FACTORS INFLUENCING THE DISCHARGE CHARACTERISTICS OF NA_(0.44)MNO_2-BASED POSITIVE ELECTRODE MATERIALS FOR ECHARGEABLE LITHIUM BATTERIES

摘要

Cathode materials with the Na_(0.44)MnO_2 structure are highly reversible towards lithium insertion processes, resist conversion to spinel and are not easily damaged by abuse through over-charging or over-discharging. While the theoretical capacity of Li_xMnO_2 made from Na_(0.44)MnO_2 is at least 200 mAh/g, in practice only about 45-60% utilization is obtained in lithium cell configurations, in part because not all of the lithium can be extracted below the oxidative stability limit of the electrolyte. Higher utilization is obtained in cells with partially exchanged materials, which have larger unit cells, but in-situ exchange processes lead to an apparent capacity fade upon cycling. The benefits of the larger unit cell without the complications of such exchange processes may be realized by replacing some of the manganese with titanium in the structure. A series of compounds with the formula Li_xTi_yMn_(l-y)O_2, where y=0.11, 0.22, 0.33, 0.44, and 0.55 was synthesized and characterized for this study. A maximum reversible capacity of 150 mAh/g between 2.5 and 3.5V vs. Li was obtained for Li/P(EO)_8LiTFSI/Li_xTi_(0.22)Mn_(0.78)O_2 cells at 85℃ in electrochemical potential spectroscopy experiments. Cells with the titanium-doped manganese oxides exhibited a fade rate of 0.12% or less per cycle, indicating that the increased utilization does not come at the expense of cyclability.