Claude Bohnke

Mechanism of ionic conduction and electrochemical intercalation of lithium into the perovskite lanthanum lithium titanate

Abstract The ionic conductivity and electrochemical intercalation properties of La 2 3−x Li 3 x TiO 3 solid solutions (for 0.07 ≤ x ≤0.13) have been studied. These compounds present a perovskite-type structure (ABO 3 ) with cation deficiency at the A-sites. The purely ionic conductivity was confirmed and the mechanism of ionic conduction investigated using impedance spectroscopy techniques. We find that the temperature dependence of conductivity can be modelized by a Vogel-Tamman-Fulcher (VTF)-type relationship. In these materials, where the high ionic conductivity may originate from the presence of vacancies in the A-sites of the perovskite structure, the VTF behavior would suggest a mechanism of conduction involving the tilting of the TiO 6 octahedra. The lithium intercalation was also investigated in LiClO 4 (M)-PC electrolyte using galvanostatic discharge and charge at very low rates (one Li/250 and /1500 h) in order to approach the equilibrium. It was shown that the lithium intercalation leads to the presence of a plateau around 1.5 V/Li in the discharge curve, it is partly reversible and the capacity of the electrode is not very high. A maximum lithium uptake of 0.15 was found. The diffusion coefficient of lithium in the intercalated material was determined by impedance spectroscopy at room temperature and found to range from 10 −8 cm 2 s −1 to 10 −9 cm 2 s −1 as intercalation proceeds. Since the experimental impedance spectroscopy data performed at room temperature follow a Warburg behavior at low frequency, the intercalation seems to proceed in a single-phase process although a plateau is observable in the discharge curve.

Electrochemical intercalation of lithium into the ramsdellite-type structure of Li2Ti3O7

The chemical and electrochemical intercalation of lithium ions into the ramsdellite-type structure of Li2Ti3O7 has been carried out by chemical reduction with n-BuLi or by galvanostatic discharge of an electrochemical cell. A maximum lithium uptake has been found to be of 0.60 and 1 per Li2Ti3O7 respectively at room temperature. During this intercalation process the ramsdellite network is preserved as shown either by electrochemical galvanostatic discharge or by X-ray powder diffraction analysis. However it is clearly shown by X-ray diffraction analysis and by electrochemical spectroscopy that two intercalation sites of very close energy are involved in this process. The potential difference between the two peaks observed in the voltammogram is 350 ± 20 mV. The variations of both the charge transfer resistance and the diffusion coefficient observed by a.c. impedance spectroscopy as intercalation proceeds can be explained by the presence of these two different intercalation sites.

Electrochemical intercalation of lithium into the perovskite-type NbO2F: influence of the NbO2F particle size

Abstract Niobium(V) oxyfluoride, NbO2F, has a perovskite structure and presents the property of lithium intercalation by topotactic chemical reaction either with n-butyllithium dissolved in n-hexane or by electrochemical reaction. The intercalation leads to the reduction of the transition metal from the oxidation state Nb(V) to the oxidation state Nb(III). This allows a theoretical Li/NbO2F intercalation ratio of 2. In this paper we will show that this theoretical value can be approached by using micron the sized active material particles. Moreover, the electrical properties of the cathode studied by the galvanostatic intermittent titration technique and a.c. impedance spectroscopy are explained in terms of structural and grain size considerations. Results of cycling experiments are also described.

Synthesis of thin-film electrodes based on LiPON and LiPON-LLTO-LiPON

Thin lithium-conducting amorphous films of lithium phosphorus oxynitride (LiPON) are synthesized by high-frequency (HF) magnetron sputtering. The effect of sputtering conditions (HF power, support temperature, working gas pressure, and deposition time) on the microstructure of LiPON films is studied. The optimal conditions of HF magnetron sputtering that allow dense, uniform, crack-free, air-stable, amorphous thin LIPON films to be formed are studied. The composite solid electrolyte LiPON-LLTO-LiPON is synthesized based on LiPON films and the La2/3 − xLi3xTiO3 (LLTO) ceramics characterized by the high conductivity with respect to lithium ions but unstable in contact with metal lithium. Its microstructure and electrophysical properties are studied. It is found that the LiPON film prevents the chemical reaction of metal lithium with the LLTO solid electrolyte. Due to the high chemical stability and the enhanced conductivity, the composite solid electrolyte based on LiPON-LLTO-LiPON can be used in electrochemical devices.

Electrochemical Insertion of lithium into perovskite structure NbO2F

Abstract Electrochemical insertion of lithium into perovskite structure NbO2F has been investigated. Lithium insertion leads to a structural transformation from the initial cubic unit cell to the LiNbO3-type structure which presents a hexagonal symmetry. The maximum lithium uptake is 1.2 Li/Nb at room temperature. It corresponds to 60% of the expected maximum. Insertion has been followed by discharge experiments associated with impedance spectroscopy. The experimental results show that the electrochemical process can be characterized by an electrode reaction - layer formation - diffusion sequence. An equivalent circuit model which includes charge transfer reaction, layer formation, constant phase elements and diffusion impedance is proposed. The mechanism of insertion is discussed on the basis of the crystallographic structure of the perovskite. The layer may be viewed as a shell of a high inserted compound surrounding a relatively unaffected nucleus which is formed as insertion proceeds.