High-Performance Ionic Liquid Gel Polymer Electrolytes for Solid-State Lithium Metal Batteries

Abstract

Lithium metal is considered as a “Holy Grail” anode material for secondary batteries because of its extremely high theoretical specific capacity (3860 mA h g−1), low electrochemical potential (-3.04 V vs. SHE), and low density (0.59 g cm-3). Ionically conductive polymer electrolytes represent a class of environmental-friendly and inflammable electrolytes for solid-state batteries that can enable the use of lithium metal. In our study, we fabricated lithium metal batteries based on transparent self-standing ionic liquid gel polymer electrolytes (ILGPEs) and LiFePO4cathodes. We evaluated the properties of ILGPEs by engineering the compositional space between the constituting components: poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The ionic conductivity of ILGPE increases with the increase of PP13TFSI and LiTFSI, and a maxima conductivity was obtained when the mass ratio of PP13TFSI, PVDF-HFP, and LiTFSI was 3:1:1, and the value was 1.3 mS cm-1at 22 ºC, and then increased to 5.82 mS cm-1at 80 °C. The optimized ILGPE showed excellent compatibility with lithium metal, determined by the interfacial resistance stability with the elapsed time. The solid-state LiFePO4cells delivered a practical capacity approaching 89.5% of the theoretical capacity and a capacity retention of 95.1% after 200 cycles with a current density of C/5 at 22 ºC. The solid-state lithium metal batteries using various cathodes materials, such as NMC, Co-free layered oxides, disordered rocksalt were also evaluated.