Interaction between High-Voltage Cathode Materials and Ionic Liquids for Novel Li-Ion Batteries

Abstract

The fast-growing market on electronic portable devices is possibly due to the development of Li-ion batteries. Besides, such batteries are the most promising candidates as energy storage media in (hybrid) electric vehicles, in the near future. However, improvements on electrochemical performances and on safety need to be achieved. With respect to energy density, positive electrodes with a high voltage vs. Li/Li+ are favourable, provided they are stable against the rest of the battery materials, thereby reflecting safety issues. Regarding this safety, misuse of a Li-ion battery often leads to promoting side reactions between the electrodes and the electrolyte. This then results in gas formation, packaging breaking and fire evolution due to exposure to air by reaction of highly reactive materials with oxygen. Therefore, novel electrolytes, such as ionic liquids are proposed, which may significantly reduce these parasitic reactions with the electrodes, and further allows the battery to operate at higher voltages due to its high voltage stability window. In this respect, the presented work concerns the research on a high voltage positive electrode material in combination with ionic liquids as electrolyte solvents. In Chapter 1, a brief history about the Li-ions intercalation concept, which is at the basis of Li-ion batteries, is introduced. Afterwards, the most important components of a Li-ion battery are discussed. Examples of negative and positive electrodes are shown, emphasizing the reasons why nano structures are to preferred in the beginning. Besides, ionic liquids are introduced and explained, as possible candidates as electrolytes or electrolyte additives. In Chapter 2, the synthesis of magnesium-doped nickel-based high-voltage positive electrode materials (LiMg0.05Ni0.45Mn1.5O4) via four different synthesis routes is shown: a solid-state method; a sol-gel method; a xerogel route and an auto-ignition method. A preliminary structural analysis is performed with XRD and SEM, showing the crystallinity and the agglomerations of the powders, respectively. Furthermore, TEM analysis on the powders showed agglomerated nanoparticles ranging from 10 nm to 200 nm. Finally, electrodes made with the synthesized materials are tested with a charge-discharge galvanostatic technique, showing the existence of impurity - mainly LiMn2O4, increasing from sol-gel, xerogel, to auto-ignition. It also gives promising results in terms of capacity retention at high charge-discharge rates. In Chapter 3, the powders made via the auto-ignition method, which showed the most promising result in the previous chapter, were further studied with an in situ charge-discharge galvanostatic technique coupled with X-ray absorption spectroscopy (XAS), which proved the presence of manganese ions in LiMg0.05Ni0.45Mn1.5O4 being reduced and oxidized, explained by the presence of manganese-rich nanodomains within the particles. Chapter 4 concerns certain safety issues of Li-ion batteries. Here, ionic liquids as possible novel electrolyte additives are studied as possible candidates to contribute to safer batteries. In this respect, the CO2-adsorption ability of the ionic liquid N-buthyl-N-methylimidazolium tetrafluoroborate ([BMIm][BF4]) is tested with a Cailletet apparatus. The interaction between ionic liquids and CO2 has been analyzed from a thermodynamical point of view. The most common theories to study the gas and gas-liquid mixtures were introduced, together with a novel approach based on the Langmuir adsorption theory. In contrast to most models, the Langmuir isotherm has a very general approach: the solvent is seen as a storage medium, in which the number of available sites depends on the number of available solvent species which adsorb gas molecules. The Langmuir model can be used in a wide concentration range and the dissolution energies, together with the entropy values, can be calculated. In the case of the presence of a lithium salt, the Langmuir model could easily be extended to the Langmuir-Hinselwood model, so as to take the adsorption/association of lithium ions to the solvent sites into account. In the [BMIm][BF4] + CO2 system studied, both the Langmuir and Langmuir-Hinselwood models could be very well used to explain the results. The obtained solubility energy and entropy do not differ so much from the evaporation energy and entropy of pure CO2, showing small solvent-CO2 interaction. When to the mixture [BMIm][BF4] + CO2 a lithium salt, i.e., LiBF4 is added, a Langmuir-Hinselwood isotherm describes the obtained data very well. However, addition of the salt reduces the amount of CO2 that can be dissolved. Actually, the results of applying this model show that the lithium ions rather stay at the anion, preventing dissociation of the salt in this IL. Consequently, it can be argued that the lithium salt, even if undissociated provides sites which can be exploited by either the lithium ions or by the CO2 molecules, as they contribute to the equilibrium equations. In Chapter 5, the electrochemical behavior of PYR14TFSI + LiTFSI is studied in relation to electrodes made from LiMg0.05Ni0.45Mn1.5O4 (LMNMO). In this respect, cyclic voltammetry (from 3.5 to 4.9 V) and electrochemical impedance spectroscopy have been used. For further comparison, also batteries made with a commercially available electrolyte has been exploited. The cyclic voltammetry showed slower redox processes regarding the cell with the ionic liquid. Another cell with the ionic liquid has been tested at a lower cycle scan rate up to 5.2 V, proving that the current is significantly hampered by kinetics in this ionic liquid. However, it showed the stability of the system a higher voltages compared to the battery with the commercial electrolyte. The electrochemical impedance spectroscopy (EIS) measurements of the system with the ionic liquid showed a process which was not present in the cell with the commercial electrolyte, possibly introduced by the slow kinetics. However, in symmetrical cells with metallic lithium at both sides, it was possible to ascribe the EIS features of the cell with the commercial electrolyte to the lithium-electrolyte interface. On the other hand, EIS measurements of the battery with the ionic liquid reveals the presence of a layer also at the LMNMO-electrolyte side, which continuously grows upon cycling. In summary, while the positive electrode materials can be employed in novel Li-ion batteries, provided they have a higher purity, the use of ionic liquids as electrolytes is still hampered by their poor electrochemical performances. Nevertheless, they are still promising, due to their electrochemical stability at high voltages . Besides, thanks to their high gas-adsorption ability, ionic liquids can be taken into consideration as electrolyte additives, for safety purposes.