Computational Insights into the Working Mechanism of the LiPF₆–Graphite Dual-Ion Battery

Abstract

The emerging field of dual-ion batteries (DIBs) show better advantages compared to the commercial Li-ion batteries. Thus, the on-going experimental studies of DIBs require a clear understanding of the reaction mechanism as well as the resulting structural variation in the involved anions and cathode system. Therefore, in this work, using the first-principles calculations, we have studied the intercalation mechanism of PF6– intercalation from the organic electrolyte into graphite. The intercalation energy characteristics indicate the favorable intercalation of PF6– into graphite following the staging mechanism, also confirmed by X-ray diffraction simulations. PF6– intercalation relatively acquiring a small interlayer distance in graphite than AlCl4– and FSI– guarantees reduction in exfoliation of graphite to have a long battery cycle life, which is in accordance with the experimental reports (2000 cycles with 97.9% capacity retention). The cell voltage determined in the range 5.28–5.49 V having a maximum specific capacity of 124 mA h g–1 is in good agreement with experimental values. Through charge transfer analysis, we found that there is 0.97 |e| charge transfer from graphite to PF6–, which clarifies that PF6– intercalation into graphite is the charging process. Moreover, the metallic character of the PF6– intercalated graphite system and a small diffusion barrier of 0.14 eV indicate a constant electronic conductivity and better rate performance, respectively. These results provide the clear understanding of PF6– intercalation into graphite and also describe the role of staging behavior to obtain the precise values of electrochemical properties.