Rational design of nitroxide radical polymers for enhanced electrochemical performance in rechargeable batteries

Abstract

The single electron characteristics and ambient stability of nitroxide radicals make them ideal for use in many different fields. This is exemplified in diverse applications ranging from energy storage materials, antioxidants in biomedicine, catalysts in organic synthesis, contrast agents for imaging, biospin-labeling, to organic magnets. Utilisation of nitroxide radicals in energy storage, especially as electrode active materials in secondary batteries, have attracted substantial interest over the past decade. Nitroxide radical electrodes have the virtue of fast redox kinetics, considerable theoretical capacity (mAh/g), and a high redox potential, positioning them as a promising replacement for traditional metal oxide materials in the fabrication of flexible and implantable electronic devices. However, some challenges still remain in utilizing nitroxide radicals or their corresponding polymers to make exceptional electrode active materials. First, the dissolution of both small molecules and polymers consisting of nitroxide radicals into organic electrolyte solution reduces its cycle stability; and second, poor miscibility between the nitroxide radical polymers and carbon materials compromises their polymer loading ratio in the final electrode and thus rate performance. These challenges greatly impede further applications and implementation of nitroxide radical compounds in energy storage applications. In this thesis, several strategies have been developed to produce welldefined nitroxide radical small molecules and polymers to overcome the current challenges. By introducing new groups in the nitroxide radical structure, increasing polymer chain length or forming non-covalent polymer gels, we can effectively reduce or eliminate the dissolution of these nitroxide radical materials into the electrolyte. Furthermore, by improving the interaction between the polymer and carbon, we could significantly increase the material loading within the electrode while maintaining high miscibility. Controlling the design of the nitroxide radical electrode, we could produce a battery that delivers high capacity, maintains excellent cycling stability and rate capability at high material loadings.In Chapter 2, a series of nitroxide compounds with different functional groups, ring sizes, and heteroatoms were synthesized. The redox potentials of nitroxide radicals synthesized were measured by cyclic voltammetry and correlated to DFT theory to reveal the effect of electron withdrawing/donating groups on the redox potential. In Chapter 3, 1,2,3-triazole ring functional TEMPO radicals were synthesized via the CuAAC reaction. The orthogonal CuAAC ‘click’ reaction ensures the absence of radical side reactions with the exclusive formation of 1,2,3-triazole ring, producing structures with low solubility in common electrolyte solvents. Directly applying the 1,2,3-triazole ring functional radical structures as a lithium battery cathode with a high loading ratio shows good cycle stability and C-rate capability. In Chapter 4, a series of poly(2,2,6,6- tetramethylpiperidinyloxyl-4-yl methacrylate) (PTMA) with different degrees of polymerization ranging from 66 to 703 are synthesized via single electron transfer ‘living’ radical polymerization (SET-LRP). The PTMAs with different molecular weights are tested as cathode material in a lithium battery, demonstrating that the higher molecular weight PTMA provides better cycling stability. Furthermore, in Chapter 5, the high molecular weight PTMA is covalently functionalized with pyrene groups and bound to rGO to form a ‘sandwich’ polymer/rGO nanostructure. This strategy improves the polymer/carbon miscibility and electron transfer on the polymer/carbon interface. Electrochemical results indicate the sandwich-like polymer/rGO nanostructure significantly enhances its C-rate capability and cycle stability at polymer loading up to 45 wt%. Finally, in Chapter 6, a hydrophilic nitroxide radical gel with a polyether backbone is synthesized via anionic copolymerization of 4-glycidol TEMPO and 1-glycidol pyrene. The gelation of the polymer resulted from the π-π interaction of pyrene groups that efficiently prevents polymer dissolution. This gel shows excellent cycle stability over 100 cycles in comparison to the non-gel polymer in a lithium battery.The objective of this Thesis is to study the impact of the chemical structure of nitroxide radical compounds and microstructures as electrode composites on the electrochemical properties. The results provide insights into the (micro)structure-property relationship upon the applications of nitroxide radicals as a secondary battery. The results support that the strategies developed in this Thesis effectively alter and enhance the electrochemical performance of nitroxide radical compounds with respect to cycle stability and rate capability. The knowledge and methodologies developed in this Thesis will not only benefit nitroxide radical based active materials, but also be applicable to other redox-active organic and polymer materials in energy storage applications.