Correlating capacity fading and structural changes in Li1+yMn2-yO4-δ 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 were synthesized. These samples had different Li/Mn ratios (by varying the Li/Mn ratio used in stirling materials) and various oxygen contents (by controlling synthesis conditions, including temperature, heat-treatment time, and purging gas during both the solid-state reaction and annealing). In systematic studies of charge-discharge cycling behavior and in situ X-ray diffraction (XRD) at room temperature, it was found that both the charge/ discharge profile and the structural changes during cycling are closely related to the degree of oxygen deficiency created in the synthesis process. Their effects on the capacity fading are much more important than the Li/Mn ratio or other factors. A higher degree of oxygen deficiency is accompanied with a faster fading of capacity during cycling. In cells using spinel cathodes with 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 found in cathodes without an oxygen deficiency, where most of the capacity fading occurs on the 4.7 V plateau region only. Our in situ XRD results indicate 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) structure, while the capacity fading on the 4.0 V plateau is related to the phase transition between the cubic I and cubic II spinel structures. The effects of oxygen deficiency on the structural phase transition of Li1+/-yMn2O4+/-delta-type materials at temperatures around 10 degreesC were also studied. It was found that this phase transition is closely related to the degree of oxygen deficiency of the material. In samples with no oxygen deficiency, this phase transition disappeared. (C) 2001 The Electrochemical Society.