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

Abstract

Abstract Herein, graphite is proposed as a reliable Ca 2+ ‐intercalation anode in tetraglyme (G 4 ). When charged (reduced), graphite accommodates solvated Ca 2+ ‐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 ·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 Å. Density functional theory calculations also reveal that the most stable conformation of Ca‐G 4 has a planar structure with Ca 2+ 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 Ca 2+ ‐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.