Ion regulation of electrolytes for supercapacitors
Supercapacitors charge and discharge more rapidly,and deliver higher power density and longer cycle life compared to the secondary batteries,but their practical application is limited due to their lower energy density.Electrolytes,as an indispensable part of supercapacitors,can be reasonably optimized to promote the performance-and functionality-oriented applications of supercapacitors.Herein,from the perspective of electrolyte ion regulation,the energy storage mechanism,electrochemical performance improvement,and novel functionality exploitation of supercapacitors are discussed.Firstly,the effects of electrolyte ions on electrochemical characteristics of electric double-layer capacitive materials and pseudocapacitive materials are introduced along with their classification.The intrinsic properties of cations and anions and their deviation in diffusion dynamics have a key influence on the charge storage mechanisms of electric double-layer capacitive materials,including counter-ion adsorption,ion exchange/ion swapping,and co-ion desorption.On the other hand,the charge storage mechanisms of pseudocapacitive materials also greatly rely on the electrolyte ions.which can be changed from a surface pseudocapacitive to an intercalation pseudocapacitive and even to a batte y-like behavior.Then,strategies to increase the energy density of supercapacitors are addressed from the operating voltage and specific capacity aspects.Higher operating voltage of supercapacitors fundamentally requires wider electrochemical stability windows of electrolytes,which can be achieved by using highly-concentrated electrolytes and aqueous/organic hybrid electrolytes because of the suppressed solvent activity and formation of stable solid electrolyte interphased.After the determination of the upper and lower limit potentials of a given supercapacitor system,the effective potential range should be fully utilized by various methods such as electrolyte ion adjustment for maximizing the practical operating voltage.Moreover,higher specific capacity of supercapacitors can be obtained by strategies such as designing ion distribution on electrode surfaced,matching solvated ion size with material structured,and adding extra Faradaic contribution.Meanwhile,from a kinetic point of view,the ion transfer in bulk electrolytes and reaction rate at the electrode-electrolyte interface should be accelerated by weakening the ion-solvent interactions,which are also significantly important for higher specific capacity.Next,functional supercapacitors are introduced,focusing on the research progress of supercapacitor diodes.The supercapacitor diodes integrate the characteristics of a diode into a supercapacitor,representing a crucial breakthrough for extending conventional supercapacitors to new functionality applications.They can be constructed based on the ion confinement effects of electric double-layer capacitive materials,or based on the ion-selective redox reactions of metal oxides with pseudocapacitive mechanisms.Finally,our perspective on present challenges and future directions in this field are provided.
supercapacitorelectrolyte ion regulationenergy storage mechanismenergy densitysupercapacitor diode
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