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. © 2019 American Chemical Society.