Electrode and Electrolyte Design for High Energy Density Li-Ion and Mg-S Batteries

The climate change caused by the burning of fossil fuels has become a challenge for the development of modern society. To decrease the carbon emission, the development of electrical vehicles has accelerated in recent years. However, the electrical vehicles still suffer from the short operation distance and the safety problems caused by the Li-ion batteries. Therefore, a new battery system which is safer and possess a high energy density is needed. To achieve this goal, this thesis focused on the development of the Magnesium-Sulfur battery (Mg-S) and the next-generation Li-ion batteries (LIBs).

The Mg-S battery attracts great interest because of its high capacity and low cost. Like other sulfur batteries, the electrochemical process of the Mg-S battery is complicated. During the discharge, the sulfur ring which consists of eight sulfur atoms will break to form magnesium polysulfides (MgS_x, 4 ≤ x ≤ 8) which can dissolve into the electrolyte and move to the Mg anode side. The side reaction between MgS_x and Mg can cause fast capacity decay and the passivation of Mg anode.At the end of discharge, solid discharge products MgS₂ and MgS will form. The slow Mg²⁺ diffusion in the solid product will result in poor reversibility and rate performance of the Mg-S battery. To solve this problem, copper nanoparticles grown on carbon nanofibers (Cu@CNF) were synthesized and added into sulfur cathode. The interaction between copper and sulfur result in the formation of small discharge products and facilitate the redox process in Mg-S batteries. Thus, high reversibility is achieved. The influence of Cu:S molar ratio is also investigated. Although higher sulfur utilization can be obtained using high Cu:S ratio, the total capacity (capacity calculated based on Cu and S total mass) will decrease. There is an optimal Cu:S ratio for both high total capacity and sulfur utilization.

The feasibility of Mg-S flow battery for grid-energy storage was also investigated. A prototype Mg-S flow battery was built and tested with various Mg electrolytes. It was found that the MgS_x solutions used in the Mg-S flow battery is unstable. The low solubility and instability of MgS_x in Mg electrolytes result in poor reversibility of the Mg-S flow battery. It was also proven that the MgS_x speciation and stability was affected by the Mg electrolytes composition.

For the research of LIBs, in-situ polymerized gel polymer electrolytes (GPEs) were investigated. One of the reasons for the safety issues of the LIBs is the flammability of organic solvents in liquid electrolyte. Solid polymer electrolytes (SPEs) without organic solvents are applied to improve the safety of LIBs. However, the low ionic conductivity of SPEs results in poor rate performance and low material utilization. The GPEs with a polymer matrix are safer than liquid electrolytes. GPEs also have a higher ionic conductivity than SPEs due to the solvent used in the electrolyte. In this work, macromonomers with different chain chemistry and ionic monomers are crosslinked in the presence of solvents or liquid electrolyte to in-situ fabricate GPEs. The effect of chain chemistry and the percentage of macromonomers in GPEs on conductivity and lithium transference number are investigated. It is found that both the chain chemistry and ratio of free and bound salt have a strong influence on ion transport properties.