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 (G₄). When charged (reduced), graphite accommodates solvated Ca²⁺-ions (Ca-G₄) and delivers a reversible capacity of 62 mAh g^-1 that signifies the formation of a ternary intercalation compound, Ca-G₄·C₇₂. Mass/volume changes during Ca-G₄ intercalation and the evolution of in operando X-ray diffraction studies both suggest that Ca-G₄ 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-G₄ has a planar structure with Ca²⁺ surrounded by G₄, 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.