Asymmetric gel polymer electrolyte with high lithium ion conductivity for dendrite-free lithium metal batteries

Abstract

Lithium metal has been intensively investigated as a promising anode for next generation rechargeable Li metal batteries (LMBs). However, the safety concern on Li anodes caused by uncontrolled Li dendrite growth in liquid electrolytes hinders their application. Herein, a novel poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) based gel polymer electrolyte (GPE) with an asymmetric structure has been designed and developed to effectively retard the growth of lithium dendrites. Atomistic simulations confirm the strong interactions between PF₆⁻ and dipoles in the polymer matrix, which can anchor PF₆⁻ in the GPE and slow down its mobility to prevent space charge formation. In addition, this unique asymmetric membrane with a channel upper layer greatly enhances the mobility of Li⁺ in the GPE due to its low tortuosity and high porosity. The synergistic effect of the ion-dipole interaction and asymmetric structure increases the Li⁺ transference number to 0.66 and ionic conductivity to 3.36 mS cm⁻¹ (20 °C). Using this superior asymmetric GPE, Li|Li symmetric cells show more stable cycle performance than those using a liquid electrolyte. Li|LiFePO₄ batteries with the asymmetric GPE also deliver an impressive electrochemical performance, i.e., coulombic efficiency of 99.5% at 2C after 600 cycles. In consequence, this novel asymmetric GPE possesses potential application in high energy LMBs.