This is the first time that a mixed transition metal oxide is used for the negative electrode of a sodium-ion battery. The NiCo2O4 spinel was prepared by thermal decomposition of oxalate precursors at 320 °C. The electrochemical reactions of this material with sodium lead to the complete reduction of the transition metals and the formation of Na2O. The new electrode material shows reversible capacities of ca. 300 mA h g-1 in sodium-ion cells versus Na x CoO2.

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Lithium has been inserted into the spinel Li 4 Ti 5 O 12 by both chemical and electrochemical methods. The cation distribution in the lithiated phases has been analyzed by 6,7 Li NMR, Raman spectroscopy, and X-ray diffraction, and the distribution in the chemically inserted compound has been analyzed additionally by neutron diffraction. A refinement of structural parameters has been carried out by applying the Rietveld method to the neutron diffraction pattern. It is shown that the two insertion methods are based on different mechanisms. Chemically inserted lithium ions are trapped in the (48f) sites of the spinel structure from which they cannot be extracted by electrochemical means. In contrast to the electrochemical Li-insertion, which is accompanied by a spinel to rocksalt phase transition, no such structural change is found for chemical insertion. The consequences of the two different mechanisms for the reversibility of the insertion process are discussed.