No abstract

Several series of Li 1Ϯy Mn 2 O 4Ϯ␦ samples with the spinel structure were synthesized. These samples had different Li/Mn ratios ͑by varying the Li/Mn ratio used in starting materials͒ and various oxygen contents ͑by controlling synthesis conditions, including temperature, heat-treatment time, and purging gas during both the solid-state reaction and annealing͒. In systematic studies of charge-discharge cycling behavior and in situ X-ray diffraction ͑XRD͒ 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 4.2 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 (-MnO 2 ) 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 Li 1Ϯy Mn 2 O 4Ϯ␦ -type materials at temperatures around 10°C 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.

We have prepared the intermetallic compound Cu 6 Sn 5 using mechanical-alloying, gas-atomizing, and melt-spinning techniques. The electrochemical performance of the compound is critically dependent on its morphology due to different preparation methods. The Cu 6 Sn 5 alloy created by mechanical alloying, consisting of Ͻ1 m thick flake powder, has the best battery performance of all compounds. It delivers a rechargeable capacity of 200 mAh/g ͑2000 Ah/L͒ over 50 cycles when the cycled voltage range is restricted to 0.2-1.5 V. The effect of the mechanical-alloying time and Cu/Sn ratio on its battery performance was further investigated. The presence of excess Cu in alloy, relative to Cu 6 Sn 5 , showed improved cyclability at the expense of capacity, whereas an excess of Sn resulted in poor cyclability. A lithium-ion cell based on a flaked Cu-Sn microcomposite alloy negative electrode and a 5 V LiNi x Mn 2Ϫx O 4 positive electrode was assembled. The cell showed an average working voltage at 4.0 V and cycled well with a reversible capacity of ca. 200 mAh/g based on the pure Cu-Sn alloy when a cell was cycled between 3.5 and 4.6 V.

No abstract

We have used a lithiated MnO 2 , Li 0.33 MnO 2 , with ordered alternating one-dimensional [1 ϫ 2] and [1 ϫ 1] channels as a cathode material in solid-state lithium/polymer cells. An optimized cell can operate at moderate temperatures (40-80ЊC). Li 0.33 MnO 2 delivers a rechargeable capacity of 160 mAh/g with a flat potential plateau at ca. 3.0 V vs. Li/Li ϩ at the C/3 rate and 65ЊC, corresponding to a specific energy of 450 Wh/kg of the pure oxide. Cells show good rate capability and excellent cyclability when cycled between 2.7 and 3.5 V at 80% depth of discharge, whereas a capacity decline was observed when cycled between 2.0 and 3.5 V. Capacity fading upon cycling is believed to be due to the formation of a thin layer of spinel phase (transformation to Li 0.5 MnO 2 from Li 0.33 MnO 2 ) on the particle surfaces, as well as to increased cell resistance during charge/discharge cycling. The cell self-discharge at high temperature and the thermal stability of Li 0.33 MnO 2 in contact with the polymer electrolyte are also discussed.