Chemical investigations of spinel lithium manganese oxides.

For rechargeable lithium batteries, the most widely used cathode material is LiCoO2, which is gradually being replaced by other lithium metal oxides due to the high cost and toxicity of cobalt. Oxide members of the Li-Mn-0 system present themselves as attractive replacement candidates which offer all the properties for good electrode materials in addition to their low cost and low toxicity. Despite its limited practical capacity of about 120 mA h g-1, LiMn2O4 is considered to be a very attractive cathode material, mainly due to its ease of manufacturing, low cost and environmental compatibility and it has started to appear in some commercial products. However, LiMn2O4 suffers from significant capacity fading upon cycling especially at high temperatures. Worldwide research efforts have been devoted toward the understanding of this class of oxides and to produce better materials for battery applications. The work presented in this thesis is part of this effort. In this work, a preparation method has been developed to obtain a pure, single-phase, and high surface area spinel LiMn2O4. Characterisation of the product confirmed that it is a thermally stable, single-phase spinel LiMn2O4 with surface area of 7.6 m2 g-1. The relatively high surface area found for samples prepared during the course of this study indicates a positive step when compared with those prepared using other techniques. Another member of the spinel family, lambda-MnO2, has been prepared by the topotactic extraction of lithium from the previously prepared LiMn2O4. The spinel lambda-MnO2 has further chemically reduced to yield the oxide hydroxide form of manganese. The manganese oxide hydroxide obtained above has not been reported before and has been designated lambda-MnOOH to indicate the relationship to its precursor oxide. Both manganese oxides were thoroughly characterised using various analysis techniques. Surface area measurements of the oxides gave 6.3 and 11.2 m2 g-1 for lambda-MnO2 and lambda-MnOOH respectively. Kinetic investigation of the oxidation of normal spinel LiMn2O4 by chemical extraction of Li+ revealed that the extraction process is diffusion-controlled with a diffusion coefficient of 3.35-3.66 X 10-7 cm2 s-1 and activation energy of 34.81 -40.22 kJ mol-1. It was found that due to the low activation energy of reaction, Li+ is the only diffusing cation in the system and the process is taking place using the lattice's tetrahedral interstitial sites. The reversibility of the extraction of lithium from LiMn2O4 in aqueous solutions was investigated. It has been found that it is possible to re-introduce lithium ions into the Mn204 framework only if Mn2+ is present in the solution. It was confirmed by x-ray diffraction that the result of the insertion reaction is the spinel LiMn2O4. Other members of the Li-Mn-O system were investigated. Various oxide members were obtained by varying the Li:Mn ratio of the reactants. Some oxide members were successfully synthesised by mixing stoichiometric amounts of Li2CO3 and Mn2O3. A method to prepare the electrochemically active LiMnO2 was proposed. The oxide was positively identified as orthorhombic LiMnO2 with unit cell dimensions a = 2.790A, b = 5.614A and c = 4.564A. Other oxide members were prepared and characterised by means of x-ray diffraction and thermal analysis techniques