Molecular crowding bi-salt electrolyte for aqueous zinc hybrid batteries

Aqueous zinc metal batteries (AZMBs) represent a promising option for a low-cost, safe and sustainable energy storage system for large-scale applications. However, substandard Zn metal anode performance (hydrogen evolution, dendrites) and cathode instabilities, attributed to water activity and/or water-induced side reactions, still plague their practical deployment. Herein, we investigate the performance of an aqueous Zn//LFP hybrid battery by using molecular crowding electrolytes (MCE) combining a low concentrated bi-salt (Zn(TFSI)2 and LiTFSI) electrolyte with a crowding agent, namely polyethylene glycol (PEG400). The composition of MCE electrolyte was optimized as a function of the PEG content and the molality of LiTFSI salt. The PEG confines water molecules, thereby reducing the fraction of "free water" molecules as evidenced by combined LSV, Raman, DSC and molecular dynamics simulations studies. Consequently, the reversibility of Zn plating/stripping anode reaction and the long-term cyclability of the Zn//LFP hybrid battery was significantly improved in MCE compared to the conventional electrolyte. Specifically, in our optimized MCE containing 1 m Zn(TFSI)2 and 4 m LiTFSI in 70% wt. PEG, the Zn//LFP not only delivers high rate performance but also sustains high capacities (120 mAh g-1) with superior long-term cycling stability retaining 65% of its initial capacity over an extended period of 882 h (vs 5% after 205 h) over 200 cycles at 0.2 C compared to the conventional electrolyte. Furthermore, Zn//LFP demonstrates a much lower self-discharge rate (only 5% per month vs 100% in the conventional electrolyte) and superior float charge behavior inferring that the detrimental parasitic reactions in the aqueous Zn-hybrid battery are potentially suppressed in our MCE. These combined results show that this MCE strategy can be used to design high reversible, safe and long-cycling AZMBs for practical applications.