New electrolytes for high power Li-ion batteries : electrochemical stability, Li-ion insertion kinetics and corrosion inhibition properties

A new generation of high power Li-ion batteries that avoid carbon as anodic material replacing it with the spinel Li4Ti5O12 has been studied in this work. For these new Li-ion cells new electrolyte compositions can be considered. The main work of this thesis was focused on the electrolyte. Electrochemical techniques such as Cyclic Voltammetry and Amperometry were used to characterize the behaviour of electrolytes containing LiSO3CF3 or LiN(SO2CF3)2 (LiTFSI) or LiN(SO2C2F5)2 (LiBETI) salts. It was confirmed that LiTFSI determines a severe corrosion of the aluminium current collector at potential around 3.7 V (vs. Li). A protective effect induced by solvents having a cyano-group has been shown. The increase in repassivation potential (ER) observed with nitriles is in the range 0.4-0.5 V, and the value diminishes with increasing the temperature. Based on the experimental observations made and information collected from the literature, a mechanism to explain the inhibiting properties of nitriles was proposed. This was based on the assumption that aluminium undergoing corrosion have highly reactive intermediates, e.g. Al+, capable of reducing the electrolyte. The extension of the experiments to prototype Li-ion cells demonstrated that corrosion inhibiting properties combine with superior Li-ion insertion kinetics. Prototype Li-ion cells were made using Swagelok-type cells. Tests of Li4Ti5O12/LiCoO2 cells were performed for high charge rate and under extensive cycling. Charge/discharge curves showed that the new electrolytes offer properties suitable for high power batteries. Particularly, electrolytes containing LiTFSI salt were found to be compatible with LiCoO2 cathode materials on an aluminium current collector under cycling of up to 4.15 V (vs.Li.) at ambient temperature. Electrochemical stabilities of some solvents from the lactone and nitrile families were studied as well as kinetics. Substitutions in the α and γ positions of the γ-butyrolactone (γ-BL) were not successful in increasing the electrochemical stability of the γ-BL. UVvis spectra as well as HPLC analyses on electrolyzed γ-BL/LBF4 electrolytes suggested that the electrochemical decomposition of γ-BL involves a complex mechanism. Likely the reactions: occurs 'in parallel', 'in sequence' and exhibits 'feedback'. Electrochemical stability of some nitriles tested: only oxidipropionitrile exhibited a resistance to oxidation higher than γ-BL. All the electrolytes under investigation showed higher currents on glassy carbon than on tin oxide electrodes. LiMn1.5Ni0.4Ti0.1O4 electrode for high voltage Li-ion battery were also tested. The use of cyclic voltammetry in combination with nano-composite TiO2 electrodes proved suitable for kinetics evaluation. The nature of the salts as well as the solvents provided an effect on the Li-ion insertion kinetics. The observed differences were explained by means of a computational approach (MESP) relating its results to the "hard soft/acid base" concept.