An Organic Cathode Based Dual-Ion Aqueous Zinc Battery Enabled by a Cellulose Membrane

Aqueous zinc batteries (AZBs) have recently garnered considerable interest due to their potential cost benefit and safety. Use of an abundant and high-capacity zinc metal anode and inexpensive and safe aqueous electrolytes make them suitable for large-scale energy storage applications. However, the sluggish solid-state diffusion of divalent zinc puts stringent requirements on the choice of inorganic host structures. Organic solids, which are presumably sustainable, offer unique versatility, as they possess a soft lattice for facile ionic diffusion and diverse redox functions. Here, we tap into that prospect with a novel organic cathode, namely, 1,4 bis(diphenylamino)benzene (BDB), which delivers nearly a 2-electron redox capacity of 125 mA h g–1, at an average voltage of 1.25 V in an AZB. The two tertiary nitrogens reversibly oxidize/reduce in two steps, with accompanying anion insertion/release from/into a highly concentrated aqueous electrolyte possessing a high oxidative stability. Reversible plating/stripping of zinc on the anode side complements the anion (de)insertion on the cathode side, yielding a rechargeable dual-ion system. Paired with a cellulose nanocrystal membrane to suppress the active material diffusion into the electrolyte, the BDB cathode delivers 112 mA h g–1 of capacity with 82% retention after 500 cycles at a 3C rate (1C = 130 mA g–1), and 1000 cycles with 75% capacity retention at a 6C rate, at nearly 100% Coulombic efficiency. Reversible electrochemistry is accompanied by two reversible biphasic transformations and reversible chemical evolution between BDB, BDB+, and BDB2+ species, as made evident by operando X-ray diffraction and solid-state operando ultraviolet–visible spectroscopy studies. These results highlight a new avenue and understanding of organic cathode hosts development for AZBs.