No abstract

Li(x)CoO(2) and Li(x)NiO(2) (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of Li(x)CoO(2) and Li(x)NiO(2) as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Li(x)(Co,Ni)O(2) (0 < x < or = 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.

In this study, a comprehensive experimental in situ analysis of the evolution of the occupied and unoccupied density of states as a function of the charging state of the Li x≤1CoO2 films has been done by using synchrotron X-ray photoelectron spectroscopy (SXPS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and O K- and Co L 3,2-edges XANES. Our experimental data demonstrate the change of the Fermi level position and the Co3d–O2p hybridization under the Li+ removal and provide the evidence for the involvement of the oxygen states in the charge compensation. Thus, the rigid band model fails to describe the observed changes of the electronic structure. The Co site is involved in a Co3+ → Co4+ oxidation at the period of the Li deintercalation (x ∼ 0.5), while the electronic configuration at the oxygen site is stable up to 4.2 V. Further lowering of the Fermi level promoted by Li+ extraction leads to a deviation of the electronic density of states due to structural distortions, and the top of the O2p bands overlaps the Co3d state which is accompanied by a hole transfer to the O2p states. The intrinsic voltage limit of LiCoO2 has been determined, and the energy band diagram of Li x≤1CoO2 vs the evolution of the Fermi level has been built. It was concluded that Li x CoO2 cannot be stabilized at the deep Li deintercalation even with chemically compatible solid electrolytes.

No abstract

22] The results presented in this paper have been obtained using the silicon parameter set Si-I of Ref. [7] [23] Note that the initial arrangement of the particles has little effect on the outcome of the simulation. [18] [24] The term relative importance denotes the number of times a certain reaction occurs normalized by the total number of reactions considered.Performance degradation of electronic devices due to electric fatigue of functional materials is investigated for alkali containing transition metal oxide electrodes used in Li-ion batteries [1] or for interfaces between functional materials and transparent conductive oxides (TCO) used in organic light emitting diodes (OLEDs) or ferroelectric devices. Two reasons for electric fatigue can be discussed: performance loss due to long-term use like charging/discharging cycles in batteries, light emission from OLEDs or switching of ferroelectric devices and device failure because of misuse, like overcharging, overcurrents, over-voltages or thermal stress. While misuse can be excluded by electronic safety circuitry degradation under normal working conditions can only be avoided by smart material development. A prerequisite is the identification of degradation paths like irreversible changes in electronic structure and contact potentials leading to the formation of inactive regions in the materials. Experiments under welldefined conditions are necessary to unambiguously identify the origin of degradation. This work presents an experimental approach for in-situ studies of the prototypic Li-ion battery cathode material LiCoO 2 and the important ITO/organic interface in OLEDs and first experiments concerning stoichiometries, electronic structure and band alignment. Both oxide materials have in common a mixed conduction (electronic and ionic): In LiCoO 2 the conductivity for Li + and e ± is and must be in the same order. For ITO the electronic conductivity is only two orders of magnitude smaller than those of highly conducting metals and thus dominates the parasitic O 2± conduction via intrinsic oxygen defects. Interfaces of ITO, however, are expected to be strongly influenced by a low dopant concentration near the contact. [2,3] As a result of a low dopant concentrations, the interfaces of ITO have to be considered like those of semiconductors, as indicated in Figure 1(b,c). A change of dopant concentration by O 2± diffusion is then expected to change the dopant concentration and the barrier for hole injection, which is a key parameter for the function of the device.Experimental: Darmstadt Integrated System for Materials Research (DAISY-MAT): The experiments have been carried out in the Darmstadt integrated system for Materials Research (DAISY-MAT) (Fig. 2) combining different preparation chambers via a cyclic distribution chamber with a photoelectron spectrometer. It is a multi-chamber ultrahigh vacuum (UHV) system with a base pressure of p = 10 ±9 mbar allowing sample transfer between analysis and preparation facilities without leaving well defined vacuum conditi...