Graphite as a Long-Life Ca2+-Intercalation Anode and its Implementation for Rocking-Chair Type Calcium-Ion Batteries

Herein, graphite is proposed as a reliable Ca2+-intercalation anode in tetraglyme (G(4)). When charged (reduced), graphite accommodates solvated Ca2+-ions (Ca-G(4)) and delivers a reversible capacity of 62 mAh g(-1) that signifies the formation of a ternary intercalation compound, Ca-G(4)center dot C-72. Mass/volume changes during Ca-G(4) intercalation and the evolution of in operando X-ray diffraction studies both suggest that Ca-G(4) intercalation results in the formation of an intermediate phase between stage-III and stage-II with a gallery height of 11.41 angstrom. Density functional theory calculations also reveal that the most stable conformation of Ca-G(4) has a planar structure with Ca2+ surrounded by G(4), 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(-1) at 1.0 A g(-1) s vs 62 mAh g(-1) at 0.05 A g(-1)) and shows no capacity decay during as many as 2000 C/D cycles. As the first Ca2+-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.