The cathode in rechargeable lithium-ion batteries operates by conventional intercalation; Li+ is extracted from LiCoO2 on charging accompanied by oxidation of Co3+ to Co4+; the process is reversed on discharge. In contrast, Li+ may be extracted from Mn4+-based solids, e.g., Li2MnO3, without oxidation of Mn4+. A mechanism involving simultaneous Li and O removal is often proposed. Here, we demonstrate directly, by in situ differential electrochemical mass spectrometry (DEMS), that O2 is evolved from such Mn4+ -containing compounds, Li[Ni(0.2)Li(0.2)Mn(0.6)]O2, on charging and using powder neutron diffraction show that O loss from the surface is accompanied by diffusion of transition metal ions from surface to bulk where they occupy vacancies created by Li removal. The composition of the compound moves toward MO(2). Understanding such unconventional Li extraction is important because Li-Mn-Ni-O compounds, irrespective of whether they contain Co, can, after O loss, store 200 mAhg(-1) of charge compared with 140 mAhg(-1) for LiCoO(2).

Rechargeable lithium batteries represent one of the most important developments in energy storage for 100 years, with the potential to address the key problem of global warming. However, their ability to store energy is limited by the quantity of lithium that may be removed from and reinserted into the positive intercalation electrode, Li(x)CoO(2), 0.5 < x < 1 (corresponding to 140 mA.h g(-1) of charge storage). Abandoning the intercalation electrode and allowing Li to react directly with O(2) from the air at a porous electrode increases the theoretical charge storage by a remarkable 5-10 times! Here we demonstrate two essential prerequisites for the successful operation of a rechargeable Li/O(2) battery; that the Li(2)O(2) formed on discharging such an O(2) electrode is decomposed to Li and O(2) on charging (shown here by in situ mass spectrometry), with or without a catalyst, and that charge/discharge cycling is sustainable for many cycles.

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The rate capability of various lithium-ion half-cells was investigated. Our study focuses on the performance of the carbon negative electrode, which is composed of TIMREX SFG synthetic graphite material of varying particle size distribution. All cells showed high discharge and comparatively low charge rate capability. Up to the 20 C rate, discharge capacity retention of more than 96% was found for SFG6. The rate capability of the half-cells is a function of both the particle size distribution of the graphite material and the preparation method of the electrode. A transport limitation model is proposed to explain the restrictions of the high current performance of graphite electrodes. The key parameters found to influence the performance of a graphite negative electrode were the loading, the thickness, and the porosity of the electrode.Lithium-ion batteries have attracted considerable scientific and technological attention for more than a decade. Being already broadly available for small portable electronic devices like mobile phones, laptop computers, video camcorders, and personal digital assistants, the field of commercial applications of lithium-ion batteries is gradually extending to large battery systems. A particular field of growing importance is the transportation sector, where the lithium-ion battery technology is envisaged for traction batteries in overall electric vehicles and for onboard ͑42 V͒ batteries in conventional, hybrid, and fuel cell vehicles. For these purposes, the vehicle battery must deliver sufficient power. At the same time, the automotive industry imposes good cycle and calendar lifetime 1,2 for the systems used with moderate material costs. Therefore, high-ratecapable and comparatively cheap electroactive materials are required for the development of high-power lithium-ion batteries. [3][4][5] Graphite materials with a high degree of graphitization based on synthetic or natural sources are attractive candidates for negative electrodes of lithium-ion batteries due to the relatively high theoretical specific reversible charge of 372 mAh/g. The electrochemical insertion of lithium into graphite leads to an intercalation compound with a chemical composition of LiC 6 . It was generally believed that graphite negative electrodes have only a moderate rate capability. 6,7 Slow kinetics 8,9 and a solid-state diffusion limitation during charge and discharge reactions were suggested as rationalities of why the graphite electrode does not deliver high currents. 10-12 Arora et al. 13 suggested that one of the most important parameters that limits the performance of lithium-ion batteries at high rates is the transport of lithium ions in the electrolyte. The transport of lithium ions in the solid phase takes place mainly within single particles, where diffusion lengths are much shorter than the thickness of the whole electrode. 13,14 However, recently we demonstrated that graphitebased electrodes support a much higher current density than believed before. 15 There are also other studies that could s...