Defect-Triggered Reversible Phase Transformation for Boosting Electrochemical Performance of Coordination Polymers

Abstract

Coordination polymers (CPs) hold promise for reliable and powerful supercapacitors (SCs) to overcome the energy crisis. However, CP-SCs face the daunting challenge of maintaining high pseudocapacitance after long-term charge/discharge cycling. Generally, if introducing defects exerted a positive effect on the property, eliminating defects would show a negative effect, and vice versa. Contrary to this common sense, here we demonstrate that both implanting defects and eliminating defects can significantly boost the specific capacitance of the defect-engineered CPs (DECPs), which are about 1.23 and 1.62 times that of the pristine CP, respectively, without loss of rate capability even after 10,000 charge–discharge cycles. The aqueous (A-ASC) and solid-state asymmetric supercapacitor (SS-ASC) devices based on DECPs deliver high energy densities of 80.3 and 61.5 Wh kg^–1, superb power densities of 8471.0 and 8430.6 W kg^–1, and long cycling lifespan of up to 2000 cycles with 92.0 and 80.0% capacity retention, respectively. Moreover, the SS-ASC exhibits excellent flexibility, verified by 99.0% maintenance of its initial capacitance when it is twisted and bent at 180°. Importantly, this work has certified that stepwise increasing/decreasing the concentration of ordered defects gradually triggered reversible phase transformation of CP from nonporous to microporous by charge–discharge cycling, in situ addition of the modulator, and postsynthetic treatment. The mechanism of forming/eliminating defects and their effects on supercapacitive performances of CP-SCs have been unprecedentedly clarified. These findings offer insight into the relationship between defective structure and electrochemical behavior for developing efficient long-cycling CP-SCs.