Organic Electrode Materials For Rechargeable Lithium Batteries

Energy and environmental issues are recognized as two major challenges to human being in the21st century. The desire for efficient and clean power source, has promoted the development of chemical batteries, especially rechargeable lithium batteries. Currently, the rapid development of consumer electronics, electric vehicles, and energy storage stations, puts forward higher requirements on rechargeable lithium batteries in the aspects of charge/discharge performance, safety, sustainability and environmental benignancy, which provide great opportunities and challenges to the research and application of rechargeable lithium batteries.The conventional inorganic cathode materials for Li-ion batteries are restricted by the theoretical specific capacity and structure stability, making it difficult to further improve their energy density. The Large-scale production and use of Li-ion batteries, cause more and more concern on resources and environmental issues brought by the conventional inorganic electrode materials. As an alternative, the electroactive organics or polymers are promising candidates as electrode materials for the new generation of "green lithium battery" because of their higher theoretical specific capacity, sustainability and environmental benignancy. To achieve this goal, a series of high performance polymer electrode materials based on conjugated carbonyl were designed and synthesized, and the relationship between the material structure and electrochemical performance is investigated in terms of physical characterization and electrochemical measurements. Based on these, the application of these materials in some novel lithium battery techniques is further proposed. The results are detailed as follows.1. Poly(anthraquinonyl sulfide)(PAQS) cathode material.Quinone possesses high electrochemical redox activity and can be used as cathode for rechargeable lithium batteries, but the unwanted dissolution in the nonaqueous electrolyte leads to poor cycling stability. Aiming to this, we put forward a simple polymerization method by linking the quinone unit with thioether bond. Thus obtained PAQS cathode material can not only retain the high specific capacity and good reversibility of anthraquinone, but also essentially avoid the dissolution problem and show excellent cyclability.2. Polyimide (PI) cathode materials.PI is one type of engineering plastics with excellent mechanical and thermal properties, and has been widely used as structural material in many fields. Base on the electrochemical redox mechanism of anhydride structure, we proposed using PI as cathode material for rechargeable lithium batteries. We designed and synthesized five PI samples with different structure, found that the electrochemical performance of PI is mainly associated with the dianhydride component, which is explained qualitatively from the aspect of theoretical calculation. Among the five PI samples, PI-4shows the best cyclability and PI-5shows the highest energy density, comparable to that of LiCoo2.Besides, the easy preparation, safety and degradability characteristics make it more acceptable for practical application than other organic cathode materials.3. Polymer/graphene nanocomposite cathode materialsGraphene has been considered as an excellent conductive additive in composites ofmany inorganic electrode materials for improving rate performance, due to their particularly high electronic conductivity and surface area. We take PAQS and PI-4as examples to demonstrate polymer/graphene nanocomposites as cathode materials for the first time, which show excellent ultrafast-charge and-discharge ability. Based on the good dispersibility of graphene in NMP solvent, we obtained well-dispersed polymer/graphene nanocomposites through a simple "one pot" in-situ polymerization process. Benefiting from the greatly increased electronic conductivity and surface area, both the specific capacity and rate capability of polymer active material are significantly improved. Since the synthesis is very easy, it gives important insights into improving battery performance of other polymer electrodes. 4. Polyanthraquinone (PAQ) cathode materialsTo further improve the specific capacity of quinone based polymer cathode, we synthesized P(1,5-AQ) and P(1,4-AQ) through an organometallic polycondensation process. Compared to PAQS, PAQ possesses higher theoretical specific capacity and structure conjugacy. After being composited with graphene, P(1,4-AQ) shows high discharge capacity of234mAh/g, excellent cycling stability and rate capability. In particular, although P(1,4-AQ) is insoluble in organic electrolyte, it is soluble in chloroform, and can be made into a flexible thin film by evaporating the solvent, giving rise to its potential application in flexible thin film lithium batteries.5. Organic anode materials for aqueous Li-ion batteriesThe above polymer electrodes we have studied all show redox potentials between2-3V, and are initially in the oxidation state. Therefore it is possible to match them with LiCOO2or LiMn2O4cathode in aqueous Li-ion batteries. In this dissertation, for the first time, we use conjugated carbonyl based polymers instead of inorganic materials as anodes for aqueous Li-ion batteries, and focus on the investigation of PI-4/LiCoO2and PI-5/LiCoO2systems. Because of the higher specific capacity and structure stability of polymer anodes in aqueous electrolyte compared to inorganic materials, the aqueous Li-ion battery constructed with polymer anode show much higher energy density and cycling stability than previously reported ones.Because the organic electrode materials have many distinct advantages over inorganic materials in structure diversity, flexibility and processability, we will continue to develop more promising organic electrode materials with higher electrochemical performance and investigate their potential use into various novel energy storage devices. We believe that organic electrode materials will show their talents in the "post Li-ion battery" period.