the ultra-thin graphene oxide shells, the expansion of the silicon can be accommodated.

We solidify the composite to effectively trap the electrochemically active silicon nanoparticles within the graphene oxide shells, preventing segregation of the active components from the conductive matrix. These composites are thermally reduced to convert GO to a graphitic structure capable of providing high electrical conductivity as well as high lithium ion conductivity.

We present the results of both thermally reduced and chemically reduced graphene oxide based anodes. An electrode with high cycling stability was made which exhibited a storage capacity of 1450 mA h/g after 200 cycles and greater than 1300 mAh/g after 350 cycles, with a coulombic efficiency reaching 99.9%. For some anodes, capacities greater than 1200 mA-hr/g were possible at a current of 3200 mA/g. The electrode materials were characterized via laser diffraction, battery cycling, FE-SEM, TEM, Raman, and XRD.