Mechanosynthesis of Energy Materials

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This proposal focuses on mechanochemical synthesis as a relatively new method to prepare oxide and mixed anion materials for energy applications. Ball milling is widely used to reduce particle size of reagents, increase their surface areas and increase their reactivity before subsequent high temperature reaction. By using high energy, planetary ball mills, reaction can be carried out to completion at ambient temperatures without the need for follow-on high temperature reaction. A major advantage is the possible preparation of new materials that are unstable at high temperatures and cannot be prepared by traditional routes. The method has been used with some recent success to synthesise new cathode materials for lithium batteries. This proposal aims to establish the conditions for successful use of mechanochemical synthesis as a versatile synthesis procedure and to achieve this by a targetted programme on materials with disordered rock salt crystal structures. There is much scope, by doping and compositional control to achieve reproducible, high charge-discharge capacity materials that have long cycle lives.Rechargeable lithium batteries provide a major source of energy worldwide that are used to power a wide range of portable electrical devices, hybrid and all-electric vehicles, together with large-scale power station load levelling installations linked to the increasing use of wind and solar renewable sources of energy. The batteries range from miniature-scale devices that can be implanted into the human body to megawatt-scale energy storage as part of electricity grid networks. Although current lithium battery technologies are well-established, there is still a great need for improved systems with the objectives of higher energy storage capacity, increased cycle lifetimes and improved environmental compatibility through development of non-toxic and lower cost alternatives to the use of cobalt as a key redox active component of cathodes.The development of new lithium cathodes requires synthesis methods which can be scaled up to give materials that behave reproducibly during battery charge - discharge over many cycles without loss of performance. Several groups of materials have undergone intensive research and development as potential substitutes for LiCoO2, including three groups with rock salt-related crystal structures. These are: layered structures similar to LiCoO2, but with Co replaced by other transition metal combinations; more recently, similar layered structures with a higher Li content that are of particular interest for their high capacities and oxygen redox contributions to capacity; most recently, cubic disordered rock salt structures which have a deceptively simple crystal structure that has cations distributed at random over octahedral sites in a cubic close packed anion sublattice. These latter materials are the main focus of this proposal, both to optimise operational variables using mechanosynthesis and to target new compositions in a poorly-studied family of materials that have considerable potential as lithium battery cathodes.Understanding of these disordered rock salt structure materials is incomplete due to a combination of their nanoscale size, compositional uncertainty, especially concerning their oxygen stoichiometry, their local crystal structures with possible ordered domains or defect clusters and their surface structures which in many cases are atmosphere sensitive. Many have high charge storage capacities but correlations between electrochemical performance, materials synthesis procedures and structural / compositional characteristics are not well established. Many new materials will be prepared during this project, better understanding of the electrochemical properties of disordered rock salt structures gained and guidelines established for wider use of the mechanosynthesis technique.