Exceptional Sodium-Ion Storage by an Aza-Covalent Organic Framework for High Energy and Power Density Sodium-Ion Batteries

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.

Multifunctional Electrocatalytic Cathodes Derived from Metal-Organic Frameworks for Advanced Lithium-Sulfur Batteries

The rechargeable lithium-sulfur (Li-S) battery is a promising candidate for the next generation of energy storage technology, owing to the high theoretical capacity, high specific energy density, and 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 nanocages containing cobalt phosphide (CoP) nanoparticles embedded in highly porous nitrogen-doped carbon (CoP-N-GC) by thermal annealing of ZIF-67 in a reductive atmosphere 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, act as electrocatalysts to suppress the shuttle of soluble polysulfides through strong chemical interactions and catalyze the sulfur redox. As a result, the S@CoP-N-GC electrode delivers an extremely high specific capacity of 1410 mA h g^-1 at 0.1 C (1 C=1675 mA g^-1 ) with an excellent coulombic efficiency of 99.7 %. Moreover, capacity retention from 864 to 678 mA h g^-1 is obtained after 460 cycles with a very low decay rate of 0.046 % per cycle at 0.5 C. Therefore, the combination of the CoP catalyst and polar conductive porous carbon effectively stabilizes the sulfur cathode, enhancing the electrochemical performance and stability of the battery.

Redox-Active Porous Organic Polymers as Novel Electrode Materials for Green Rechargeable Sodium-Ion Batteries

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.