Differences between cation and anion storage electrochemistry of graphite and its impact on dual graphite battery

Abstract

The redox-amphoteric nature of graphite is tactically utilized to store cations and anions simultaneously in a dual graphite battery (DGB). The major differences between cation and anion storage electrochemistry of graphite are identified here and their impacts on the electrochemical performances of DGBs are analyzed. All the differences are found to originate primarily from ionic size mismatch (76 p.m. of Li+vs. 254 p.m. of PF6−) that further induces kinetic and thermodynamic limitations upon intercalation. The electrochemical studies reveal that the anion intercalation lags behind the cation because of a higher energy barrier for the interconversion among (PF6)Cx stages, the occurrence of a valley-type region in the discharge profile that delays deintercalation by 500 mV, dynamic changes in organic-rich cathode electrolyte interphase, and sluggish diffusion inside graphite bulk. In contrast, cation intercalation affects DGBs' performance negatively due to higher desolvation barriers and lithium plating on the anode surface at higher current densities. Therefore, the cathode limits the capacity and cycling efficiency, whereas the anode restricts cycle life and power density. This work also explores the strategies to mitigate the degradation pathways by electrolyte modification.