Exfoliation of Electrolyte-Intercalated Graphene: Molecular Dynamics Simulation Study

Ethylene carbonate (EC) is one of the most common electrolyte for lithium ion batteries, but it has a narrow working temperature range. Despite the structural similarity, propylene carbonate (PC) has a wider working temperature range as an electrolyte, but it induces exfoliation of the graphite anode. To understand the different behavior of EC and PC electrolytes at the atomistic level, we performed molecular dynamics (MD) simulations of electrolyte intercalated graphene sheets. We observed no diffusion of electrolyte between graphene sheets when interlayer distance is less than 6 A, but both of EC and PC form monolayer between graphene sheets with comparable density when interlayer distance is 7-8 angstrom. Because of the size difference, the intercalated PC molecules induce a longer separation distance between graphene sheets compared to that of EC. The longer separation with PC intercalant induces more frequent sliding-exfoliation movement. We found that the exfoliation diffusion coefficient of the graphene sheet with PC intercalant is similar to 200 times larger than that with EC intercalant. One graphene diffuses and exfoliates from other graphene through sliding displacement rather than vertical separation because of steric interaction with electrolyte molecules in the bulk phase. For calculating the free energy changes of exfoliation, we constructed potential of means force using steered molecular dynamics simulations and found that the energy barrier of exfoliation of EC intercalated graphene sheets is similar to 45 kcal/mol where it is similar to 4 kcal/mol for PC intercalated graphene sheets. We also analyzed the static and dynamic properties of electrolyte confined between two graphene sheets. The self-diffusion coefficient of confined PC is larger than that of EC, but smaller in the bulk phase. We also found that the decaying of the dipole rotation autocorrelation of confined electrolyte is slower than that in the bulk phase. The dynamic properties of the graphene in two different electrolytes reported in this paper can be used for designing new anode materials with better performance.