Silicon underpotential deposition on defective graphene and its relevance for lithium-ion batteries: First-Principles studies

G.L. LUQUE, ; M. OTERO, ; E.P.M. LEIVA; M.L. URQUIZA, ; D. BARRACO,

Congreso; 3rd International Workshop on Lithium, Industrial Minerals and Energy; 2016

In terms of sustainable development and environmental issues, the use of efficient storage systems is an increasingly critical point. The world has undergone a great technological revolution in the past 20 years led by lithium ion batteries, due to their relatively high energy density. However, in order to meet the energy demand of our modern life, a new generation of lithium batteries, more powerful, efficient and cheaper, is desirable. Silicon has shown to be a very promising anode material to fulfill the needs of a lithium-ion battery. It has a high theoretical capacity, of around 3579 mAh/g, low cost and natural abundance. However this material runs with the great disadvantage that during the formation of a Si-Li alloy corresponding to the insertion of Li in the anode for the charging process, it suffers a change of volume that can reach 380%. This expansion, followed by contraction of the material upon battery discharge, quickly leads to irreversible damage of the electrode, causing a rapid decrease in capacity during cycling. Furthermore, Si usually has a low electrical conductivity. In this regard, some current studies propose the addition of Si in graphene sheets to improve conductivity and provide a high flexibility to tolerate volume changes. In the present work first-principles calculations are undertaken to analyze the properties of carbon-Silicon hybrid materials consisting of silicon modified graphene and defective graphene to evaluate the stability of the structure and their interactions with lithium. We evaluate the effect of the presence of simple (SV) and double vacancy (DV) defects in the interaction of silicon and of these hybrids structures with lithium. We discuss this information in the context of underpotential deposition, which could be relevant to seek an electrochemical alternative for the controlled decoration of graphenic surfaces. The ab initio calculations were performed with Quantum Espresso package. For the calculations we used ultrasoft pseudopotentials with the Perdew-Wang approximation for the exchange correlation functional in the PW91 functional. The simulation box consisted in a 4x4 graphene supercell containing 32 C atoms with periodic boundary conditions in all directions. We found that the binding of Si clusters to defective graphite surfaces are very strong. This binding energy, could contribute to stabilize the interaction between Si nanostructures and graphene layers, thus being very beneficial to stabilize composites used to store lithium-ions. In the case of the incorporation of Si atoms into double vacancies, it is found that the Si atoms increase the density of states at the Fermi level, thus improving the conductivity of the system, and probably its catalytic activity.Regarding the lithiation potential structures, we found that in the case of SV and DV-defective graphene sheets where the silicon clusters form stable structures, the lithiation potential shows a positive value indicating a favorable reaction. Results also show that the structures that present the larger values of lithiation potential are the double vacancy type. So, we can state, that the presence of this kind of defect not only promotes the formation of stable hybrid Si-graphene structures, but also favors the lithiation process.