Development of Hybrid Polymer Electrolytes with Tailored Mechanical Properties for Lithium Batteries

The current energy and climate crisis calls for a fast transition of the energy sector. The storage of renewable energy using lithium-ion batteries builds the foundation for this transition. However, current lithium-ion batteries pose major safety risks. Solid-state batteries using polymer electrolytes are one of the candidates to make lithium-ion batteries safer and allow for higher energy densities due to the use of metallic lithium as the anode material. However, the current ionic conductivities and high voltage stability values of polymer electrolytes are insufficient to displace liquid electrolytes. As a special class of polymer electrolytes, hybrid polymer electrolytes are promising candidates to overcome these shortcomings of classical polymer electrolytes. This thesis describes the synthesis and characterization of ORMOCER®-based hybrid polymer electrolytes. The synthesis was systematically altered to manipulate the connectivity of the inorganic backbone. Additionally, three different organic crosslinkers based on vinyl, epoxy, and methacrylate groups were used. The polymers were characterized in terms of their mechanical and electrochemical properties. Their rheological behavior was analyzed by stress-strain experiments. The lithium-ion conductivity and the transference number were measured and their stability of the polymers against lithium metal, as well as oxidation and reduction stability were determined. Additionally, several synthesis parameters were varied, and polymer additives were mixed into the polymer formulation to further improve the electrochemical properties of the best performing electrolyte. The curing time of the polymers was altered, as well as the curing temperature. Different lithium salts were added to the polymer formulation, the effect of added ionic liquid was investigated, and the addition of LLZO particles. All hybrid polymer electrolytes synthesized exhibit comparable mechanical properties and similar ionic conductivities. However, the epoxide-based polymer showed superior electrochemical stability values. This polymer was used to design two different types of cathode electrolyte interfaces either by infiltration of porous cathodes or by incorporation of the polymer into the cathode slurry. Both methods showed comparable cyclability results with the infiltration method being less labor intensive during the cell preparation. Full cell tests at 60 °C using infiltrated cathodes were performed using LFP and NCM as cathode active materials. Both cell types proved to be electrochemically active with good cyclability results for the LFP cells and moderate stability in the NCM cells. The electrochemical results show that the performance of ORMOCER®-based hybrid polymer electrolytes exceed the performance of traditional polymer electrolytes, which makes them a valid candidate to replace liquid electrolytes in multi-electrolyte solid-state battery.