Extended Data Fig. 7: Representative structures of stage-I [Br_0.5Cl_0.5]C_3.5 complex obtained from DFT simulations.

All structures have intercalation voltages within 0.02 V per ion of a structure with homogenous Br–Cl–Br–Cl bond lengths of 2.45 Å (top left). The bottom right structure is simulated on the basis of the reported Br₂ structure²⁸. Quantum chemistry calculations performed on a Cl–Br∙∙∙Cl–Br cluster surrounded by conductive polarized continuum also yielded a zig-zag configuration for the Cl–Br∙∙∙Cl–Br complex with a Cl–Br∙∙∙Cl–Br angle of around 110°, which has lower energy than the linear Cl–Br∙∙∙Cl–Br configuration by 0.1 eV according to MP2/aug-cc-pvTz and PBE/aug-cc-pvTz calculations. The most stable geometry obtained from these cluster calculations is similar to that found in the stage-I complexes shown above.