Sodium-ion batteries are considered to be a main contender to lithium-ion as they are cheaper and made from more abundant, sustainable materials. However, Na-ion batteries cannot currently deliver the high volumetric and gravimetric energy densities that are required for long-range electric vehicles. A key constraint is the cathode. Due to the larger ionic radius of Na+ compared with Li+, Na-ion cathodes exhibit more severe structural transitions. This limits the compositional range over which they can be cycled reversibly leading to lower energy density. Another consequence of the difference in size is that Na+ ions are typically too large to be substituted into the transition metal sites of 3d-based layered oxide cathodes, meaning Na-rich cathodes such as Na2MnO3 and Na1.2Ni0.2Mn0.6O2 are not synthetically viable. The primary known examples of Na-rich materials which have been used as cathodes, Na2RuO3 and Na2IrO3, involve expensive rare earth metals which are not suitable for mass market application.
This project aims to explore new synthetic routes to make Na-rich transition metal oxides in order to achieve Na-ion cathodes with higher capacities. In one strategy, high valence transition metal ions which are closer in size to Na+ (1.02 A) than Mn4+ (0.53 A) will be selected to favour the presence of Na+ ions in the transition metal layer. These will be combined with low valence, transition metal ions to act as charge compensating redox centres. Another strategy will involve using disorder to access Na-rich compositions. Transition metal ions such as Ti4+ and Nb5+ with d0 electron configurations and zero crystal field stabilisation energy are found to promote cation disorder in Li-rich rocksalt cathodes. This same principle will be explored in Na-ion systems to examine whether the same effect translates to Na-ion cathodes to yield Na-rich materials.
To make these phases, conventional solid state synthesis under different atmospheres and temperature regimes will be explored. The resulting materials will be investigated with a range of characterisation tools to confirm structure, morphology and composition and verify the extent of success of each strategy. They will be examined as cathode materials in Na-ion cells and the structural and chemical changes during the charge and discharge reactions will be studied with diffraction and spectroscopy. A particular focus will be paid to the role of oxide anion redox which is anticipated at high degrees of desodiation in these Na-rich materials.
The materials and understanding established as a result of this PhD project will be important in advancing the development of higher energy density Na-ion batteries. This technology could provide a viable alternative to Li-ion in cheap, mass market electric vehicles and help to side-step the issues of resource scarcity and price volatility that Li-ion batteries face.
This project falls within the EPSRC Energy Storage, Electrochemical Sciences and Materials for Energy applications research areas. It is fully funded by the EPSRC.