Wu, Qi-Hui (2003)
Photoelectron Spectroscopy of Intercalation Phases.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

V2O5 and LiMn2O4 are promising cathode materials for lithium-ion batteries due to their high capacities and battery voltages. The several work was mainly focused on the study of electrochemical and structural properties during lithium intercalation. But there is no detailed knowledge of the changes in electronic structure and the intercalation mechanism itself. Especially no general agreement has been reached on the nature and the extent of the interactions between host material and alkali guest atoms. This thesis addresses the electronic stucture of transition-metal oxides and its changes during the intercalation of alkali metals. The intercalation of Na and Li into V2O5 thin films and LiMn2O4 powder samples have been studied mainly using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and resonant photoemission spectroscopy (RPES). The PVD prepared V2O5 thin films are nearly stoichiometric with only about 4% oxygen deficiency. After deposition onto the HOPG substrate, the oxide surface is smooth and polycrystalline. The valence band of V2O5 is formed by the hybridisation of the O2p and V3d electron states. It has been shown by RPES that the V3d admixture to the valence band is about 20%. Therefore, the real electron occupation number of the 3d electron state of the V ions in V2O5 is about 2 instead of 0, and the simple ionic model is not valid. The influence of heating on the V2O5 films has also been studied. Elevated temperatures lead to sub-stoichiometric V2O5-x that can be probed by chemically shifted components in the V2p3/2 emission line, and a decrease of O1s/V2p intensity ratios. Annealing of the sub-stoichiometric films at 400°C in an oxygen atmosphere lead to the reoxidation of vanadium to its higher oxidation state. The alkali metals are instantaneously intercalated into V2O5 when they are deposited onto the surface at room temperature. Only a small amount (about 10%) of the alkali metal atoms remain adsorbed on the surface due to the intercalation kinetics. The results obtained in this work demonstrate that the electrons of intercalated alkali metals s orbitals are mostly transferred to the transition-metal 3d orbitals and cause the reduction of the transition-metal ions as proven by the XPS and UPS data. The values of effective electron transfer for Na3s and Li2s are about 0.42 and 0.55 electrons per alkali atom, respectively. With low content of intercalated alkali metals, the electronic and crystalline structure of the host do not change considerately. For Na, an alkali saturation concentration of V2O5 films can be reached as Na1.4V2O5 without decomposition of the host, for Li, this saturation value is about Li2.5V2O5. When this limit for alkali intercalation is reached, further deposition of alkali atoms will not intercalate into the host but form oxides, peroxides and even metallic alkali on the surface. The formation of surface oxide films on the electrodes would have a severe impact on battery performance. A better understanding of such films can be essential to solve the stability problem of lithium-ion batteries, such as capacity loss, power-fade, poor cyclability, and self-discharge. After the over-intercalated samples have been kept in the ultra-high vacuum chamber for few days, the alkali metal will react further with vanadium oxides and form alkali oxides and peroxides on the surface. In this work, we have clearly demonstrated the formation of alkali oxides and peroxides species, which are probably part of the so-called solid electrode interface (SEI) layers. Finally, the electronic structure and surface composition of LiMn2O4 powder has been studied. The results show that manganese ions exist in two oxidation states: a trivalent state (Mn3+) as well as a tetravalent state (Mn4+). The photoemission intensity ration of Mn3+ to Mn4+ is about 0.9, so that the average oxidation state is 3.55 which is a little higher than the expected value of +3.5 which is probably due to small amounts of lithium oxides formed on the surface. UPS and RPES indicate that the Mn ions are in a high spin configuration, and O2p and Mn3d orbitals are strongly hybridised.

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