Extended Data Fig. 4: Cell design and the physical and electrochemical characterizations of Te and Se electrodes.

a, Schematic drawing of our in-house-made cell. b-e, Scanning electron microscopy image and energy-dispersive spectroscopy (EDS) (c), thermogravimetric analysis (d), X-ray diffraction pattern (e), transmission electron microscopy and EDS mapping (Se, N, C) (b) of the Se composite prepared by pyrolysis of polyacrylonitrile/selenium. The weight fraction of selenium in this composite is confirmed to be ~62.5% from both EDS and TGA analysis. The selenium composite has a uniform distribution of Se, N and C, where the N is from the residue of pyrolyzed polyacrylonitrile. f, The first-cycle voltage-time trace and rate performance (varying charging rate and constant discharge at D/10) of the Se-free carbon material, prepared in the same way except without selenium. g, h, Voltage-time traces (g) capacity retention (h) of Se in EMIC-AlCl₃ at different temperatures and cycling rates, characterized by low capacity and fast fading even at very low cycling rates.