Abstract :
[en] Electrical energy consumption has increased dramatically and its mode of consumption has diversified over the years, with the development of mobile energy storage devices such as lithium batteries. The present thesis is dedicated to the synthesis and study of the compound Li4Ti5O12 as negative electrode in Li-ion batteries. The main objective is the development of innovative synthesis techniques based on soft chemistry in order to control and study the effect of structural and microstructural parameters on the electrochemical properties of the compound.
Because of its ability to intercalate/deintercalate Li+ ions in its structure without stress-generating volume changes of the structure, Li4Ti5O12 is considered as a so-called "zero-strain" electrode, favouring excellent cycling performance. Therefore, this compound appeared to us a most interesting candidate in this study.
The first part of this work investigates a method involving the freeze-drying of a gel precursor followed by self-ignition. Hydroxypropylmethylcellulose is used as an additive and the synthesis of Li4Ti5O12 takes place in four steps : Gelling – Freeze-drying – self-ignition – Calcination. Ammonium nitrate was also added to the reactants to determine the impact on the self-ignition process and the consequences on the structure, microstructure and electrochemical properties of the compound. Indeed, a major issue usually encountered in self-ignition is controlling the amount of heat released, and therefore the temperature during self-ignition. These factors greatly influence the structural and microstructural properties of the compound. The results show that addition of ammonium nitrate leads to an excessive crystal growth which is unfavorable to electrochemical properties of the compound.
In the second part of the thesis, the spray-drying method has been investigated using both a lab-scale and a pilot-scale equipment, to take into account possible developments on an industrial scale.
For both devices, the influence of concentration on particle morphology was observed. The lab spray-dryer allowed us to obtain a convex to mushroom morphology. For the pilot spray-dryer, the morphologies vary from cauliflower to smooth hollow spheres, and can be further modified by addition of a carbonate source which decomposes during calcination. Depending on the particle morphology and its surface for intercalation of Li+ ions, the electrochemical performances have been improved, sometimes considerably. The drying temperature also affects the performance of the compound through the agglomeration rate of the particles.
In conclusion, a wide panel of microstructures has been obtained by investigating two synthesis methods with various sets of experimental conditions. Although all these powders are Li4Ti5O12, there is a significant variation of their electrochemical performance and we have attempted to correlate these with particle morphology. Some prominsing results have been obtained and suggest that performance can be further improved through future work, on both fundamental and applied aspects.
