  In this study, we conducted experiments on CaC6 for elucidating the
Na-catalyzed formation mechanism and achieving rapid mass synthesis of graphite
intercalation compounds (GICs). Rapidly synthesized CaC6 was characterized by
analysis of its crystal structure and physical properties. We found that the
formation of the reaction intermediate Na-GIC (NaCx, x = 64) requires a larger
amount of Na than is intercalated between the graphite interlayers. The
requirement for excess Na may provide insights into the mechanism of
Na-catalyzed GIC formation. A Na-to-C molar mixing ratio of 1.5-2.0:6 was
suitable for the efficient formation of CaC6 under heat treatment at 250{\deg}C
for 2 h, and the catalytic Na remaining in the sample was demonstrably reduced
to a Na:Ca ratio of approximately 3:97. The upper critical field Hc2 was
enhanced approximately three times compared to those of previous reports. Based
on X-ray diffraction and experimental parameter analysis, we concluded that the
enhancement of Hc2 was attributed to the disordered stacking sequence in CaC6,
possibly because of the rapid and low-temperature formation. Physical
properties derived from specific heat measurements were comparable to those of
high-quality CaC6, which is slowly synthesized using the molten Li-Ca alloy
method. This study provides new avenues for future research and exploration in
the rapid mass synthesis of GICs as practical materials, for applications such
as battery electrodes and superconducting wires.
