Revealing the anion intercalation behavior and surface evolution of graphite in dual-ion batteries via in situ AFM

Graphite as a positive electrode material of dual ion batteries (DIBs) has attracted tremendous attentions for its advantages including low lost, high working voltage and high energy density. However, very few literatures regarding to the real-time observation of anion intercalation behavior and surface evolution of graphite in DIBs have been reported. Herein, we use in situ atomic force microscope (AFM) to directly observe the intercalation/de-intercalation processes of PF₆^- in graphite in real time. First, by measuring the change in the distance between graphene layers during intercalation, we found that PF₆^- intercalates in one of every three graphite layers and the intercalation speed is measured to be 2 μm·min^-1. Second, graphite will wrinkle and suffer structural damages at high voltages, along with severe electrolyte decomposition on the surface. These findings provide useful information for further optimizing the capacity and the stability of graphite anode in DIBs.