Toward this end, it is useful to consider the evolution of the electronic structure from single particles (gaseous molecules) to the condensed phase and relate the oxidation potential to the resulting energy level structure. Nevertheless, none of these models presents by itself a detailed view of the electronic states referenced to the vacuum level, which would be needed for an improved conceptual understanding of oxidation potentials. This last approach has been found to be consistent with experimental data in the past 35–37 and is also currently applied to battery electrolytes with reasonable results.
Lithium nmc cathode cei free#
33,34 Finally, the oxidation potential of the organic solvent is typically evaluated via the ionization potential of the isolated solvent molecule diminished by the difference in free energies of solvation between the compound and its cation. 33 On the other hand, the distribution of electronic states around the redox potential is treated according to the Marcus model, which states that the electronic levels of reduced and oxidized species are separated due to solvent reorganization and are subject to statistical broadening due to the fluctuations of the solvent shell. The electrochemical potential of redox electrons is commonly established via a Born-type of cycle, including the ionization potential of the isolated species as well as the solvation energies of the reduced and oxidized species. There are several approaches for the evaluation of energy levels in the electrolyte, which are mainly derived from thermodynamical considerations and address different aspects of the energy level structure. 26, and the perspectives for further work are discussed. Here, an integrated overview with key insights regarding the electronic structure and reactivity is given based on Ref. Different aspects of the interface formation of LiCoO 2 electrodes have been published previously, especially in Refs. As a consequence, interfacial energy level diagrams are proposed, which contain electronic states in the electrolyte phase arising from the coordination of solvent molecules with salt anions. The results are linked to literature data of oxidation potentials for solvent–salt complexes, which demonstrate that the presence of salt significantly reduces the oxidation stability compared to the pure solvent phase. The data were obtained using a surface science approach, 27,28 allowing the determination of the electronic structure of the interface including effects from the electrochemical double layer. In this article, we present an overview on our experimental results on the electronic structure of LiCoO 2 –solvent interfaces. The perspectives for further investigations of the electronic structure of Li-ion cathode–liquid electrolyte interfaces are discussed. The results demonstrate that the simple energy level approach commonly used to evaluate the stability window of Li-ion electrolytes has very limited applicability. In agreement with thermodynamic considerations, our data show that surface layer formation on pristine electrodes occurs inside the electrochemical window as defined by the measured oxidation and reduction potentials, which can be attributed to electrode surface interactions. We find LiCoO 2 valence band–solvent highest occupied molecular orbital offsets that are in agreement with expectations based on ionization potentials, polarization effects, and solvent–salt interactions. In this article, we present an overview on our results obtained with a surface science approach and discuss the implications for the stability window of Li-ion electrolytes under consideration of calculated oxidation potentials from the literature. Although electrolyte decomposition is a key issue for the stability of Li-ion batteries and has been intensively investigated in the past, a common understanding of the concepts and involved processes is still missing.
0 kommentar(er)
