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Project

Electrodeposition of Anode and Cathode Materials for Lithium Batteries

Rechargeable lithium-based batteries are mainly used in portable devices, due to their high specific energy. In the future they will also play an important role for grid-scale storage and the electrification of vehicles. In this PhD thesis, three types of batteries were investigated in the frame of three research projects. Each type of battery has certain improved properties compared to current lithium-ion batteries.

In a first project, high voltage cathode materials were investigated. Materials like Li2MSiO4 result in a 5 V lithium-ion battery. Therefore new electrolytes are required with a high electrochemical stability. Ionic liquids are known for their broad electrochemical window, which makes them suitable for high voltage lithium-ion batteries. Unfortunately due to their high viscosity, they have slow kinetics. Preceding research within the Binnemans and Fransaer groups resulted in the development of liquid metal salts. These ionic liquids contain the metal ion of interest in the cationic and/or anionic structure of the ionic liquid, resulting in a very high metal ion concentration. By coordinating neutral ligands around the lithium ion, new complexes were discovered and characterized. These coordination compounds showed an improved (electro)chemical stability compared to the non-coordinated neutral ligands, and a broad electrochemical window of approx. 5 V vs. Li+/Li was obtained. From these ionic liquids it was also possible to deposit lithium metal at high current densities, which makes them suitable as battery electrolyte for high power applications.

Batteries based on solids improve the safety of a battery, due to the avoidance of flammable, liquid electrolytes. In a second project, it was the challenge to deposit electrode materials conformally on 3D substrates, in our case silicon pillars. Therefore, the deposition process needs to be self-limiting which is not commonly observed for electrodeposition. Here the proof-of-concept is given for the deposition of thin manganese oxide films (80 nm). This oxide was deposited by the electro-precipitation via oxygen reduction, similar to the deposition of Li2O2 in lithium-air batteries. Since the manganese oxide is less conductive than the current collector, the deposition of new oxide material is preferential on the non-coated current collector. As a result the electro-precipitation of metal oxides is a self-limiting deposition technique, where the electrochemical reduction of dissolved oxygen is followed by the chemical formation of the metal oxide.

In the third project, lithium-air batteries were investigated due to their very high specific energy. This type of batteries is still in an early development stage and the focus of our research was on the deposition of the cathode material. Here, oxygen from the atmosphere is used as reagent for the formation of Li2O2. The efficient deposition and dissolution of Li2O2 is one of the major challenges for the cycle life of a lithium-air battery. The deposition/dissolution mechanism was investigated from a mass point-of-view by an in-situ technique, using an electrochemical quartz crystal microbalance. It was found that in dimethylsulfoxide (DMSO) there are two possible pathways, a solution or a surface pathway, which is determined by the applied potential. The followed pathway for deposition also influenced the ease of dissolution. Where the solution pathway was suitable for deposition and dissolution, it was found that the surface pathway led to the formation of insoluble lithium oxide species. A redox mediator was used for the dissolution of the latter lithium oxide species.

Another challenge of lithium-air batteries is the limited discharge rate, or in other words, the deposition rate of Li2O2. For this, the electrolyte plays an important role, due to crucial properies such as a low volatility, a high chemical stability and a high oxygen solubility. Ionic liquids were investigated as electrolyte because they are known for their low volatility and high stability. However the oxygen solubility in ionic liquids is rather low and the Li2O2 deposition rate highly depends on the dissolved oxygen concentration. To improve the latter, ionic liquids were functionalized with a fluorinated alkyl chain attached to the cation or anion. As a result the oxygen solubility of the ionic liquids could be increased up to six times compared to the solubility in the commercial ionic liquid without the fluorinated chain. Although these results are promising, future research will confirm whether the rate performances of the lithium-air battery can be improved by using these fluorinated ionic liquids.

The involvement in the three research projects and knowledge about the three types of batteries, led to a cross-fertilization. Ionic liquids, originally developed for the deposition of materials for all-solid-state batteries, were used for high voltage batteries. The deposition mechanism at the cathode of a lithium-air battery was used for the deposition of electrode materials for all-solid-state batteries. Finally, ionic liquids were developed for the improvement of the cathode reaction in a lithium-air battery. The uniqueness of this PhD thesis lies in the combination of the three research projects, that led to new insights and lifted the research to a higher level.

Date:1 Jul 2013 →  29 Jun 2017
Keywords:Anode materials, Cathode materials, Li-ion battery, Ionic liquids, Li-air battery, Electrodeposition, Solid-state Li-ion battery
Disciplines:Ceramic and glass materials, Materials science and engineering, Semiconductor materials, Other materials engineering
Project type:PhD project