The positive discharge time for the cell with OMIM-Cl extends over 1 hour after charging for 3.8 hours. With the bottom axis in Fig. , the data as a function of the specific capacity in units of mAh/g correspond the galvanostatic charge-discharge profiles. The specific capacity of the cell reaches 10.5 mAh/g which is 21 % of a theoretical specific capacity of 49.9 mAh/g expected for the MOF that accommodates one Cl -per trinuclear unit. This is increased to 33 %, taking a mulliken charge of -0.64 for the chlorine atom into account (see Supplementary Information 2, Section S3). If all water molecules coordinated to copper atoms were replaced by Cl -, the specific capacity can reach as high as 300 mAh/g. The power density of the cell increases monotonically with charging time up to ≈ 7 Wh/kg. The coulombic efficiency values are 28.6, 28.4, 28.1 and 26.6% after charging at a positive current of +50 µA for 0.5, 1.0, 2.0, and 3.8 hours, respectively.