Herein, we applied a novel superconcentrated aqueous electrolyte in calcium-ion batteries for the first time to improve electrochemical performance by decreasing the hydration number and radius of calcium ions. Their charge/discharge capacities with the superconcentrated electrolyte were ca. 13% higher than that with the dilute electrolyte. The cycling performance improved quite remarkably in the superconcentrated electrolyte, because of the suppression of structural collapse of copper hexacyanoferrate used as the electrode. The results have been confirmed through X-ray diffraction results.