The voltage plateau with a sharp kink is clearly visible in the positive discharge curves, while it is shorter in time with no sharp kink in the negative discharge curves. Similar asymmetric properties are observed for a battery cell with OMIM-Br as shown in Fig. , and with HMIM-Cl (see Supplementary Information 2, Sections S2). These results demonstrate that the battery uses halogen ions (Cl -or Br -) shuttling between the electrodes to exploit the redox reactions of the Cu-triazole MOF for energy storage.