PH'D Defense M Yunfan Shao

Abstract :
Lithium batteries are considered as the most successful energy storage systems due to their high energy and power density. However, there are concerns over the current lithium batteries, including safety, sustainability, etc. On the one hand, the safety issue involves thermal runaway phenomena which is mainly ascribed to the currently used flammable organic liquid electrolytes. To improve the safety of batteries, polymer electrolytes with better thermal stability and mechanical strength have been developed to substitute the organic electrolytes. Among them, single-ion polymer electrolytes (SIPEs) with an immobilized anionic group at the polymer chain exhibit extra advantages owing to their high lithium transference number. This special design reduced the polarization inside the electrolyte to avoid salt precipitation, concentration gradient and more importantly lithium dendrite growth. On the other hand, the sustainability issue of current batteries comes from the use of critical minerals such as Cu, Co, and Ni which are widely used in positive electrode materials and current collector. Organic active materials are composed of earth abundant elements (C, H, N, O and S) with a lower environmental footprint and toxicity. And the production of organic active materials could be independent from the price and geopolitical limits of such elements, rendering them potentially greener and more sustainable candidates for battery applications.
In this thesis, organic active materials were applied with SIPEs to answer these concerns. SIPEs with different structures were designed, synthesized, and characterized to reach high ionic conductivity, lithium transference number close to 1 and good electrochemical stability. The SIPEs were then applied in lithium-metal batteries (LMBs) with an organic positive electrode with long-term cycling stability and high Coulombic efficiency. Additionally, the interaction between the lithium-metal electrode and the SIPEs were also studied by lithium plating/stripping tests. With lithium nitrite doping in the SIPE, I observed improved cycling stability and reversibility with lithium-metal electrodes by modifying the solid electrolyte interphase (SEI) to reduce the interfacial resistance and improve surface morphologies of the lithium metal. The rational designed SIPEs could deliver high performance facing both organic and metallic lithium electrodes to promote safe and sustainable organic batteries, rendering them a powerful candidate for future energy storage devices.