Joint Theoretical and Experimental Study on the Effects of the Salts in the Graphite-Based Dual-Ion Batteries

Graphite-based dual-ion batteries show great promise in the next-generation energy storage application. However, in-depth understanding of this novel class of batteries like the effects of the salts and solvents on battery performance is still lacking. In this work, we perform density functional theory calculations to investigate the effects of different salt ions, including four anions (BF₄^–, ClO₄^–, PF₆^–, and TFSI^–) and three cations (Li⁺, Na⁺, and K⁺), on the onset voltage of a dual-ion battery. It is shown that the size of the cation has a significant effect on the voltage whereas the anion size is of minor importance. The calculated onset voltage results agree well with the experimental results. For further analysis, we divide the overall discharging process into three independent steps (extraction, ionization, and solvation). We find the extraction and ionization energetics for each anion nearly compensate each other, resulting in the small differences of their net voltage contributions. Therefore, the solvation energy of the anion is the main contribution to the voltage difference among the different anions. However, the distinguishable and strong interaction between the cations and the graphene layers results in the significant contribution from the extraction and ionization steps to the onset voltage.