Bimetal Fluorides\nwith Adjustable Vacancy Concentration\nReinforcing Ion Transport in Poly(ethylene oxide) Electrolyte

The poor ambient ionic transport properties of poly(ethylene\noxide)\n(PEO)-based SPEs can be greatly improved through filler introduction.\nMetal fluorides are effective in promoting the dissociation of lithium\nsalts via the establishment of the Li–F bond. However, too\nstrong Li–F interaction would impair the fast migration of\nlithium ions. Herein, magnesium aluminum fluoride (MAF) fillers are\ndeveloped. Experimental and simulation results reveal that the Li–F\nbond strength could be readily altered by changing fluorine vacancy\n(V<sub>F</sub>) concentration in the MAF, and lithium salt anions\ncan also be well immobilized, which realizes a balance between the\ndissociation degree of lithium salts and fast transport of lithium\nions. Consequently, the Li symmetric cells cycle stably for more than\n1400 h at 0.1 mA cm<sup>–2</sup> with a LiF/Li<sub>3</sub>N-rich\nsolid electrolyte interphase (SEI). The SPE exhibits a high ionic\nconductivity (0.5 mS cm<sup>–1</sup>) and large lithium-ion\ntransference number (0.4), as well as high mechanical strength owing\nto the hydrogen bonding between MAF and PEO. The corresponding Li//LiFePO<sub>4</sub> cells deliver a high discharge capacity of 160.1 mAh g<sup>–1</sup> at 1 C and excellent cycling stability with 100.2\nmAh g<sup>–1</sup> retaining after 1000 cycles. The as-assembled\npouch cells show excellent electrochemical stability even at rigorous\nconditions, demonstrating high safety and practicability.