First-principles study of solvation effects on propylene carbonate reduction on defective hard carbon, for advanced sodium-ion battery anodes.

Hard carbon (HC) is an attractive anode material for grid-scale sodium-ion batteries (SIBs), but its structural design must be improved through understanding the reaction mechanism. In this work, we investigate the adsorption characteristics of Na atoms with a propylene carbonate (PC) molecule, and the decomposition reaction mechanism of the Na-PC complex on a HC sheet, under PC solvent conditions using first-principles calculations. Using perfect and defective graphene cluster models, which include point defects such as mono-vacancy (MV), di-vacancy (DV) and Stone-Wales (SW), we calculate the binding energies of one or two Na atoms, revealing that the binding strength of the Na atom is in the order of SW < perfect < DV < MV and adding another Na atom lowers the binding energy due to the Na-Na interaction. For adsorption of the Na-PC complex, our calculations demonstrate that binding energies of Na2PC are lower than those of NaPC, where the PC solvent molecules enhance the binding strength of the adsorbate complex on the HC surface. Furthermore, we investigate the decomposition reaction mechanism of the PC molecule by calculating reaction heats and activation barriers, finding that one-electron reduction reactions are endothermic while two-electron ones are exothermic without reaction barriers at room temperature. These findings contribute to our atomistic understanding of the initial charge process of the HC anode and thus aid improvement of the first cycle efficiency of SIBs.