ABSTRACT
Understanding the properties that govern the kinetics of charge storage will enable informed design strategies and improve the rate performance of future battery materials. Herein, we study the effects of structural ordering in organic electrode materials on their charge storage mechanisms. A redox active unit, N,N'-diphenyl-phenazine, was incorporated into three materials which exhibited varying degrees of ordering. From cyclic voltammetry analysis, the crystalline small molecule exhibited diffusion-limited behavior, likely arising from structural rearrangements that occur during charge/discharge. Conversely, a branched polymer network displayed surface-controlled kinetics, attributed to the amorphous structure which enabled fast ionic transport throughout charge/discharge, unimpeded by sluggish structural rearrangements. These results suggest a method for designing new materials for battery electrodes with battery-like energy densities and pseudocapacitor-like rate capabilities.
ABSTRACT
Developing cathodes that can support high charge-discharge rates would improve the power density of lithium-ion batteries. Herein, the development of high-power cathodes without sacrificing energy density is reported. N,N'-diphenylphenazine was identified as a promising charge-storage center by electrochemical studies due to its reversible, fast electron transfer at high potentials. By incorporating the phenazine redox units in a cross-linked network, a high-capacity (223â mA h g-1 ), high-voltage (3.45â V vs. Li/Li+ ) cathode material was achieved. Optimized cross-linked materials are able to deliver reversible capacities as high as 220â mA h g-1 at 120 C with minimal degradation over 1000â cycles. The work presented herein highlights the fast ionic transport and rate capabilities of amorphous organic materials and demonstrates their potential as materials with high energy and power density for next-generation electrical energy-storage technologies.
ABSTRACT
The development of next-generation materials is coupled with the ability to predictably and precisely synthesize polymers with well-defined structures and architectures. In this regard, the discovery of synthetic strategies that allow on demand control over monomer connectivity during polymerization would provide access to complex structures in a modular fashion and remains a grand challenge in polymer chemistry. In this Article, we report a method where monomer selectivity is controlled during the polymerization by the application of two orthogonal stimuli. Specifically, we developed a cationic polymerization where polymer chain growth is controlled by a chemical stimulus and paired it with a compatible photocontrolled radical polymerization. By alternating the application of the chemical and photochemical stimuli the incorporation of vinyl ethers and acrylates could be dictated by switching between cationic and radical polymerization mechanisms, respectively. This enables the synthesis of multiblock copolymers where each block length is governed by the amount of time a stimulus is applied, and the quantity of blocks is determined by the number of times the two stimuli are toggled. This new method allows on demand control over polymer structure with external influences and highlights the potential for using stimuli-controlled polymerizations to access novel materials.
ABSTRACT
Control of polymer initiation, propagation and termination is important in the development of complex polymer structures and advanced materials. Typically, this has been achieved chemically, electrochemically, photochemically or mechanochemically. Electrochemical control has been demonstrated in radical polymerizations; however, regulation of a cationic polymerization has yet to be achieved. Through the reversible oxidation of a polymer chain end with an electrochemical mediator, temporal control over polymer chain growth in cationic polymerizations was realized. By subjecting a stable organic nitroxyl radical mediator and chain transfer agent to an oxidizing current, control over polymer molecular weight and dispersity is demonstrated and excellent chain end fidelity allows for the synthesis of block copolymers.
ABSTRACT
The oxidation of four benzylic alcohols employing hydrogen peroxide and TBHP as oxidants, catalyzed by an iron(III) complex bearing a 14,28-[1,3-diiminoisoindolinato]phthalocyaninato (diiPc) ligand has been studied and found to proceed with good selectivity, high turnover numbers, and high turnover frequencies in the absence of organic solvents other than the substrates themselves.
