Functional design of advanced polymer architectures for improved lithium-ion batteries

Lithium ion batteries (LIBs) are ubiquitous for applications in consumer electronics, electric vehicles, and grid-scale energy storage. Despite rapidly increasing demand, modern LIBs face significant challenges with regards to their safety and energy density. Additionally, the rigid nature of existing LIBs precludes their use in emerging applications in flexible/wearable electronics. Polymeric materials promise to address many of the issues facing LIBs, yet the existing polymers used commercially fall short of this goal. In this work, we design functional polymer materials to address three major challenges for next-generation LIBs. We explore the structure-property relationships of these polymer architectures in the context of ion transport, mechanical properties, and electrochemical performance. In the first project, a new polymer electrolyte is designed to replace the flammable liquid electrolyte in conventional LIBs. We study the effect of lithium ion coordination in polymer electrolytes and discover a modified polymeric backbone that loosely coordinates to lithium ions. The loose coordination of this new polymer electrolyte enables an improved lithium transference number of 0.54, compared to 0.2 achieved in conventional polymer electrolytes. This polymer electrolyte is demonstrated to operate effectively in a battery with a lithium-metal anode. In the second project, the learnings of the lithium coordination environment from the first project are used to design a multifunctional polymer coating to stabilize high energy density lithium metal anodes. We combined loosely-coordinating fluorinated ligands dynamically bonded with single-ion-conductive metal centers. The resulting supramolecular polymer network functions as an excellent lithium metal coating, allowing for achievement of one of the highest-reported coulombic efficiencies and cycle lives of a lithium metal anode. A systematic investigation of the chemical structure of the coating reveals that the properties of dynamic flowability, single-ion transport, and electrolyte blocking are synergistic in improving Li-metal coating performance. This coating is applied in a commercially relevant lithium metal full-cell and increases the cycle life over two-fold compared to an uncoated anode. The final project uses supramolecular polymer design to create ultra-robust ion transport materials. We show that when soft ion conducting segments are combined with strong dynamically bonded moieties in the polymer backbone, the ion transport properties can be decoupled from the mechanical properties. This decoupling enables for the creation of polymer electrolytes with extremely high toughness and high ionic conductivity. These supramolecular materials enable the fabrication of stretchable and deformable batteries that demonstrate respectable energy density even when stretched to 70% of their original length. Overall, the work demonstrated in this thesis provides a robust understanding towards designing polymer networks with tunable ion transport and mechanical properties. Additionally, the polymer materials demonstrated here provide promising avenues toward improving the safety, energy density, and flexibility of LIBs

Keywords: Chemical Engineering.
Advisor: Bao Zhenan
Cui Yi
Qin Jian (Professor of Chemical Engineering)