Computational insights into working mechanism of LiPF6 graphite dual-Ion battery

Abstract

emerging field of dual-ion batteries (DIBs) show quite better advantages compared to the commercial Li-ion batteries in terms of high working voltage, high energy density and low cost. To improve performance, the on-going experimental studies of DIBs require a clear understanding of the reaction mechanism occurring during both charging and discharging processes as well as the resulting structural variation in the involved anions and cathode system. Therefore, in this work using the first principle calculations, we have studied the staging mechanism of PF6- intercalation from organic electrolyte into graphite. We have modelled four different stages stage-1, stage-2, stage-3 and stage-4 of PF6- intercalated graphite with four varying concentrations. The binding energy characteristics of all stages indicates the favorable intercalation of PF6- into graphite. The cell voltage is determined in range 5.28 to 5.49 V at specific capacity 124 mAhg-1, in good agreement with experimental values. Through charge transfer analysis, we found that there is significant amount of charge transfer from graphite to PF6-, which clarify the PF6- intercalation into graphite is charging process. For high charging/discharging rates battery electrodes ought to have metallic character (for fast electron transfer) and low diffusion barrier (for fast ion transfer). Our density of states calculations confirms the metallic character of PF6- intercalated graphite system and from the diffusion barrier calculations, we observe that PF6- diffuse with an energy barrier of 0.14 eV, low enough to provide trouble free diffusion. These all 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.