Correlating capacity fading and structural changes in Li1+yMn2-yO4-delta spinel cathode materials - A systematic study on the effects of Li/Mn ratio and oxygen deficiency

Several series of Li1+/-yMn2O4+/-delta samples with the spinel structure we re synthesized. These samples had different Li/Mn ratios (by varying the Li /Mn ratio used in stirling materials) and various oxygen contents (by contr olling synthesis conditions, including temperature, heat-treatment time, an d purging gas during both the solid-state reaction and annealing). In syste matic studies of charge-discharge cycling behavior and in situ X-ray diffra ction (XRD) at room temperature, it was found that both the charge/ dischar ge profile and the structural changes during cycling are closely related to the degree of oxygen deficiency created in the synthesis process. Their ef fects on the capacity fading are much more important than the Li/Mn ratio o r other factors. A higher degree of oxygen deficiency is accompanied with a faster fading of capacity during cycling. In cells using spinel cathodes w ith an oxygen deficiency. the capacity fading during cycling occurs on both the 4.2 and 4.0 V plateaus. This behavior is quite different from that fou nd in cathodes without an oxygen deficiency, where most of the capacity fad ing occurs on the 4.7 V plateau region only. Our in situ XRD results indica te clearly that the capacity fading on the 4.2 V plateau is related to the phase transition between the cubic II and cubic III (lambda -MnO2) structur e, while the capacity fading on the 4.0 V plateau is related to the phase t ransition between the cubic I and cubic II spinel structures. The effects o f oxygen deficiency on the structural phase transition of Li1+/-yMn2O4+/-de lta-type materials at temperatures around 10 degreesC were also studied. It was found that this phase transition is closely related to the degree of o xygen deficiency of the material. In samples with no oxygen deficiency, thi s phase transition disappeared. (C) 2001 The Electrochemical Society.