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Poly (ethylene oxide) (PEO)‐based electrolytes are extensively applied to LiFePO4/Li solid‐state batteries on account of their high safety and good flexibility. Nevertheless, the unsatisfactory energy density and inferior lifespan of the batteries still inhibit their large‐scale applications. Herein, graphitic carbon nitride nanosheets (GCN)‐assisted‐alkali metal iodides are proposed as multifunctional fillers for PEO‐based electrolytes. GCN is introduced to fix the location of iodine species through adsorption to reduce iodine migration and promote the kinetics of iodine redox reactions. Besides reducing the PEO crystallinity and elevating the ionic conductivity of electrolyte, the incorporation of GCN‐assisted‐alkali metal iodides facilitates lithium migration, constructs a stable interphase on the Li anode, and enables uniform deposition of lithium. Of particular note, extra capacity is provided through iodine redox reactions. The all‐solid‐state LiFePO4/Li battery using GCN‐LiI modified PEO electrolyte (GCN‐LiI‐PEO) delivers a high discharge capacity of 228.4 mAh g−1 at 0.1 C. Superior rate capability with 99.2 mAh g−1 at 10 C and high‐capacity retention of 87.3% after 900 cycles at 0.5 C are also achieved. Moreover, the LiFePO4/Li batteries using GCN‐assisted‐KI or NaI also achieve high capacity and prolonged cycle life. This work opens a promising avenue for pursuing high‐energy density solid‐state batteries. [ABSTRACT FROM AUTHOR]