(71b) Core-Shell Graphene/Silicon Nanoparticles for Use As Lithium-Ion Battery Anodes

Although lithium-ion batteries have the highest specific energy capacity of any rechargeable battery chemistry in wide-spread use today the cycling stability and specific energy capacity of these batteries are still problematic for all-electric vehicles and a host of other high energy applications. Increasing the capacity of these batteries is crucial to keep pace with the demands of these new applications.

To decrease the weight of the anode, researchers have turned to materials with higher specific capacity than graphite, the most commonly used anode material in commercially available batteries. Silicon, with the highest known lithiation capacity of any material, has about 10 times the theoretical capacity of graphite. It has been noted that increasing the capacity of the anode up to about 1000 mAh/g greatly increases the total capacity of a lithium-ion battery cell.[2]

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.

Silicon has previously been used by a number of researchers, however simple admixture tend to have poorer cycling stability. Other more complicated techniques use hazardous materials or are difficult to scale up. Our approach consists of encapsulating silicon nanoparticles inside hollow graphene oxide membrane particles (HGOM) and thus, it avoids the uneven distribution of the silicon particles, where the HGOM particles act as nanoparticle carriers. When silicon nanoparticles are encapsulated inside these hollow sacks, they do not occupy the entire volume leaving some “void space” necessary to accommodate the physical change in volume of the silicon nanoparticles during cycling of the batteries. Such particles are often called “yolk-shell” particles to distinguish these from simpler core-shell particles in which there is no void space. In our method, the solid electrolyte Interface (SEI) forms around the graphene shells rather than around the silicon particles, which has a much smaller surface area providing larger stability and enhancement of capacity.

We have recently published a method to obtain these hollow graphene oxide membrane particles with controlled size and thus, controlled void space. Here, we propose a new facile method, for engineering the required void space and controlling the formation of the SEI around HGOM particles, which are assembled into anodes for Lithium ion batteries.

We employ a different technique based upon the newly discovered method of encapsulating silicon nanoparticles within graphene oxide shells, through the process of emulsification-diffusion, followed by sublimation of naphthalene and assembly into anodes. To our knowledge this method has never been accomplished before. This method has the potential to be scalable, while being able to control the void space within the core-shell structure. Although a sacrificial scaffold is used to create the void space, there is a great potential to recycle this material since no reactions occur during sublimation of the sacrificial scaffold.

Thermal reduction (pyrolysis) of the silicon/HGOM core shell particles yields highly conductive composites. Since the silicon nanoparticles have very high specific capacity, the resulting composite also has high specific capacity, thus increasing the specific capacity of lithium-ion batteries which employ these composites. We obtained an electrode with high cycling stability and a storage capacity greater than 1800 mA h/g after 200 cycles and greater than 1200 mA h g-1 after 450 cycles. The coulombic efficiency of the anode rapidly rose as cycling continued, eventually approaching 99.9%. The electrode materials were characterized via laser diffraction, zeta potential, EIS, cyclic voltammetry, FE-SEM, TEM, Raman Spectroscopy, BET, and, XRD.