Rechargeable batteries are ubiquitous, and their usage is projected to grow tremendously over the years. They are vital to curbing the fossil fuel dependency and are destined to play a significant role in the future energy landscape. However, rising demand will eventually strain raw material resources, and potentially cripple the development and deployment of associated technologies. In this regard, alongside materials and methods involving sustainable resources, greener manufacturing and recycling potential, renewable resource derived biodegradable materials, i.e., natural biopolymers-are attracting interest for sustainable and eco-friendly future batteries. While they are being studied for nearly all aspects of a battery cell development, their exploration as an electrolyte component has been the subject of widespread investigations. These studies include, but are not limited to, their utilization as the building block for electrolyte wettable and more thermally resistant separators, as an alternative to synthetic polymers in gel polymer and solid polymer electrolytes, and as functional membranes to inhibit the loss of electroactive species in diverse battery chemistries. Here, a summary of natural biopolymer-based electrolytes and separators development that incorporate novel concepts is presented to tackle specific issues in various battery systems and future perspective underpinning their further advancement.

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 BDB 2+ 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.

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