Lithium metal batteries offer a promising route to higher energy density than conventional lithium-ion batteries, but they suffer from severe lithium dendrite growth and rapid capacity decay. To address these issues, quasi-solid-state composite electrolytes (QSCEs) have emerged as a promising option by bridging the performance and processing gaps between inorganic and polymer solid electrolytes. Utilizing polymer matrices provides essential flexibility, facilitates lithium-ion transport, and ensures compatibility with commercial roll-to-roll manufacturing. Nevertheless, state-of-the-art QSCEs still fail to meet practical demands due to several intertwined challenges: (1) low room-temperature ionic conductivity (< 1 mS/cm) that limits fast charging; (2) a narrow electrochemical stability window (ESW) imposed by the poor high-voltage tolerance of conventional polymers; (3) the chemically and mechanically unstable interface against lithium anodes that triggers parasitic side reactions, increases interfacial impedance, and accelerates dendrite propagation. These combined deficiencies severely shorten cell-cycle longevity, thereby restricting the viability of QSCEs for long-term operations.

To overcome these severe limitations, recent research focus has shifted toward integrating ionic liquids (ILs) via IL-confinement strategies. In the combination of modifying polymers or fillers, these strategies successfully incorporate the unique properties of ILs, such as a wide ESW, inherent non-flammability, and high ionic conductivity, into the solid-like composite matrices. Motivated by these advancements, the overall goal of this thesis is to develop high-performance QSCEs with improved room-temperature ionic conductivity, a widened ESW, and robust interfacial stability with Li metal anodes by combining IL-confined strategies. The key advancements of this research are summarized as follows:

1. 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 that the specific influences between confined IL and bulk IL on electrolyte properties and performance remain unclear and require further investigation.

2. The effects of confined IL on electrolyte properties and performance were investigated, together with the Li+ transport mechanism. Confined QSCEs were prepared by confining IL within SiO2 (SiO2@IL-C), combined with LiTFSI and polyethylene oxide (PEO).  Compared to unconfined systems prepared by direct mixing, the confined electrolyte exhibited better properties, including higher ionic conductivity, an improved Li+ transference number, and over 1900 hours of stable stripping/plating cycles. The improved performance is attributed to a novel Li+ transport pathway (Li+ → SiO2@IL-C) in the confined electrolyte, which facilitates faster Li+ transfer and enhances overall electrochemical performance.

3. The impact of IL-confined thickness on Li+ diffusion and overall performance was further investigated. Electrolytes with different IL-confined thickness (SiO2@IL-C, different IL content on SiO2) were prepared.  The results demonstrate that adjusting IL layer thickness on  SiO2 allows the electrolyte to achieve superior ionic conductivity, a high Li+ transference number, and enhanced cycling durability; insufficient IL thickness narrows the effective Li+ migration region due to weak confinement, which impedes ion kinetics; conversely, an excessively thick IL film induces bulk-phase characteristics, thereby reducing ion transport. The electrolytes with optimized IL thickness enhance interfacial stability and electrochemical performance; specifically, LiFePO4//Li coin cells using the optimized electrolyte maintained 77.7% capacity retention after 700 cycles at 0.5 C. Theoretical calculations confirmed that this controllable confinement accelerates ion diffusion by constructing more efficient transport networks.

4. Based on the SiO2@IL filler, a fluorine-grafted gel polymer electrolyte (F–GPE) was developed. The F–GPEs demonstrate desirable properties, including high ionic conductivity (1.21 mS cm‒1 at 25 °C), a wide electrochemical window (5.20 V), and over 4000 hours of cycling stability in the Li//Li 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 results revealed that fluorine reduces the interaction and coordination number of polymer-Li+ and forms a stable solid electrolyte interphase (SEI), thus enhancing the overall electrolyte performance.

5. The use of SiO2 may be hindered by its tendency to aggregate and its insufficient capability to form a robust SEI. To address these issues, GaN quantum dots (QDs) were introduced into PVDF-based GPEs to regulate overall performance. Due to the inherent properties of the QD materials, GaN QDs induce robust localized electronic states and internal fields an order of magnitude stronger than traditional fillers (e.g., SiO2). Benefiting from this, the GaN QD-based GPE achieves an over two-fold increase in both room-temperature ionic conductivity and Li+ transference number compared to GPEs without GaN QDs.  This regulation enhances electrochemical performance, enabling symmetric cells to cycle for over 5000 h at 1 mA cm–2 and full cells to exceed 2000 cycles in LiNi0.6Co0.2Mn0.2O2//Li.