Developing metal-salen complexes as redox mediators for lithium-air batteries

为了实现更绿色、可持续的未来，人们加大了对下一代储能设备的研究力度。预期在适当时候，诸如太阳能和风能等可再生能源将取代不可再生燃料，成为消费能源生产的主要手段。然而，它们的间歇性需要使用电网储能系统。因此，开发高效、可持续的能源存储设备对于进一步实现更绿色的未来至关重要。其中最有前途的是锂空气电池，它对目前的市场领导者锂离子电池提供了充分的竞争。锂金属负极和多孔碳空气呼吸正极的耦合导致理论重量能量密度高达3500 Wh kg，比目前的锂离子技术高出一个数量级。使用更丰富的材料，如锂和碳，是一个巨大的转变，从使用较少的，更昂贵的过渡金属在电极结构。此外，电池的化学反应依赖于锂与氧气的反应，而氧气很容易从空气中获得。然而，尽管锂空气电池具有很高的理论性能和可持续设计，但在其商业化之前仍有一些重大问题需要解决。它们的放电/充电反应动力学缓慢；放电产物过氧化锂（Li2O2）溶解度和导电性差；用Li2O2钝化正极。为了解决锂空气电池遇到的电化学难题，可以在电解质中添加氧化还原介质。这些都是均相催化剂，它们能够将电子从正极转移到溶液中的中间物质。这样做，电池在放电/充电期间的速率性能显着提高，并且可以减轻许多问题。该项目将着手对氧化还原介质在锂空气电池中的作用机制进行连贯的理解。为了做到这一点，一种被称为金属salen复合物的分子将在整个项目中使用。这类之所以被特别选择，是因为它在功能化方面的通用性很高。通过改变分子的结构，可以微调其各种性质，即：溶剂重组能、结合位点和氧化还原电位。通过改变介体的结构和性质，可以迫使其采用内球或外球的电子转移机制。在整个项目中，将合成金属salen复合物衍生物，并初步筛选锂空气电池电解质的化学和电化学稳定性。循环伏安法和核磁共振波谱等技术将用于获得介质氧化还原电位和介质/电解质降解。将对Li2O2与介质的化学反应进行评估，其中将采用UV-Vis光谱等技术，并对有前途的介质进行研究。扫描电化学显微镜（SECM）将被用来获得Li2O2在合成介质存在下的动力学信息。该技术将与其他技术如EPR和拉曼光谱相结合，以监测新中间体或键的形成。最后，将采用电池循环来评估介质在锂金属存在下的稳定性；电极间介质的穿梭；以及标准锂空气电池介质的性能。

The drive towards a greener, sustainable future is leading to increased research into next-generation energy storage devices. In due course, it is anticipated for renewable energy sources, such as solar and wind, to replace non renewable fuels as the major means of consumer energy production. However, their intermittency requires the use of electrical grid energy storage systems. As such, the development of efficient, sustainable energy storage devices is essential to further a greener future. One of the most promising is the lithium air battery, which offers ample competition to the current market leader, lithium ion batteries. The coupling of a lithium metal negative electrode and porous carbon, air-breathing positive electrode leads to a high theoretical gravimetric energy density of 3500 Wh kg 1 an order of magnitude greater than current lithium ion technology. The use of more abundant materials, such as lithium and carbon, is a monumental shift away from using less abundant, and more expensive transition metals in electrode structures. Furthermore, the cell chemistry relies on the reaction of lithium with oxygen, the latter of which is readily available from air. However, despite its high theoretical performance and sustainable design, there are significant issues to address before lithium-air batteries are commercialised. They suffer from sluggish reaction kinetics of the discharge/charge reactions; poor solubility and electrical conductivity of the discharge product, lithium peroxide (Li2O2); and passivation of the positive electrode by Li2O2. To tackle the electrochemical difficulties encountered in lithium air batteries, redox mediators can be added to the electrolyte. These are homogenous catalysts, which are capable of transferring electrons from the positive electrode to intermediate species in solution. In doing so, the rate performance of the cell during discharge/charge is significantly enhanced, and many of the issues outlined can be alleviated. This project will embark on developing a coherent mechanistic understanding of how redox mediators operate in lithium-air batteries. To do so, a class of molecule, known as metal salen complexes, will be used throughout the project. This class has been specifically chosen due to its high versatility in functionalisation. By modifying the structure of the molecule, its various properties can be fine-tuned, namely: the solvent reorganisation energy, binding site, and redox potential. It is expected by altering mediator structures and its properties outlined above, it will be forced to adopt an inner or outer-sphere electron transfer mechanism. Throughout the project, metal salen complex derivatives will be synthesised and initially screened for chemical and electrochemical stability in lithium-air battery electrolytes. Techniques such as cyclic voltammetry and nuclear magnetic resonance spectroscopy will be used to obtain mediator redox potentials, and mediator/electrolyte degradation. Chemical reaction of Li2O2 with mediators will be assessed, where techniques such as UV-Vis spectroscopy will be employed - promising mediators will be taken forward. Scanning electrochemical microscopy (SECM) will be employed to obtain kinetic information of Li2O2 in the presence of synthesised mediators. This technique will be coupled with others such as EPR and Raman spectroscopy to monitor formation of new intermediates or bonds. Finally, cell cycling will be employed to assess the stability of mediator in the presence of lithium metal; shuttling of mediator between electrodes; and performance of mediators in standard lithium-air cells.