Superionic Transition in Mixed Conducting Graphite Intercalation Compounds CsC₈ and RbC₈

Mixed ionic-electronic conductors (MIECs) have attracted considerable interest in electrode/interface materials for solid-state batteries. Here, we investigate the ionic diffusion behavior of graphite intercalation compounds XC₈ (X = Cs, Rb) through first-principles calculations combined with ab initio molecular dynamics simulations. The results show that CsC₈ and RbC₈ undergo superionic transitions near 500 and 600 K, respectively, with corresponding ionic conductivities of 0.0127 and 0.0787 S/cm. Their electrical conductivities reached up to 10⁷ S/m. Notably, RbC₈ exhibits a stacking sequence transition from ΑαAβAγAδ to ΑαAβ at 500 K. The introduction of defects further reduces the superionic transition temperature to 400 K. Cs⁺ and Rb⁺ ions migrate via a vacancy mechanism along two pathways: crossing C-C bonds or diffusing from above carbon atoms to nearby hexagon centers. Additionally, both compounds show good thermal stability and mechanical properties. These findings offer new insights into the design of high-performance MIEC materials.