High Oxidation State Alkali-Metal Late-Transition-Metal Oxides
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High Oxidation State Alkali-Metal Late-Transition-Metal Oxides David B. Currie, Andrew L. Hector, Emmanuelle A. Raekelboom, John R. Owen and Mark T. Weller* Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK ABSTRACT Li2NaCu2O4 has been prepared by solid state reaction under high-pressure (250 Atm) oxygen. A structural study, using time-of-flight powder neutron diffraction on a sample made with 7Li, shows a material isostructural with Li3Cu2O4, with sodium occupying the octahedral and lithium the tetrahedral A-cation sites. A 7Li MAS-NMR study of Li3Cu2O4, Li2NaCu2O4 and Li2CuO2 has been used to confirm the Li/Na site ordering. INTRODUCTION Portability is one of the primary concerns in the design of many new electronic devices. Thus the reduction in size and weight of primary and secondary (rechargeable) battery systems is a key concern. The main focus of current interest is the lithium battery technology based on a lithium (or intercalation host) anode and a cathode, which is usually a layered or spinel oxide of manganese or cobalt. Our aim is to synthesise interesting high oxidation state transition metal oxide systems, to investigate their structures and link these with their utility as cathode materials. High oxidation states are particularly of interest as a means of maximising cell potential. To this end our synthetic strategy involves reactions in high pressure oxygen and sealed systems with highly oxidised reagents. The number of copper based cathode systems that have been studied previously is quite small. As a primary battery system, Li-CuO cells [1] have a low but convenient working voltage of 1.5 V and are able to operate at relatively high temperatures (135°C). Use of a copper oxide phosphate (Cu4O(PO4)2) cathode increases the working voltage to 2.5 V [1], but the recharging of cells is still not possible. These systems use the Cu+/Cu2+ redox couple. Li-CuFeO2 cells have a working voltage of 1 V [2]. The cells have been shown to cycle but lose capacity rapidly. Copper K-edge X-ray absorption and 57Fe Mössbauer spectroscopy have shown [2] that copper is reduced to Cu0 and iron remains Fe3+. The mechanism by which these cells operate appears to be the replacement of copper with lithium, yielding LiFeO2, with deposition of elemental copper. In order to achieve higher cell potentials with a copper based cathode, the Cu2+/Cu3+ redox couple is a promising candidate. In the spinel Li2.02Cu0.64Mn3.34O8 the copper(III)-copper(II) reduction yields a mid-discharge potential of 4.9 V (a manganese(III)-manganese(IV) feature is observed at 4 V) [3]. A further Cu2+/Cu3+ system is derived from Li2CuO2. This system was investigated by Arai et al [4], who showed that on charging the voltage remains almost constant at around 3.1 V until the (calculated) composition Li1.2CuO2, after which it rose to about 3.5 V, reaching 3.7 V at a calculated composition LiCuO2. Our interest in the Arai system was heightened by the report of a Li1.5CuO2 phase, since the structure of Li3Cu2O4 (Figure 1)
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