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

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.

This CDT will deliver impact aligned to the following agendas:

People
A2P will provide over 60 PhD graduates with the skill sets required to deliver innovative sustainable products and processes into the UK chemicals manufacturing industry. A2P will inspire and develop leaders who will:
- understand the needs of industrial end-users;
- embed sustainability across a range of sectors; and
- catalyse the transition to a more productive and resilient UK economy.

Economy
A2P will promote a step change towards a circular economy that embraces resilience and efficiency in terms of atoms and energy. The benefits of adopting more sustainable design principles and smarter production are clear. For example, the global production of active pharmaceutical ingredients (APIs) has been estimated at 65,000-100,000 tonnes per annum. The scale of associated waste is > 10 million tonnes per annum with a disposal cost of more than £15 billion. Consequently, even a modest efficiency increase by applying new, more sustainable chemical processes would deliver substantial economic savings and environmental wins. A2P will seek and deliver systematic gains across all sectors of the chemicals manufacturing industry. Our goals of providing cross-scale training in chemical sciences with economic and life- cycle awareness will drive uptake of sustainable best practice in UK industry, leading to improved economic competitiveness.

Knowledge
This CDT will deliver significant new knowledge in the development of more sustainable processes and products. It will integrate the philosophy of sustainability with catalysis, synthetic methodology, process engineering, and scale-up. Critical concepts such as energy/resource efficiency, life cycle analysis, recycling, and sustainability metrics will become seamlessly joined to what is considered a 'normal' approach to new molecular products. This knowledge and experience will be shared through publications, conferences and other engagement activities. A2P partners will provide efficient routes to market ensuring the efficient translation and transferal of new technologies is realised, ensuring impact is achieved.

Society
The chemistry-using industries manufacture a rich portfolio of products that are critical in maintaining a high quality of life in the UK. A2P will provide highly trained people and new knowledge to develop smarter, better products, whilst increasing the efficiency and sustainability of chemicals manufacture.
To amplify the impacts of our CDT, effective public engagement and technology transfer will become crucially important. As a general comment, 'sustainability' styled research is often regarded in a positive light by society, however, the science that underpins its effective implementation is often poorly appreciated. The University of Nottingham has developed an effective communication portfolio (with dedicated outreach staff) to tackle this issue. In addition to more traditional routes of scientific communication and dissemination, A2P will develop a portfolio of engagement and outreach activities including blogs, webpages, public outreach events, and contribution of material to our award-winning YouTube channel, www.periodicvideos.com.

A2P will build on our successful Sustainable Chemicals and Processes Industry Forum (SCIF), which will provide entry to networks with a wide range of chemical science end-users (spanning multinationals through to speciality SMEs), policy makers and regulators. We will share new scientific developments and best practice with leaders in these areas, to help realise the full impact of our CDT. Annual showcase events will provide a forum where knowledge may be disseminated to partners, we will broaden these events to include participants from thematically linked CDTs from across the UK, we will build on our track record of delivering hi-impact inter-CDT events with complementary centres hosted by the Universities of Bath and Bristol.