Polyethylene glycol functionalized polyhedral cage silsesquioxane as all solid-state polymer electrolyte for lithium metal batteries

Polyethylene oxide (PEO) based solid polymer electrolyte is expected to be the safe and high energy density lithium-ion batteries owing to leakage-free and good flexibility, but high crystallinity results in low ionic conductivity at room temperature. In this paper, different molecular weights of the polyhedral oligomeric silsesquioxane-polyethylene glycol (POSS-(PEG_w)₈, w = 1 K g/mol and 2 K g/mol) is synthesized by click reaction between azido polyhedral oligomeric silsesquioxane (POSS-N₃) and different weight of alkynyl methoxy polyethylene glycol (Alkynyl-PEG) are used as doped fillers to reduce the crystallinity of the PEO and enhance the ion conductivity of the electrolyte. The effect of the molecular weight of PEG in POSS-(PEG_w)₈ on the crystallinity and conductivity of composite solid polymer electrolytes (CSPEs) is analyzed and the result shows that PEO/POSS-(PEG_1K)₈ exhibits higher ionic conductivity (0.9 × 10⁻³ S·cm⁻¹ at 50 °C) and Li⁺ transference number (0.47) than PEO/POSS-(PEG_2K)₈. Simultaneously, PEO/POSS-(PEG_1K)₈ presents a superior electrochemical window of 5.08 V. The LiFePO₄‖PEO/POSS-(PEG_1K)₈–60%‖Li cell assembly reaches 133 mAh·g⁻¹ after 100 cycles. The high capacity and good cycle performance prove the feasibility of applying POSS-(PEG_w)₈ to CSPEs, which contributes to the practical application of lithium metal batteries.