Bimetal Fluorides with Adjustable Vacancy Concentration Reinforcing Ion Transport in Poly(ethylene oxide) Electrolyte

The poor ambient ionic transport properties of poly(ethylene oxide) (PEO)-based SPEs can be greatly improved through filler introduction. Metal fluorides are effective in promoting the dissociation of lithium salts via the establishment of the Li-F bond. However, too strong Li-F interaction would impair the fast migration of lithium ions. Herein, magnesium aluminum fluoride (MAF) fillers are developed. Experimental and simulation results reveal that the Li-F bond strength could be readily altered by changing fluorine vacancy (V_F) concentration in the MAF, and lithium salt anions can also be well immobilized, which realizes a balance between the dissociation degree of lithium salts and fast transport of lithium ions. Consequently, the Li symmetric cells cycle stably for more than 1400 h at 0.1 mA cm^-2 with a LiF/Li₃N-rich solid electrolyte interphase (SEI). The SPE exhibits a high ionic conductivity (0.5 mS cm^-1) and large lithium-ion transference number (0.4), as well as high mechanical strength owing to the hydrogen bonding between MAF and PEO. The corresponding Li//LiFePO₄ cells deliver a high discharge capacity of 160.1 mAh g^-1 at 1 C and excellent cycling stability with 100.2 mAh g^-1 retaining after 1000 cycles. The as-assembled pouch cells show excellent electrochemical stability even at rigorous conditions, demonstrating high safety and practicability.