New synthetic pathways for carbon-doped oxides for battery and related applications

Abstract

Doping anions into materials for energy storage is a new approach to enhance conductivity and battery performance. In this thesis, titanium-based materials are studied, replacing oxygen with carbon. Three techniques are used to synthesize the materials with different amount of carbon-doped in titanium oxides, and two for lithium titanates using the previously synthesized titanium oxycarbides as precursors. One crucial factor is the carbon level, which needs to be controlled with the different synthetic approaches used to obtain the material. The carbon level can have an influence on the structure as a less binding atom inside the structure. This is done with X-Ray diffraction (XRD) and for application, the magnetic properties and electrochemical properties in batteries are tested.

The synthesis of TiCxOy with x between 0 and 1 in 0.1 steps was carried out with different oxygen partial pressure to help stabilize the inserted carbon in the material. Microstructural and TGA analysis help to understand the material's densification and carbon-to-oxygen ratio. The oxygen-to-carbon ratio plays an important role during the synthesis of each of the synthetic approaches used as the method influences the ratio quite significantly. Under the given conditions CO sintering has the highest oxygen partial pressure leading to a higher oxygen content in the product. Compared to 5 % H₂ in Ar the oxygen partial pressure is slightly lower so the oxygen ratio is lower as in CO synthesis as well. In a high-temperature vacuum furnace, it is the lowest oxygen partial pressure so nearly no oxidation is happening. The oxygen content at the end will influence the material properties significantly. One method to investigate that the oxygen partial pressure and so the oxygen content is different in the product from the synthetic route is the Thermogravimetric analysis (TGA), energy electron loss spectroscopy (EELS), and X-Ray absorption spectroscopy (XAS). To understand slightly more about carbon bonding SXES (Soft X-Ray emissions spectroscopy) is made. This method showed that the materials could be divided into three different regions: An oxygen-rich region, where the samples are still dominated by oxygen and carbon is doped in with 0.4 moles at max. An intermediate range, from 0.4 to 0.6 moles, and the carbon-rich regions, where carbon plays a more dominant role in the applications.

The materials for the titanium oxycarbides are unsuitable for electrode applications in batteries because the capacity is just a quarter of the theoretical capacity and fades very quickly. Still, this material's electronic conductance is relatively high, which could make them an excellent current collector. The EXAFS indicates that titanium becomes reduced with carbon insertion and also the oxygen content, which is influenced by the synthetic approach. There is a formation of unpaired electrons as the materials are all Pauli-paramagnetic regardless of the sintering method. This changes with doping carbon in lithium titanates. All materials sintered in carbon monoxide are Pauli-paramagnetic, but the sample sintered in 5 % hydrogen in argon has some superconductivity, and some stay Pauli-paramagnetic. In this case the material property changes with higher carbon doping of the precursor. The critical temperature of these materials is 11.5 K, comparable to the oxide material.

For battery testing every C-Rate is applied for 25 cycles for the evaluation. The performance of the carbon-doped lithium titanate is promising and better than the full oxygen material for some synthetic pathways. Especially the materials sintered in Argon show some oxygen redox of the sample.