Ionic Liquids for High Performance Solid-state Lithium Metal Batteries

Abstract

The quest for high energy storages has driven the growth of high-performance lithium metal batteries, but this has also raised serious safety concerns. In response, ionic liquids (ILs) have become a popular choice due to their high ionic conductivity, non-flammability, and ability to facilitate the formation of stable solid electrolyte interphase (SEI) layer. Understanding the challenges faced by lithium metal batteries and the role of ILs in them is vital to improving their performance. This study examines how ILs affect key factors such as ionic conductivity, Li⁺ ion transference number, electrochemical stability window, and the lithium metal anode/electrolyte interface. It also investigates the use of ILs with different types of cathodes, such as, including LiFePO4 (LFP), LiNi0.6Co0.2Mn0.2O2 (NCM622), LiNi0.8Co0.1Mn0.1O2 (NCM811), and LiCoO2 (LCO). A comparative study was made on the development of ionic liquid involved solid-state

electrolyte to achieve high performance solid-state lithium metal batteries. Three key aspects are addressed in this thesis:

Firstly, an ionic liquid was injected into metal-organic framework (MOF-5) nanomaterials to improve the poly(ethylene oxide) (PEO) solid electrolyte and enhance the performance of solid- state lithium batteries. The results show that the formed nano-wetted interface structure can greatly improve the interface stability, reduce the interface impedance, and inhibit the Li dendrite growth. The MOF structure accelerates the transport of lithium ions by ion confinement effect on anions inn the IL and large-size cations, thereby improving the lithium transference number. As a consequence, the overall performance of solid solid-state Li metal battery has been improved. Secondly, using electrospun polyacrylonitrile PAN membranes, the ionic liquid and liquid electrolyte monomers are combined and in situ polymerized to form a polymer electrolyte. In this system, the decomposition of the ionic liquid is involved in the formation of the solid electrolyte interface ( SEI mem brane. Through analysis at different current densities of the Li symmetric cell , it was found that the ionic liquid can significantly suppress the formation and growth of lithium dendrites. Moreover, due to the increased lithium affinity of the ionic liqui d, Li ion transport is accelerated, resulting in a high lithium transference number, which improves conductivity and allows the battery performing within a wide temperature range. Additionally, L i F e P O 4 /Li batteries can run steadily for 100 0 cycles at high rate of 2 C. T hirdly, through the combined action of fluorine containing additives and ionic liquids, the in situ formed polymer lithium battery can operate stably at high voltage. Analysis has shown that the SEI membrane in this system is rich in LiF, whi ch effectively increases interface stability. The ionic liquid enhances the electrochemical window of the polymer electrolyte, allowing this system to match high voltage cathodes. Moreover, IL is beneficial to improve the interfacial contact and provide st able components for the interfacial layer. Results show that at room temperature, the NCM811/cell can perform at 1C, and the LCO/Li cell has good cycling performance at 4.45 V, increasing the battery energy capacity. This project contributes to the understanding of the application of ionic liquids in solid state electrolytes, the knowledge of which can be used to design the solid state electrolyte. The chemical compositions of the SEI layers formed on the surface of Li anode from this experimental work also provide valuable data that can be used in the future studies