A Study Of Lithiation Mechanism Of Si-graphene Composite Anode For Lithium-ion Batteries

Lithium-ion batteries?LIBs?have attracted great attention in recent years due to their wide range of applications.As a naturally abundant element,Silicon is a promising anode material for lithium-ion batteries.The reason why Si has attracted considerable attention is that it has the highest theoretical specific capacity among all exiting anodes,which can reach 4200mAhg^-1.Unfortunately,major obstacle hindering practical application of silicon anode is associated with its large volume change during cycles of charging/discharging.This volume change makes Si particles susceptible to rupture and break away from electric contacts,and furthermore difficult to maintain a stable solid electrolyte interphase?SEI?,resulting in quick capacity fading and short cycle life.Various approaches have been employed to overcome this obstacle.One of them is to prepare Si into various Nano scale,such as nanoparticles,nanowires and nanotubes,and then inlay these nanostructures inside carbon cages to form Si core-carbon shell composite structures,and then,an extra space is created between Si core and carbon shell to accommodate volume expansion during cycles of charging/discharging.Graphene has attracted enormous attention because of its two-dimensional?2D?crystal structure with atomic thickness,unique electronic structure,high intrinsic mechanical strength,high surface area,and superior electronic conductivity.The carbon component in the Si/Carbon composites can enhance the conductivity to maintain the electrical contact of the electrode with the current collector.In this paper,first,the mechanism of Li adsorption on a Gra/Si system is investigated using density functional theory.Based on the calculated results of binding energies,charge transfer,charge density difference,mean squared displacement and diffusion constant,it concluded that the incorporation of graphene can significantly enhance the electrochemical performance of silicon as high performance anode material,which consists with the experimental results.Moreover,the origin of irreversible capacity of Gra/Si system is investigated.The average bond lengths of Li-Si and Li-C near the Gra-Si interface in fully lithiated structure are significantly shorter than those in the structures with reversible lithium storage,it related to the formation of Si-C bonds under large volume expansion of silicon,which is also indicated by pair distribution functions.Further study on atomistic-scale mechanism of the interaction between Li and Graphene-Silicon?Gra/Si?system is very challenging for current experimental technologies.The lithiation mechanism of graphene-silicon electrode is investigated by first principle calculations.For the second part,based on the calculated results of density of states,radial distribution functions,formation energies and voltage curves,it concluded that Si/Gra with higher Si concentration is more energetically favorable for Li incorporation,which consists with the experimental results.Then the reversible/irreversible storage mechanism is investigated further,the formation of Si-C bonds near silicon-graphene interface and C-Li bonds near graphene layers are considered as the main origin of irreversible capacity loss.Based on the conclusions,the reversible capacities for the structures with different silicon contents are predicted and verified by experimental results.The proposed computational frame can be used to evaluate and design nanocomposite anode materials for lithium-ion batteries.