Abstract
A xMO 2 layered oxides (A Li, Na; M a 3d or 4d element) have aroused considerable interest as positive electrode materials in secondary batteries with an alkali metal as the negative electrode. The structure of the layered oxides A xMO 2 can be described as a packing of MO 2 layers with alkali ions intercalated between them. Starting from an A xMO 2 oxide synthesized at high temperature, new metastable phases A x−yMO 2 or A x+zMO 2 can be obtained either by deintercalation or intercalation. Depending upon the intercalation rate and the nature of the M element, the process can be reversible (the integrity of the MO 2 slabs being preserved) or partially irreversible. In this latter case the loss of reversibility from a partial migration of the M element from the MO 2 layer to the interslab space. In the case of sodium intercalation compounds the decrease of the intersheet bonding leads to several structures which differ in terms of their MO 2 packing (sheet gliding). For the applications point of view these materials are very interesting as a result of the high cell voltages (3–4 V) attainable. For LI xCoO 2 and Li xMoO 2 systems very high energy densities have been obtained. Electrons play a very important role in both the thermodynamics and the kinetics. In materials with oxygen vacancies, an electronic compensation effect has been shown.
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