超级电容器电解液的离子调控

Ion regulation of electrolytes for supercapacitors

与二次电池相比,超级电容器具有充/放电速度快、功率密度高和循环寿命长的特点,但实际应用受限于较低的能量密度.电解液作为超级电容器的重要组成部分,对其进行设计开发有助于推动超级电容器以性能和功能为导向的应用.本文从电解液离子调控的角度出发,讨论了超级电容器的储能机制阐释、电化学性能提升和新颖功能开发.首先讨论了电解液离子对双电层电容材料和赝电容材料电化学行为的影响.接着从工作电压和比容量角度讨论了提升超级电容器能量密度的策略,包括拓宽电解液的电化学稳定窗口、充分利用有效电压范围、合理匹配离子尺寸与材料结构、引入额外法拉第贡献,以及促进溶剂化离子的体相传输和界面去溶剂化动力学等.然后,介绍了功能型超级电容器,重点讨论了超级电容器二极管的研究进展.最后,提出了我们对该领域当前挑战和未来方向的看法.

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 battery-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.