a–c, Scanning electron microscope (a) and energy-dispersive X-ray spectroscopy mapping (b, c) images of an LBC-G composite cathode, showing the morphology and elemental distributions of Br (b) and Cl (c) in the cathode layer after 5 full cycles. The distributions of Br and Cl are overlapping, indicating that the two salts are well mixed as a result of their close association during co-intercalation/de-intercalation. d, Ex situ XRD patterns of LBC-G cathodes collected from disassembled cells after the 5th charge and discharge. The disappearance of the LiBr and LiCl peaks and the appearance of the GIC peaks of the LBC-G cathode confirm the BrCl intercalation reaction at the fully charged state, whereas the typical patterns of crystalline LiBr and LiCl at the fully discharged state suggest that solid LiBr and LiCl are reformed after de-intercalation of halogen anions from graphite. The (002) peak of graphite, which has very high intensity, is cut off to show the other peaks. Theta, diffraction angle. e, The potential of the LBC-G cathode during discharge, open-circuit relaxation during a 40-h rest, and charging at 0.2 C. The complete recovery of the charge capacity in the next cycle shows that all of the active LiBr and LiCl material was well confined in the LBC-G cathode and there was no capacity loss during the long rest. f, The open-circuit voltage decays in the 40-h rest of the LBC-G cathode at the fully charged state of 4.5 V at 0.2 C. Self-discharge was evaluated by comparing with the Coulombic efficiency and the capacity loss after resting.