Electrochemical Chlorozincate Anion Intercalation\ninto Layered Carbon Materials for the Cathodes of Aqueous Zinc Metal\nSecondary Batteries

Aqueous\nzinc metal secondary batteries (ZSBs) are expected to be\nnext-generation secondary batteries, and it is important to explore\ncathode materials and electrolyte solutions that exhibit excellent\nelectrochemical properties for their practical use. In this study,\nwe employed a layered carbon material named graphene-like graphite\n(GLG) as a cathode active material and a concentrated aqueous zinc\nchloride solution as an electrolyte solution, and its electrochemical\nanion intercalation reaction was investigated. As a result, GLG obtained\nat 300 °C of thermal treatment (GLG300) exhibited lower anion\nintercalation potential and better Coulombic efficiency in ZnCl<sub>2</sub>·2.33H<sub>2</sub>O compared to graphite and GLG obtained\nat 700 °C. X-ray diffraction measurement suggested that GLG300\nformed a stage-1 intercalation compound at 1.8 V vs Zn<sup>2+</sup>/Zn, and extended X-ray absorption fine structure analysis revealed\nthat the intercalated anion was hydrated [ZnCl<sub>4</sub>]<sup>2–</sup>. The initial discharge capacity of GLG300 was approximately 170\nmAh g<sup>–1</sup> in the potential range of 0.5–2.2\nV with a current density of 20 mA g<sup>–1</sup>. The charge–discharge\ncycling test showed that GLG300 had good reversibility, the discharge\ncapacity remained above 110 mAh g<sup>–1</sup>, and the Coulombic\nefficiency approached nearly 100% at the 50th cycle. These results\ndemonstrated that the system using GLG300 and concentrated aqueous\nzinc chloride solution exhibits excellent cathode properties as aqueous\nZSBs and showed great promise for their future practical use.