Although properties of binary IL electrolytes (i.e., either salt-IL or polymer-IL) have been extensively studied, the complex properties of 'ternary systems' composed of a polymer and a high concentration salt-IL mixture (HCIL) have been less explored. In this work, we report a novel PEO-in-high salt concentrated ionic liquid system (PiHCIL-SPE) based on LiFSI and C3mpyrFSI ionic liquid. We studied the effect of HCIL concentration on the physical and electrochemical properties of the composite electrolyte. By focusing on the enhancement of electrochemical window and ion transport, we seek to address a critical gap in the current understanding of PEO-based SPEs. Fourier-transform infrared spectroscopy (FTIR) and solidstate MAS NMR techniques are used to unravel the cation-polymer and cation-anion interactions and their impacts on ion dynamics within the designed SPEs. The optimal PiHCIL-SPE achieved high electrochemical stability and ambient temperature ionic conductivity. Stable and reversible cycling toward Li metal electrodes and favourable all-solid-state LMB performance were demonstrated. Based on the findings, an ion conduction network model consisting of interconnected ion clusters and pores is proposed to help understand the high ionic conductivity, and promising battery performance of the designed PiHCIL-SPE. Thus, we believe the concept of incorporating high salt-concentrated IL into high molecular weight PEO to form a robust SPE represents a promising approach toward safer and high-performing allsolid-state LMBs.