Graphite as a Long-Life Ca-Intercalation Anode and its Implementation for Rocking-Chair Type Calcium-Ion Batteries.

Herein, graphite is proposed as a reliable Ca-intercalation anode in tetraglyme (G). When charged (reduced), graphite accommodates solvated Ca-ions (Ca-G) and delivers a reversible capacity of 62 mAh g that signifies the formation of a ternary intercalation compound, Ca-G·C. Mass/volume changes during Ca-G intercalation and the evolution of in operando X-ray diffraction studies both suggest that Ca-G intercalation results in the formation of an intermediate phase between stage-III and stage-II with a gallery height of 11.41 Å. Density functional theory calculations also reveal that the most stable conformation of Ca-G has a planar structure with Ca surrounded by G, which eventually forms a double stack that aligns with graphene layers after intercalation. Despite large dimensional changes during charge/discharge (C/D), both rate performance and cyclic stability are excellent. Graphite retains a substantial capacity at high C/D rates (e.g., 47 mAh g at 1.0 A g s vs 62 mAh g at 0.05 A g) and shows no capacity decay during as many as 2000 C/D cycles. As the first Ca-shuttling calcium-ion batteries with a graphite anode, a full-cell is constructed by coupling with an organic cathode and its electrochemical performance is presented.