Safe and stable Zn-lignin batteries with a biopolymer based hydrogel electrolyte†

The safety risks associated with organic solvent-based batteries for stationary energy storage have driven scientists to reconsider aqueous electrolytes combined with ultra low-cost materials. In this context, zinc (Zn) metal and biopolymer lignin are certainly among the most abundant and economical electroactive materials on Earth, displaying compatibility in their redox activity to fit the stability window of aqueous electrolytes. But, up to now, the electrolyte solutions in those systems incorporate fluorinated organic salts or bio-ionic liquids, both of which are detrimental to the environment and expensive. In this work we use a state-of-the-art lignin electrode based on catechol functionalized lignin (LC) nano-composited with carbon black (C) and a biopolymer hydrogel electrolyte based on agarose with non-fluorinated Zn salt. The optimization of the hydrogel's composition was realized by reducing the amount of free water by promoting its bonding with additional glycerol. The hydrogel facilitates the growth of Zn in the (002) plane, preventing dendritic formation. The highest discharge capacity of 79.7 mA h g_LC⁻¹ was obtained at 0.05 A g⁻¹ charge/discharge rate for the buffered 3% agarose hydrogel electrolyte containing 25% glycerol with 1 M Zn²⁺. The hydrogel containing 25% glycerol with 1 M Zn²⁺ and 1 M K⁺ in the absence of buffering shows the best cycle performance with 78% capacity retention after 26 000 cycles at 1 A g⁻¹ with a capacity of 58 mA h g_LC⁻¹ at 0.05 A g⁻¹. This study shows the possibility of a safe, affordable, bio-based environmentally friendly energy storage system that has the potential for large-scale applications.