Constructing Functional Aqueous Electrolytes By Regulating The Molecular Structures Of Polymer Additives For Energy Storage Devices

Aqueous electrochemical energy storage devices are considered to be the ideal choice for large-scale energy storage due to their low cost,environment-friendly and safety.However,due to the narrow electrochemical window of aqueous electrolyte,devices have low output voltage and energy density,and poor cycle life,which make them less economical for large-scale energy storage and become the fatal weakness of the energy storage technology.This paper focuses on the problems and challenges of aqueous supercapacitors and zinc-based energy storage devices.Based on the design and regulate of functional aqueous electrolyte,the research was carried out from three aspects:molecular weight design of polymer additive,electrolyte composition optimization and polymer charged groups modification,so as to finally realize the construction of high-performance aqueous energy storage devices.The details are as follows:1.The development of low-cost and eco-friendly aqueous electrolytes with a wide voltage window is the key to accomplish safe and high energy density supercapacitors（SCs）.In this work,molecular crowding electrolyte is prepared by simulating the crowded environment in living cells.The ion transport of the molecular crowding electrolyte can be effectively improved via reducing the molecular weight of crowding agent,i.e.,poly（ethylene glycol）（PEG）.The results show that PEG in 200 molecular weight（PEG200）can significantly improve ionic conductivity,while maintaining a wide voltage window.Such advantages enable commercial activated carbon-based SCs to work at 2.5 V with high energy density,outstanding rate performance and good stability for more than 10000 cycles.On this basis,three series of molecular crowding electrolytes with sodium perchlorate,lithium perchlorate and sodium trifluoromethanesulfonate as salts are developed,respectively,indicating the generality of PEG200 for wide voltage aqueous electrolytes.2.Traditional SCs still face the bottleneck of low energy density,while the zinc-based energy storage devices can greatly improve the energy density.However,the narrow voltage window and unstable zinc metal anode also limited the large-scale application of zinc-based energy storage devices.Herein,a solute-solvent dual engineering strategy using lithium bis（trifluoromethane）sulfonimide（LiTFSI）and inexpensive poly（ethylene glycol）（PEG,M_n=200）as co-additive with an optimized ratio can accomplish an all-round performance enhancement of electrolyte.Thanks to the synergistic inhibition of water activity and Zn²⁺solvation structure reorganization by LiTFSI-PEG,as well as a stable F-rich interfacial layer and PEG adsorption on Zn anode surface,dendrite-free Zn plating/stripping at nearly 100%Coulombic efficiency and stable cycling performance over 2000 h at 0.5 m A cm^-2 are achieved.Importantly,the integrated Zn-ion hybrid supercapacitors are endowed a wide voltage window of 0-2.2 V,superb cycling stability up to 10000 cycles,and excellent temperature adaptability from-40 ^oC to 50 ^oC.Versatilely,the highest cut-off voltage reaches 2.1 V in Zn//LiMn₂O₄ and Zn//VOPO₄ full cells with a stable lifespan over 500 cycles.3.PEG200 as a functional additive has greatly improved the voltage window of electrolyte and the stability of Zn metal anode.However,little attention was paid to the fundamental problems of polymers as additives,especially polyelectrolytes,in improving electrolyte and Zn metal anode stability.Here,a polycation additive,polydiallyl dimethylammonium chloride（PDD）,is introduced to achieve long-term and highly reversible Zn plating/stripping.Specifically,the PDD can simultaneously regulate the electric fields of electrolyte and Zn/electrolyte interface to improve Zn²⁺migration behaviors and guide dominant Zn（002）deposition.Importantly,such electric field variation is veritably detected by Zeta potential,Kelvin probe force microscopy and scanning electrochemical microscopy.Moreover,PDD also creates a positive charge-rich protective outer layer and a N-rich hybrid inner layer,which accelerates the Zn²⁺desolvation during plating process and blocks the direct contact between water molecules and Zn anode.Above combined functions lead to substantial improvement in reversibility and long-term stability,as certified by a higher average coulombic efficiency of 99.7%for Zn//Cu cells and 22 times longer life for Zn//Zn cells compared with that of PDD-free electrolyte.