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This thesis describes the synthesis, characterisation and modification of a range of sulfate materials, of interest for applications as cathodes for Na-ion batteries. The successful synthesis and doping of sodium transition metal sulfate hydrates is reported. Two structures are possible for Na2M(SO4)2·2H2O: a monoclinic structure, favoured by larger divalent transition metal cations (e.g. Mn, Fe, Co and Cu) and a triclinic structure, which is only naturally adopted by the smaller Ni2+. The effect of substitution of (SO4)2- by isoelectronic dopants such as (PO3F)2- and (SeO4)2- was explored. The structure obtained after doping was found to be strongly dependent on the cation selected to form the framework and level/type of dopant used. In addition, the products of dehydration were also examined and it was found that systems that naturally adopt a Na2M(SO4)2 phase after dehydration, can adopt a novel alluaudite-type phase Na3M1.5(SO4)3-x(SeO4)x, once a threshold concentration of selenate is reached. The langbeinite system, K2MII2(SO4)3, has also been investigated as a potential host structure for sodium due to its structural similarity to the well-established NaSICON systems. A Na substitution study was conducted on K2Mg2(SO4)3, which showed that up to 88% of the K could be replaced by Na. This study was later extended to examine langbeinite systems containing common transition metals used in battery materials (Mn, Fe, Co and Ni). Lower sodium levels could be accommodated in these latter systems, although the amount of Na incorporation could be increased by partial substitution of the transition metal by Mg. In addition, this thesis reports an alternative synthetic approach to obtain a small family of Na-V-S-O systems. The method proposed in this work consistently produces high purity samples at relatively low temperatures |
