Etudes d'additifs susceptibles de sécuriser les electrolytes des batteries lithium-ion du point de vue : contrôle de charge, de l'effet retardateur de flammes et de la mouillabilité
Auteur / Autrice : | Mohamed Taggougui |
Direction : | Daniel Lemordant, Bernard Carre |
Type : | Thèse de doctorat |
Discipline(s) : | Chimie - Physique |
Date : | Soutenance en 2007 |
Etablissement(s) : | Tours |
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Résumé
This work aims at improving the security linked to the use of Li-batteries electrolytes. First, we tested the use of redox shuttles as an electrochemical system of internal protection against overcharge. We studied the possibility of employing ionic liquids as electrolytes for Li-ion accumulator. Finally, we tested two fluoro-alkylcarbonates as flame-retardant additives. Three classes of molecules were studied as redox shuttles : metallic complexes, aromatic and radical systems. Only a few molecules of the radical system allow an efficient protection against overcharge of the cell Li4Ti5O12 / (EC/DEC) + LiPF6 / LiFePO4. These molecules are the TEMPO (tetramethylpiperidinyl-1-oxy) and the Cyano-TEMPO, they limit the recharge voltage to 3. 5-3. 7 V vs. Li+/Li. We chose two ionic liquids whose anion is the bis(trifluoromethanesulfonyl)imide (TFSI-) ion and the cation N-trimethyl-N-hexylammonium (HTMA+) or N-trimethyl-N-propylammonium (PTMA+). Two electrolytes have been optimized regarding their transport properties and their thermal stability : HTMA+TFSI-/(EC:DEC) [ = 0. 3] + LiTFSI 1 mol. L-1 and PTMA+TFSI-/(EC:DEC) [ = 0. 25] + LiTFSI 1 mol. L-1 ( with the weight fraction of the ionic liquid in the mix). The electrochemical study shows that the electrolyte cannot be cycled on the graphite electrode. Cyclic voltammetry performed on the negative electrode demonstrates the absence of formation of the SEI and reveals the intercalation of ammonium cations into the graphene layers. The analysis of the electrode, after cycling, by X-Ray Diffraction (XRD) and NMR spectroscopy, confirms the insertion of the ammonium cations. To improve the cycling results, we replaced the EC by the Cl-EC (chloro-ethylene carbonate). Cl-EC allows the formation of the passivation layer that protects graphite against exfoliation, but does not prevent the intercalation of the ammonium cations. The cycling tests on the negative electrode Li4Ti5O12 and the positive LixCoO2 were conducted successfully. In order to improve the ininflamability of the electrolyte (EC/DMC/DEC + LiPF6 1 mol. L-1), we tested the use of two fluoro-alkylcarbonates as flame-retardant additives : the 2,2,3,3,4,4,4-hepatafluorobutyl methyl carbonate (FC2) and the 2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl methyl carbonate (FC1). These two additives were synthetized at the laboratory. The electrochemical study demonstrates that the FC1 compound does not modify the cyclability of the positive electrode LixCoO2. Nevertheless, it generates an important loss of capacity of the graphite electrode. The cycling tests conducted on the LixCoO2 cathode and the graphite anode show the good performances of the FC2 additive.