Si-CNT/Reduced Graphene Oxide Nanoheterostructures As High-Performance Lithium-Ion Battery Anodes
Authors: Mahlendorf Falko , Xiao Lisong , Sehlleier Yee Hwa , Dobrowolny Sascha , Heinzel Angelika , Schulz Christof , Wiggers Hartmut 
Published: February 2020

In the recent years, rechargeable lithium-ion batteries (LIBs) have gained in importance for electronic devices and electric vehicles. Thus, research and development focuses on improving energy and power densities as well as durability of LIBs. Currently, commercially available graphite anodes with a specific capacity of 372 mAh g–1 are used, but these cannot satisfy the abovementioned demands. Silicon is a very promising candidate as an anode material due to its high theoretical capacity of 3579 mAh g–1 at room temperature (according to the formation of the Li15Si4alloy). However, this high specific capacity owing to host up to 3.75 lithium atoms per silicon atom leads to extreme volume expansion up to 300% during lithiation, which results in pulverization and delamination of the electrode material after few cycles. Various approaches have been conducted to overcome these issues e.g. by using nanosized active material or carbon-based silicon composites. Nanosized structures can shorten the diffusion pathways of the lithium-ions, thus facilitating rapid lithiation and delithiation processes. In addition, smaller sized structures can also significantly reduce the inner-mechanical stress of the materials during lithiation/delithiation of the active material. Carbon-based additives, e.g. carbon nanotubes (CNTs) for forming nanocomposites with silicon nanostructures can not only improve the electrical conductivity of the silicon anodes, but can also effectively accommodate the large volume changes of silicon nanoparticles (Si NPs) during the electrochemical reactions.

ln this study, we propose a new strategy using reduced graphene oxide (rGO) in combination with a stabilized Si-CNT hybrid to generate a robust, highly conducting nanoheterostructure to further enhance the stability and the electrochemical performance of silicon-based electrode materials. Si NPs were synthesized in the gas phase by decomposition of monosilane in a hot-wall reactor. Si NPs and CNTs were functionalized and reacted to form the Si-CNT hybrids, utilizing the formation of peptide bonds between amine-modified Si NPs and the carboxyl-functionalized CNTs. This chemical linking is the first step to improve the binding strength between the Si NPs and an electrically conducting network. The Si-CNT hybrid was combined with GO to self-assemble to form a robust nanoheterostructure, which is subsequently reduced. Si NPs, CNTs, and rGO sheets play important roles in this unique nanoheterostructure as LIB anode. Si NPs provide high capacity, rGO ensures high electrical conductivity for the entire structure and provides sufficient void spaces to buffer the volume changes of the Si NPs, and CNTs act as the scaffolding to bind the Si NPs and provide additional electrically-conductive channels.

Here we present electrochemical investigations of silicon anodes based on Si-CNT/rGO nanoheterostructure nanocomposite material. The electrode preparation is based on a well-established wet chemical doctor blade manufacturing process using a water based binder. The nanocomposite material shows a high reversible capacity of 1665 mAh g–1 with good capacity retention of 88.6% over 500 cycles when cycled at 0.5 C, that is, a 0.02% capacity decay per cycle. The high-power capability is demonstrated at 10C (16.2 Ag-1) where 755 mAh g–1 are delivered. Full cell experiments demonstrate the applicability of improved silicon anodes for lithium-ion batteries.