Roles of film-forming additives in diluted and concentrated electrolytes for lithium metal batteries: A density functional theory-based approach

Abstract

Electrolyte for lithium-ion batteries should facilitate Li+-ion transport and control solid electrolyte interface (SEI). To improve their performance, most of the researches focused more on the effect of the solvation structure of bulk electrolytes on the SEI formation rather than adsorbed species on the Li-anode surface in the past. However, it was found recently that the adsorbed species on the Li-anode surface are strongly connected with the formation of SEI. In this study, diluted [Li+ fluoroethylene carbonate (FEC)/ethylene carbonate(EC)n, Li+(EC)n (n = 1–4)], and concentrated (Li+FEC(EC)n(PF6−) (n = 0–3) electrolyte are investigated in bulk phase and on Li-anode surface using density functional theory (DFT). The stability of the clusters is investigated by their solvation energy and Gibbs free energy change. From the Gibbs free energy change of the clusters, it is found that Li+(EC)4 cluster is the most stable structure in the bulk phase of the diluted electrolyte in the presence or absence of FEC. Pure EC solvated Li+-ion species in the dilute electrolyte in bulk solution is more stable, but the adsorption energy of pure EC-solvated Li+-ion species on Li-anode surface was found weaker than EC and FEC co-solvated Li+-ion species in the diluted electrolyte. DFT calculation suggesting that the dominant species on the Li-anode surface are found Li+FEC(EC)3 and Li+FEC(EC)2PF6− in the diluted and concentrated electrolyte, respectively. The decomposition of the anion-rich adsorbed species in the concentrated electrolyte is suggested to form a better SEI to stabilize Li-anode compared to anion-free adsorbed species in a diluted electrolyte.