Synthesis, Electrochemistry and Solid-Solution Behaviour of Energy Storage Materials Based on Natural Minerals
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Polyanionic compounds have been heavily investigated as possible electrode materials in lithium- and sodium-ion batteries. Chief among these is lithium iron phosphate (LiFePO4) which adopts the olivine structure and has a potential of 3.5 V vs. Li/Li+. Many aspects of ion transport, solid-solution behaviour and their relation to particle size in olivine systems are not entirely understood. Morphology, unit cell parameters, purity and electrochemical performance of prepared LiFePO4 powders were greatly affected by the synthetic conditions. Partially delithiated olivines were heated and studied by Mössbauer spectroscopy and solid-solution behaviour by electron delocalization was observed. The onset of this phenomenon was around 470-500 K in bulk material but in nanocrystalline powders, the onset of a solid solution was observed around 420 K. The isostructural manganese member of this family (LiMnPO4) was also prepared hydrothermally. Owing to the thermal instability of MnPO4, partially delithiated LiMnPO4 did not display any solid-solution behaviour. Phosphates based on the tavorite (LiFePO4OH) structure include LiVPO4F and LiFePO4(OH)1-xFx which may be prepared hydrothermally or by solid state routes. LiVPO4F is a high capacity (2 electrons/transition metal) electrode material and the structures of the fully reduced Li2VPO4F and fully oxidized VPO4F were ascertained. Owing to structural nuances, the potential of the iron tavorites are much lower than that of the olivines. The structure of Li2FePO4F was determined by a combined X-ray and neutron diffraction analysis. The electrochemical properties of very few phosphates based on sodium are known. A novel fluorophosphate, Na2FePO4F, was prepared by both solid state and hydrothermal methods. This material exhibited two two-phase plateau regions on cycling in a half cell versus sodium but displayed solid-solution behaviour when cycled versus lithium, where the average potential was 3.3 V. On successive cycling versus Li a decrease in the sodium content of the active material was observed, which implied an ion-exchange reaction occurred between the material and the lithium electrolyte. Studies of polyanionic materials as positive electrode materials in alkali metal-ion batteries show that some of these materials, namely those which contain iron, hold the most promise in replacing battery technologies currently available.