Boosting the Capacity of Aqueous Li‐Ion Capacitors via Pinpoint Surgery in Nanocoral‐Like Covalent Organic Frameworks

Abstract

Furthermore, in order to meet the future application requirements of electronic devices and hybrid electric vehicles, it is urgent to develop advanced energy storage devices combining high energy density and power density, as well as long cycle life. [3,4] Lithium (Li)-ion capacitors (LICs), also known as lithiumion hybrid capacitors, as a potential hybrid energy storage device, combining Faradaic feature and capacitive behavior, bridge the performance gap between lithiumion batteries (LIBs) and supercapacitors (SCs), and are expected to solve the above issues. [5] As a typical configuration, LICs can generally be constructed with a non-Faradic capacitor-type electrode (enable fast charging/discharging rates) and a Faradic battery-type electrode (enable great specific capacity and energy density) in Li-salt-containing organic/aqueous electrolytes, and the comprehensive energy performance can be achieved for LICs. [6] Generally, the focus of this research field is mainly on those LIC devices that use nonaqueous electrolytes restricted by the trouble of highly flammability, toxicity, and volatility. [7] Aqueous LICs (ALICs) using aqueous Li-salt solutions (LiCl, LiNO 3 , and Li 2 SO 4 , etc.) as electrolytes are promising candidates thanks to the high security and processability, low cost, high ionic conductivity, along with simple assembly process. [1,8] Previous researches have made some refreshing progress in ALICs mainly assembled with lithium-intercalation compounds as cathode electrode and activated carbon (AC) as anode electrode. [9,10] Unfortunately, the cycling and rate capability of these cathode materials were reported to be very poor caused by violent volume expansion and slow reaction kinetics, resulting in the inferior performances of the ALICs, [11] which make ALICs fail to attract strong interest from researchers. Thus, the urgent demands for enhanced electrochemical performance of ALICs lie in the development of innovative configuration of anode and cathode (MoS 2 @α-Fe 2 O 3 /carbon nanotube fiber (CNTF)//LiCoO 2 / CNTF, [12] boron-doped graphene (BDG)/boron-doped diamond (BDD)/TiO 2 //BDG/BDD, [13] Ti 3 C 2 T x MXene//porous carbon (PC)) [14] as well as design and synthesize high-performance electrode materials. Aqueous lithium storage devices are promising candidates for next-generation energy storage applications, featuring low-cost, safety, environmental benignness, and grid-scale merits. Developing reliable anode materials with fast Li + diffusion is paramount to stimulate their development. Herein, the electrochemical performance and mechanism of a redox-active β-ketoenamine-linked covalent organic framework (COF) (2,6-diaminoanthraquinone and 2,4,6-triformylphloroglucinol COF, DAAQ-TFP-COF) for lithium storage in aqueous electrolyte are explored for the first time. Systematic studies demonstrate that, by the conversion of neutral COF into anionic COF via a pinpoint surgery on the β-ketoenamine linkage, the resultative COF shows doubled Li + storage capacity (132 mAh g −1 at 0.5 A g −1 , 87%...

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