Stéphane Hamelet

An Electrochemical Cell for Operando Study of Lithium Batteries Using Synchrotron Radiation

A new electrochemical cell has been specially designed for operando experiments at synchrotron facilities both for X-ray diffraction and X-ray absorption. It allows the investigation of insertion materials under high current densities (up to 5C rate) and hence to study complex phenomena of structural and electronic changes out of equilibrium. The LiFePO 4 -FePO 4 system has been chosen as a case study to validate this cell, and tricky phenomena, with apparent delays in phase formation compared with the number of electrons exchanged, have been spotted.

The effects of moderate thermal treatments under air on LiFePO 4-based nano powders

The thermal behavior under air of LiFePO4-based powders was investigated through the combination of several techniques such as temperature-controlled X-ray diffraction, thermogravimetric analysis and Mossbauer and NMR spectroscopies. The reactivity with air at moderate temperatures depends on the particle size and leads to progressive displacement of Fe from the core structure yielding nano-size Fe2O3 and highly defective, oxidized LixFeyPO4 compositions whose unit-cell volume decreases dramatically when the temperature is raised between 400 and 600 K. The novel LiFePO4-like compositions display new electrochemical reactivity when used as positive electrodes in Li batteries. Several redox phenomena between 3.4 V and 2.7 V vs.Li were discovered and followed by in-situX-ray diffraction, which revealed two distinct solid solution domains associated with highly anisotropic variations of the unit-cell constants.

Electrochemical Kinetic Study of LiFePO4 Using Cavity Microelectrode

Lithium cation insertion and extraction in LiFePO4 were electrochemically studied with a cavity microelectrode (CME). Cyclic voltammetry measurements were used to characterize the kinetics of the material. LiFePO4 was successfully cycled from 0.1 mV s–1 up to 1 V s–1 and is therefore a suitable material to be used in high power applications, such as asymmetric hybrid supercapacitors. Several kinetic behaviors were observed depending on the sweep rate. The LiFePO4 was found to follow different kinetics behaviors depending of the sweep rate. The charge storage mechanisms were investigated for Liþ extraction/insertion.