Lithium Insertion and Proton Exchange in Layered and Spinel Oxides of Cobalt and Manganese

Abstract

Spinel lithium manganates are of interest as lithium ion sieves and as battery cathode materials The reversible electrochemical extraction and reinsertion of lithium into LiMn2O4 is under commercial development for new low cost, environmentally friendly, rechargeable batteries In aqueous acid solution lithium can be oxidatively extracted from LiMn2O4 to leave a metastable spinel lattice (λ-MnO2). λ-MnO2 has a high capacity and specificity to the absorption of lithium ions from aqueous solution and may be used as a lithium selective sieve. The lithium rich members of the lithium manganate spinels, Li1+yMn2-yO4(0 < y ≤ 0.33), are formed at ca. 400°C and contain manganese primarily in the +4 state. Li1+yMn2-yO4 cannot be oxidatively delithiated, instead undergoing lithium < proton ion exchange to give a defect spinel similar to λ-MnO2, but containing a large amount of inserted proton. This thesis investigates the effect of substituting cobalt into the LiMn2O4 and Li1+yMn2-yO4 spinels. The short, medium and long range structure of the cobalt doped spinels was investigated by X-ray absorption (EXAFS, XANES) and powder X-ray and neutron diffraction. Proton insertion was investigated by infrared and incoherent inelastic neutron scattering (INS) vibrational spectroscopies. In conjunction with investigations into the effect of substituting cobalt for manganese in the LiCoxMn2-xO4 and Li1.33-x/3CoxMn1.67-2x/3O4 spinel systems, the acid delithiation of a layered lithium cobaltate (LiCoO2) was studied as a possible aid in the understanding of the effects of cobalt substitution on the spinel lithium manganates, and also as a potential lithium ion exchange material in its own right. These studies showed that cobalt can be substituted into the lithium manganate spinels to give homogeneous phases which have improved crystallinity over the non-substituted spinels. Doping the stoichiometric LiMn2O4 spinel with cobalt reduces the structural stress caused by oxidation and reduction of Mn3+. However, doping also reduces the amount of lithium that can be extracted in acid solution or in a 4 V battery. The end member of cobalt substitution is LiCoMnO4, a material that may be one of the first examples of a new generation of high voltage rechargeable lithium batteries, using improvements in electrolyte technology to exploit the Co3+ / Co4+ redox couple and give a cell that operates at above 5 V. Lithium rich Li1.33-x/3CoxMn1.67-2x/3O4 spinel can be prepared at high temperature (750°C) by addition of ca. 5% cobalt to the preparation. The inclusion of cobalt significantly reduces the formation of Li2MnO3 impurity. The changes in stoichiometry caused by cobalt substitution reduces the amount of lithium that may be extracted and cobalt appears to influence the sites of proton insertion so that a greater proportion are unexchangeable in the doped materials. Acid extraction of lithium from R3m layered LiCoO2 is accomplished by oxidation of the cobalt, with no evidence of lithium ↔ proton ion exchange. When LiCoO2 is incompletely formed acid treatment causes phase separation and a reduction in the layered character of the material. The presence of structural defects allows a small amount of incidental proton insertion to occur.