Influence of Porous Separators on the Ionic Conductivity of Nonflammable Liquid Perfluoropolyether Electrolytes

Abstract

Conventional Lithium-ion coin cell batteries contain a volatile flammable liquid electrolyte, impregnated in a porous separator. This leads to a serious safety issue with, for example, the possibility of flammable reaction products and internal short-circuits.[1] Commercial microporous polyolefin membranes, made of semi-crystalline polymers as polyethylene (PE) and polypropylene (PP), have been widely used as battery separators. [2] The main functions of the separator are to act as a physical barrier to prevent short-cut between the two electrodes and to allow ionic transport. Ionic transport within the separator depends on several factors such as the separator geometry, porosity and tortuosity, the electrolyte resistance and wettability.[3] Thus, the presence of the separator leads to an effective ionic conductivity lower than the bulk electrolyte conductivity, and an increase of the Li battery internal resistance. The recent development of non-flammable liquid electrolytes based on functionalized perfluoropolyethers (PFPEs) opened the path of safe Li-ion batteries.[4, 5] PFPE is chemically resistant, non-crystalline, and nonflammable exhibit low glass transition temperature and low toxicity. It was demonstrated that a methyl carbonate-terminated perfluoropolyether (see Figure 1) is suitable for lithium battery operation, but optimization is necessary to improve the performance. So far little is known on the transport properties of this new class of non-flammable liquid electrolytes within the separator. As for any conventional liquid electrolyte, investigating the nature of the separator and its effect on the effective ionic conductivity is required to achieve an optimal battery design. We investigated the physico-chemical and electrochemical properties of three commercial separators: a PP monolayer, a trilayer PP-PE-PP and a polytetrafluoroethylene (PTFE). The ionic conductivities of low molecular weight PFPEs, with dimethyl carbonate or ethoxylated alcohol end-groups, doped with the lithium salt LiTFSI was measured as a function of the temperature either in the bulk or in the presence of the separators. For comparison, a conventional liquid electrolyte made of 1M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate was also studied. In addition, the effective ionic conductivity of each electrolyte impregnated in the separator is modeled based on the bulk electrolyte conductivity and a separator morphological form factor.