Hybrid chitosan-MOF gel electrolytes for durable aqueous zinc-ion batteries

Efficient energy storage with non-critical materials are increasingly important for decarbonization strategies. Biobased polymers are viable alternatives to the conventional liquid electrolyte and separator pair with safer and stable gel-like electrolytes. We demonstrate the suitability of chitosan, a polysaccharide found in arthropod exoskeletons, fungi, and other living organisms, for aqueous hybrid gel-electrolytes in zinc-ion batteries (ZIBs). Inclusion of a zinc-containing metal-organic framework (MOF), Zn-MOF-74, enhances the electrochemical performance. The hybrids fabricated by acetic acid dissolution and coagulation, combine micron-meter pores from chitosan with nanometer-sized pores of MOFs. The electrolytes outperformed conventional glass microfiber separators regarding electrolyte wettability, operating at current densities of 20 mA·cm^-2 as opposed to the glass microfiber separator that underwent short-circuiting at 0.2 mA·cm^-2. When assembled into Zn||α-MnO₂ cells, the chitosan-MOF gels enable 106 mA·h·g^-1 after 50 cycles at increasing current rates from 0.1 to 1.0 A·g^-1, representing a 71 % increase over glass microfiber. To understand the origin of the improved cycling stability and short-circuit resistance, in-situ electrochemical impedance spectroscopy and post-mortem analyses of the electrodes and electrolytes were done using electron microscopy, X-ray diffraction and elemental mapping. This study demonstrates that chitosan-MOF gels have the attributes for efficient aqueous electrolytes, enabling ZIBs with long-term stability.