Preparation Of Aqueous Electrolyte With Wide Voltage Window And Its Application Research

In the recent years,solar photovoltaic power generation,solar thermal power generation and wind power generation technologies have become the hot spot and focus of new energy development and utilization.However,owing to the unsynchronized time between the acquisition and actual use of these new energy sources,the realization of"peak and valley reduction"through high-density energy storage technology has become a key technology in the development and utilization of new energy sources.The electrochemical energy storage represented by lithium-ion battery and supercapacitor has the advantages of high storage energy density,good sustainability and mobile utilization,which has received wide attention.Compared with lithium-ion batteries,supercapacitors have the advantages of faster charging and discharging capability,long cycle life and low cost.However,the low energy density of supercapacitors has always limited their widespread use in energy storage.According to the formula of energy density,increasing the operating voltage window can effectively improve the energy density of supercapacitors.The use of organic electrolyte with wide voltage window can achieve high energy density,but its flammability,toxicity and volatility will lead to serious environmental pollution and safety problems.Therefore,the development of inexpensive,safe and environmentally friendly aqueous electrolytes is an ideal solution to these problems.However,due to the poor overpotential resistance of water,aqueous electrolytes exhibit a narrow electrochemical stability window,and supercapacitors made with aqueous electrolytes tend to have lower operating voltage and energy density.In this thesis,we designed and prepared various functionalized electrolytes with wide electrochemical stability windows,starting from improving the electrochemical stability of aqueous electrolytes,and applied them in supercapacitors and zinc ion hybrid supercapacitors to test their operating performance.The specific research contents and results are as follows:（1）An aqueous hybrid electrolyte with low cost was designed and prepared.The electrolyte uses Na Cl O₄ as the electrolyte salt and acetonitrile and polyethylene glycol as additives,resulting in an electrochemical stability window of 3.12 V.It also has a high conductivity(16 m S cm^-1)and excellent flame retardant properties.The molecular dynamics simulation and structural characterization of the electrolyte have revealed that there are almost no free water molecules in the electrolyte.The electrochemical performance test results show that the double-layer supercapacitor using this electrolyte can operate at a high operating voltage of 2.6 V,which is the highest operating voltage reported for aqueous carbon-based supercapacitors so far.In addition,the constructed supercapacitor exhibited good multiplicity performance,high energy density(31.2 Wh kg^-1),and ultra-long cycling stability（81%capacity retention after more than 11,000 cycles）.（2）The development of high concentration“water-in-salt”WIS electrolytes has broken the barrier of narrow electrochemical window of conventional dilute electrolytes.However,as the salt concentration increases,the viscosity of the electrolyte also increases while the ionic conductivity gradually decreases,which is unfavorable for supercapacitors pursuing high multiplicity performance as well as power density.Here we developed a new molecularly crowded electrolyte additive,polyethylene glycol dimethyl ether,which has superior physicochemical properties（low viscosity,low freezing point,low density,and high ionic conductivity）compared to conventional polyethylene glycol additives due to the absence of intermolecular hydroxyl interactions.By introducing this additive,a wide electrochemical stability window（3 V）was achieved at a low concentration（3 m Na Cl O₄）,and a safe,high voltage（2.4 V）and high multiplicity performance（72.3%capacity retention with a 10-fold increase in current density）double layer supercapacitor was successfully constructed.（3）Based on the previous experiments that molecular crowding agents can effectively broaden the electrochemical stability window of electrolytes,this thesis further introduces the molecular crowding agent polyethylene glycol dimethyl ether into Zn（Cl O₄）₂ electrolyte and uses it to assemble zinc ion hybrid capacitors in order to further improve the energy density of zinc ion hybrid capacitors.The electrochemical test results show that the electrolyte can stably achieve the zinc deposition and stripping process without the appearance of dendrites,which makes it much more reversible.The operating voltage of the zinc ion hybrid capacitors made with this electrolyte is comparable to that of organic electrolytes,and no capacity degradation was observed at a high voltage of 2.1 V for 10,000 cycles.