The use of redox-active organic materials in rechargeable batteries has the potential to transform the field by enabling lightweight, flexible, green batteries while replacing lithium with sodium would mitigate the limited supplies and high cost of lithium. Herein, we report the first use of highly porous azo-linked polymers (ALPs) as a new redox-active electrode material for rechargeable sodium-ion batteries. ALPs are highly cross-linked polymers and therefore eliminate the solubility issue of organic electrodes in common electrolytes, which is prominent in small organic molecules and leads to fast capacity fading. Moreover, the high surface area coupled with the π-conjugated microporous nature of ALPs facilitates electrolyte adsorption in the pores and assists in fast ionic transport and charge transfer rates. An average specific capacity of 170 mA h g −1 at 0.3 C rate was attained while maintaining 96% Coulombic efficiency over 150 charge/discharge cycles.
Redox-active covalent organic frameworks (COFs) are a new class of material with the potential to transform electrochemical energy storage due to the well-defined porosity and readily accessible redox-active sites of COFs. However, combining both high specific capacity and energy density in COF-based batteries remains a considerable challenge. Herein, we demonstrate the exceptional performance of Aza-COF in rechargeable sodium-ion batteries (SIBs). Aza-COF is a microporous 2D COF synthesized from hexaketocyclohexane and 1,2,4,5-benzenetetramine by a condensation reaction, which affords phenazine-decorated channels and a theoretical specific capacity of 603 mA h g −1 . The Aza-COF-based electrode exhibits an exceptional average specific capacity (550 mA h g −1 ), energy density (492 W h kg −1 ) at 0.1 C, and power density (1182 W kg −1 ) at 40 C. The high capacity and energy density are attributed to swift surface-controlled redox processes and rapid sodium-ion diffusion inside the porous electrode. Rate capability studies showed that the battery also performs well at high current rates: 1 C (363 mA h g −1 ), 5 C (232 mA h g −1 ), 10 C (161 mA h g −1 ), and 20 C (103 mA h g −1 ). In addition, the long-term cycling stability test revealed very good capacity retention (87% at 5 C) and Coulombic efficiencies near unity over 500 cycles.
The rechargeable lithium-sulfur (Li-S) battery is a promising candidate for the next generation of energy storage technology,o wing to the high theoreticalc apacity,h igh specific energy density,a nd low cost of electrode materials. The main drawbacks in the development of long-life Li-S batteries are capacity fading and the sluggish kinetics at the cathode caused by the polysulfides shuttle. These limitations are addressed through the design of novel nanocagesc ontaining cobalt phosphide (CoP) nanoparticles embedded in highly porousnitrogen-doped carbon (CoP-N-GC) by thermal annealing of ZIF-67 in ar eductivea tmosphere followed by a phosphidation step using sodium hypophosphite. The CoP nanoparticles, with large surface area and uniform homogeneous distribution within the N-doped nanocage graphitic carbon,a ct as electrocatalysts to suppress the shuttle of soluble polysulfides through strong chemical interactions and catalyze the sulfur redox. As ar esult,t he S@CoP-N-GC electrode delivers an extremely high specific capacity of 1410 mA hg À1 at 0.1 C(1C= 1675 mA g À1)w ith an excellent coulombic efficiency of 99.7 %. Moreover,c apacity retention from 864 to 678 mA hg À1 is obtained after 460 cycles with a very low decay rate of 0.046 %p er cycle at 0.5 C. Therefore, the combination of the CoP catalyst and polar conductive porousc arbon effectively stabilizes the sulfur cathode, enhancing the electrochemical performance and stabilityo f the battery.
Covalent organic frameworks (COFs) hold great promise for electrochemical energy storage because of their high surface area, readily accessible redox-active sites, and environment-friendly chemical composition. In this study, the synthesis of a redox-active pyrene-containing polyimide COF (PICOF-1) by linker exchange using an imine-linked COF as a template is reported and its performance in sodium-ion batteries (SIBs) is demonstrated. The reported synthetic route based on linker exchange mitigates the challenges typically encountered with crystallizing chemically stable polyimide COFs from typical condensation reactions; thus, facilitating their rapid synthesis and purification. Using this approach, PICOF-1 exhibits high crystallinity with very low refinement parameters R P and R WP of 0.415% and 0.326%, respectively. PICOF-1 has a high Brunauer-Emmette-Teller (BET) surface area of 924 m 2 g −1 and well-defined one-dimentional (1D) channels of 2.46 × 1.90 nm, which enable fast ion transport and charge transfer, reaching a capacity at 0.1 C of almost nearly as its theoretical capacity and maintaining 99% Coulombic efficiency over 175 cycles at 0.3 C. The study demonstrates that imine-linked COFs are effective templates for integrating carbonyl-rich polyimide moieties into high-surface COFs to advance electrochemical energy storage applications.
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