Charaf Cherkouk

Cathodes - Technological review

Lithium cobalt oxide (LiCoO2) was already used in the first commercialized Li-ion battery by SONY in 1990. Still, it is the most frequently used cathode material nowadays. However, LiCoO2 is intrinsically unstable in the charged state, especially at elevated temperatures and in the overcharged state causing volume changes and transport limitation for high power batteries. In this paper, some technological aspects with large impact on cell performance from the cathode material point of view will be reviewed. At first it will be focused on the degradation processes and life-time mechanisms of the cathode material LiCoO2. Electrochemical and structural results on commercial Li-ion batteries recorded during the cycling will be discussed. Thereafter, advanced nanomaterials for new cathode materials will be presented.

Harmonic Principles of Elemental Crystals—From Atomic Interaction to Fundamental Symmetry

The formation of crystals and symmetry on the atomic scale has persistently attracted scientists through the ages. The structure itself and its subtle dependence on boundary conditions is a reflection of three principles: atomic attraction, repulsion, and the limitations in 3D space. This involves a competition between simplicity and high symmetry on the one hand and necessary structural complexity on the other. This work presents a simple atomistic crystal growth model derived for equivalent atoms and a pair potential. It highlights fundamental concepts, most prominently provided by a maximum number of equilibrium distances in the atom’s local vicinity, to obtain high symmetric structural motifs, among them the Platonic Solids. In this respect, the harmonically balanced interaction during the atomistic nucleation process may be regarded as origin of symmetry. The minimization of total energy is generalized for 3D periodic structures constituting these motifs. In dependence on the pair potential’s short- and long-range characteristics the, by symmetry, rigid lattices relax isotropically within the potential well. The first few coordination shells with lattice-specific fixed distances do not necessarily determine which equilibrium symmetry prevails. A phase diagram calculated on the basis of these few assumptions summarizes stable regions of close-packed fcc and hcp, next to bcc symmetry for predominantly soft short-range and hard long-range interaction. This lattice symmetry, which is evident for alkali metals as well as transition metals of the vanadium and chromium group, cannot be obtained from classical Morse or Lennard-Jones type potentials, but needs the range flexibility within the pair potential.

Positive electrodes based on Ion-implanted SrTiO3

Abstract An O2-electrode was fabricated using a metal ion implanted SrTiO3 single crystal. The time resolved oxygen exchange rate of ion implanted strontium titanate (SrTiO3) single crystals was studied by means of oxygen solid electrolyte coulometry (OSEC). Transmission electron microscopy (TEM) was performed in order to determine structural changes after ion implantation. Moreover, theoretical modelling based on defect chemistry under equilibrium conditions was applied for determining of effective rate constants. OSEC measurements turn out to be a damage and calibration free method, which was used for the first time in order to characterize kinetic parameters of oxygen exchange on single crystalline surfaces.

Energy Storage in crystalline Materials based on multivalent Ions

Energy conversion and storage has become the main challenge to satisfy the growing demand for renewable energy solutions as well as mobile applications. Nowadays, several technologies exist for the conversion of electric energy into e. g. heat, light and motion or vice versa. Among a large variety of storage concepts, the conversion of electrical in chemical energy is of great relevance in particular for location-independent use. Main factors that still limit the use of electrochemical cells are the volumetric and gravimetric energy density, cyclability as well as safety. The concept for a new thin-film rechargeable battery that possibly improves these properties is presented. In contrast to the widespread lithium-ion technology, the discussed battery is based on the redox reaction of multivalent Al-ions and their migration through solid electrolytes. The ion conduction and insertion processes in the crystalline materials of the suggested cell are discussed under a crystallographic point of view to identify suitable electrode and separator materials. A multilayer-stack of all-solid-state batteries is synthesized by pulsed laser deposition and investigated in situ, i. e. during charge and discharge, by X-ray reflection and diffraction methods. The correlation between crystal structure, morphology and electrical performance is investigated in order to characterize the ion diffusion and insertion process.

Categorization of electrochemical storage materials en route to new concepts

Because of their broad range of applications, electrochemical energy storage devices are the subject of a growing field of science and technology. Their unique features of high practical energy and power densities and low prices allow mobile and stationary applications. A large variety of electrochemical systems has been tailored for specific applications: Lithium-ion batteries for example have been optimized for mobile applications ranging from mobile phones to electric vehicles. On the other hand, sodium-sulphur accumulators – among others – have been developed for stationary applications to account for the capricious nature of renewable energies. Chemistry, physics and materials science have led to the optimization of existing cell-chemistries and the development of new concepts such as all-liquid or all-solid state batteries as well as high-energy density metal-air batteries. The aim of the BMBF (Federal Ministry of Education and Research, Germany)-financed project “CryPhysConcept” is to develop new concepts for electrochemical energy storage applying a crystallographic approach. First, a categorization of the main solid components of batteries based on their underlying working principles is suggested. Second, an algorithm for the identification of suitable new materials and material combinations, based on economical, ecological and material properties as well as crystallographic parameters, is presented. Based on these results, new concepts using multi-valent metal ions are proposed. Theoretical as well as experimental results including an iron-ion approach are presented.