Poly(ethylene oxide) (PEO) has become the leading material for SPEs since the 1970s because of its exceptional coordination and dissociation ability towards Li salt, excellent interfacial stability with Li metal, and high mechanical strength to form flexible membranes. Specifically, high molecular weight PEO shows the advantages of preventing Li dendrites and improved cycling stability due to its prominent mechanical property compared to lower molecular weight PEO. However, PEO-based SPEs are often criticized for their limited electrochemical stability window (ESW) primarily owing to the low oxidative stability of ether oxygen and limited room temperature ionic conductivity (up to 10 -6 S cm -1 ) due to their crystallization tendency that restricts polymer segmental motion for ion transport. These limitations significantly restrict their operational voltage range (< 4V) and fast cycling capability in solid-state LMBs. Intensive research has been centred on enhancing ionic conductivity and oxidative stability of PEO-based SPEs, such as applying inorganic fillers, plasticizing ionic liquids, designing complex polymer architecture and single-ion conducting polymers, and introducing crosslinked networks. Among these, ionic liquid (IL)-based electrolytes have received significant interest because of their good electrochemical and thermal stability, low vapor pressure, and non-flammability. Early works from Howlett et al. and other researchers demonstrated promising physical and electrochemical properties of ILs as the electrolyte solvent for LMB applications. Susan et al. have successfully incorporated ionic liquids with compatible polymers for the preparation of "ion gels", showing increased thermal stability and high ambient temperature conductivity. In recent years, enlightening from the idea of concentrated electrolyte in liquid electrolytes, research has identified high salt concentration ionic liquid electrolyte as a novel approach to effectively enhance Li ion transport, electrochemical stability, and support fast charge-discharge. Pal et al. and co-workers investigated an ether-aided super concentrated ionic liquid electrolyte that could effectively enhance ion transport and interfacial stability toward Li metal and high voltage materials. With the ever-increasing attempt to boost the performance of SPEs, the use of high concentrated salt-IL mixtures is predicated to improve both electrochemical stability and ion transport of PEO-based SPEs by altering the ion dynamics and creating more efficient Li transport pathways within the polymer matrix.