Disordered rocksalt cathode materials for Na-ion Batteries

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Disordered rocksalt cathode materials for Na-ion BatteriesThere is a pressing need to develop rechargeable batteries that are cheaper and more sustainable than the Li-ion battery. Na-ion offers an attractive alternative, but this technology falls short of Li-ion on performance, particularly energy density, due to a lack of suitable high energy Na-ion cathode materials. The key challenges are related to the fact that Na+ ions are larger than Li+. The increased size of Na+ induces changes in the host structure that are much more severe than Li+ when the ions are removed and reinserted, leading to degradation over cycling. There is also a limit to the amount of Na+ that can be incorporated into layered transition metal oxide hosts when they are synthesised, as they are too large to be substituted into sites in the transition metal layers. This means that the best cathodes typically only utilise a fraction of the Na+ ions that may be possible, thus restricting the energy density.This project aims to investigate a new class of Na-ion cathodes based on the disordered rocksalt crystal structure, following the recent discovery of several highly promising Li-ion analogues. The disordered nature of this structure permits the constraints on Na content to be relaxed, unlocking the possibility of synthesising "Na-rich" compositions with the potential to achieve higher energy densities than current state-of-the-art cathodes. The disordered rocksalt structure has also been shown to exhibit much less pronounced structural change during the charge-discharge reaction than in layered phases, allowing stable cycling over much wider compositional ranges. To make these phases, a range of synthetic approaches will be employed including mechanochemical ball-milling, in-situ electrochemical preparation and ion exchange. To understand the chemical and structural changes taking place during cycling, a range of advanced characterisation techniques will be employed, including diffraction, electron microscopy and spectroscopy. Diamond Light Source and ISIS neutron and muon source along with other international research facilities will also be used as part of this research to provide insight into the structural and electronic properties of these materials. The specific PhD thesis aims are: 1) to identify, synthesise and characterise new Na-ion disordered rocksalt compositions based on earth abundant elements, 2) to define the limits of synthetic viability of disordered rocksalt phases, 3) to develop a comprehensive understanding of the chemical and structural stability of these materials during charge and discharge in Na-ion cells, 4) to identify and optimise promising candidates for commercialisation in partnership with Faradion.This project falls within the EPSRC Energy Storage, Electrochemical Sciences and Materials for Energy applications research areas. It is part-funded by Faradion, the UK-based non-aqueous sodium-ion cell technology company.This is a 3.5-year EPSRC DTP-CASE studentship in association with Faradion Ltd.