Synthesis Of High-concentration Electrolytes And Their Application In Supercapacitors

Supercapacitors are used in a wide range of applications in aerospace,rail transportation,engineering machinery,and military industries due to their long operating life,high power characteristics,wide application temperature range,and exemplary safety.However,compared to energy storage devices such as batteries,supercapacitors’energy density is an order of magnitude lower,which limits further applications of supercapacitors.The current research in supercapacitors focuses on improving energy density,and the two ways to improve energy density are to increase the specific capacitance and the operating voltage.In this paper,the effects of aqueous electrolyte and organic ultra-high concentration electrolyte on the working voltage and particular capacitance of supercapacitors are discussed to improve the specific capacitance and working voltage.We successfully synthesized 1-propyl-3-methylimidazolium chloride（PMIm-Cl）,dissolved it in deionized water to form an aqueous electrolyte.The effect of concentration on ionic solvation capacity,voltage withstand,and physicochemical properties are investigated.With the increase in the PMIm-Cl/H₂O ratio,the aqueous supercapacitor’s operating voltage increased significantly,reaching 2.3 V at a concentration of 30 mol/kg of electrolyte.Meanwhile,the organic solvent N-methyl pyrrolidone（NMP）was introduced as a diluent to reduce the electrolyte’s viscosity and cost.The introduction does not damage the solvation structure of the highly concentrated PMIm-Cl/H₂O and thus still enables stable operation of the supercapacitor at 2.3 V.A small-sized electrolyte salt,N,N-dimethylpyrrolidinium tetrafluoroborate（DMP-BF₄）,was successfully synthesized and dissolved in the high-k dielectric solvent acetonitrile to form a highly concentrated DMP-BF₄/AN electrolyte.The DMP-BF₄/AN electrolyte’s negative window widened with increasing concentration,while the positive window was almost unaffected by the concentration.The negative window of4 mol/L DMP-BF₄/AN was extended while maximizing the supercapacitor device’s electrochemical window.The maximum operating voltage of the supercapacitor assembled with the 4 mol/L DMP-BF₄/AN electrolyte was tested at 3.4 V using a three-electrode device.Simultaneously,the ionic conductivity of the 4 mol/L DMP-BF₄/AN electrolyte is still high at 39.6 m S/cm,which ensures that the supercapacitor achieves high energy density while maintaining good multiplication performance.The maximum energy density and power density of the supercapacitor assembled with 4 mol/L DMPBF₄/AN electrolyte were calculated to be 49.8 Wh/kg and 14.6 Kw/kg,respectively.