Amine-Functionalized Boron Nitride Nanosheets: A New Functional Additive for Robust, Flexible Ion Gel Electrolyte with High Lithium-Ion Transference Number

Ion gel electrolytes show great potential in solid-state batteries attributed to their outstanding characteristics. However, because of the strong ionic nature of ionic liquids, ion gel electrolytes generally exhibit low lithium-ion transference number, limiting its practical application. Amine-functionalized boron nitride (BN) nanosheets (AFBNNSs) are used as an additive into ion gel electrolytes for improving their ion transport properties. The AFBNNSs-ion gel shows much improved mechanical strength and thermal stability. The lithium-ion transference number is increased from 0.12 to 0.23 due to AFBNNS addition. More importantly, for the first time, nuclear magnetic resonance analysis reveals that the amine groups on the BN nanosheets have strong interaction with the bis(trifluoromethanesulfonyl)imide anions, which significantly reduces the anion mobility and consequently increases lithium-ion mobility. Battery cells using the optimized AFBNNSs-ion gel electrolyte exhibit stable lithium deposition and excellent electrochemical performance. A LiFePO4|Li cell retains 92.2% of its initial specific capacity after the 60th cycle while the cell without AFBNNSs-gel electrolyte only retains 53.5%. The results not only demonstrate a new strategy to improve lithium-ion transference number in ionic liquid electrolytes, but also open up a potential avenue to achieve solid-state lithium metal batteries with improved performance.