(Invited) From Solid State Chemistry to Battery Materials Electrochemistry: A Tool for Solid State Chemistry

Abstract

Researches on battery materials has considerably increased during the last 30 years due to the huge development of lithium-ion batteries for portable devices (laptops, cellular phones, …) and more recently with the need to store energy in order to optimize its consumption. The next goal is the development of batteries for electric vehicles and their use into the grid. Nevertheless, the aqueous batteries (Ni-Cd, Ni-MH and Lead Acid) are always used in numerous applications thanks to their low price. In most of the batteries, the intercalation (deintercalation) of monovalent cations (H+, Li+, Na+) and electrons is the basic electrochemical reaction. The cell voltage is equal to the difference in Fermi level between the two electrodes. If one electrode exhibits a constant voltage it can act as reference and therefore, the cell voltage reflects all structure modifications which occurs on the material upon intercalation. The change is cell voltage depends on: (i) the electronic band filling, (ii) the change in the band structure due to change in composition, (iii) the modification of the Magdelung energy. In some cases the monotonous decreases of the voltage during the intercalation reaction indicates that the reaction occurs through a monophasic domain. In numerous cases, the reaction mechanism is more complicated and involves biphasic domains and/or formation of materials with a specific composition. The voltage vs composition curve is much more complicated and gives directly the phase diagram of the studied system. For the solid state chemist, the studies of the electrochemical reaction using a battery opens new possibilities to determine phase diagram at RT, but also to synthesize new metastable phases from a precursor made by classical solid state chemistry. The in situ or in operando experiments allow to follow directly the structural modifications occurring during the electrochemical reaction. Using synchrotron experiments it is possible to make experiments at high rate and therefore evidence intermediate states and understand the reactions mechanism. In the recent years, XRD diffraction and XAS analysis were intensively used.On the opposite way, very sophisticated Solid State Chemistry techniques like solid state MAS NMR, Mossbauer spectroscopy and high resolution electron microscopy in aberration corrected microscope allowed to characterize battery materials at the atomic level. In this presentation, some typical results obtained in Bordeaux concerning lithium and sodium layered oxides will be presented to emphasize the dual approach: batteries to solid state chemistry and solid state chemistry to batteries.