Composite solid electrolytes (SCEs) are developed to overcome the limitations of inorganic solid-state electrolytes (ISSEs) and solid polymer electrolytes (SPEs). ISSEs have high stability and conductivity but suffer from high interfacial impedance and poor machinability, while SPEs are flexible but have low room-temperature conductivity and a narrow electrochemical window. SCEs combine the benefits of both by integrating polymers with inorganic materials like oxides. Despite some improvements, SCEs still do not fully meet practical needs. To enhance performance, researchers are developing quasi-solid-state composite electrolytes (QSCEs) by adding ionic liquids (ILs), utilizing IL-confinement strategies, and modifying polymers for better suitability in lithium metal batteries.

The overall goal of this thesis is to develop QSCEs with improved ionic conductivity, a wide electrochemical stability window, and good interface stability to lithium metal, using the IL-confinement strategy. The key advancements achieved in this thesis are as follows:

1. In the first part, the review summarized and explored IL confinement in various quasi-solid-state electrolytes (QSSEs), including polymer/IL, host/IL, and polymer/filler/IL systems, discussing the impact of factors like substrates and confinement methods. It compared IL confinement in QSSEs with general IL confinement in other fields, noting that IL confinement enhanced electrolyte performance and differed significantly in the battery context. The study highlights the specific differences between confined IL and bulk IL on electrolyte performance remain unclear and require further investigation.

2. In the second part, the effects of confined IL on electrolyte properties and performance were investigated, with a focus on the Li⁺ transport mechanism. Confined QSCEs were prepared by confining IL within SiO₂ (SiO₂@IL-C), combined with LiTFSI and PEO. For comparison, unconfined QSCEs were produced by directly mixing all compositions. The electrochemical results showed that confined QSCEs outperformed unconfined QSCEs, exhibiting higher ionic conductivity, Li⁺ transference number, and more stable stripping/plating cycling over 1900 hours. Molecular dynamics (MD) simulations and density functional theory (DFT) calculations revealed that the confined QSCEs create a novel Li⁺ transport pathway (Li⁺ → SiO₂@IL-C), enhancing Li⁺ transfer and thereby improving the performance of the confined QSCEs.

3. In the third part, a fluorine-grafted QSCE (F–QSCE) with the SiO₂@IL filler was developed. For comparison, a non-fluorinated QSCE was prepared. The F–QSCE demonstrates good properties, including high ionic conductivity (1.21 mS cm^‒1 at 25 °C), wide electrochemical window (5.20 V), and over 4000 h of cycling stability in Li//Li symmetric cells. When paired with a LiFePO₄ cathode, it retains 98.8% capacity after 460 cycles, while with a high-voltage NCM622 cathode, it retains nearly 100% capacity after 350 cycles. The theoretical studies revealed that the fluorine reduces the interaction and coordination number of polymer-Li⁺, thus enhancing overall electrolyte performance.