Preparation of 3DOM PI–Ion Gel Composite Electrolyte Membranes for Li Metal Batteries
Authors: Shimbori Yuma , Kanamura Kiyoshi 
Published: December 2024

The electrolyte used in Li metal batteries (LMBs) contributes to high cycle performance, coulombic efficiency and safety. In particular, highly flammable electrolytes limit an operating temperature of LMBs around room temperature to keep safety, so that an introduction of cooling system is necessary, resulting in a reduction of the energy density of LMBs. Therefore, a use of safer electrolytes is required. Ion gel (IG) electrolytes have attracted attention as a candidate for electrolytes with high safety. Ionic liquid (IL) electrolytes have been incorporated in a polymer matrix. A large amount of polymer matrix has been needed to prepare thinner membranes of IG electrolytes, due to achieve high mechanical strength of mebranes. In contrast, the amount of polymer matrix is minimized to achieve higher ionic conductivity. This trade-off relationship between mechanical strength and ionic conductivity of IG electrolyte membranes must be solved by different approaches. This problem may be solved by IG electrolyte membranes incorporated into a separator framework. This separator does not contributes only to mechanical strength and ionic conductivity, but also to electrochemical performance of Li metal anode, such as Li metal deposition and dissolution and its coulombic efficiency. Therefore, a selection of separator used in the composite electrolyte membrane is very important. We have developed a three-dimensionally ordered macroporous polyimide (3DOM PI) separator that has a good affinity with IL electrolytes. In this study, the mechanical strength and ionic conductivity of the IG electrolyte membranes are improved by using a 3DOM PI separator as the framework. In addition, the more uniform porous structure of the 3DOM PI separator provides a high reversibility of Li metal dissolution and deposition.

Three types of electrolytes were used in this study: pristine IG electrolyte membrane, surfactant-coated polypropylene (PP 25 μm)–IG composite electrolyte membrane, and 3DOM PI (30 μm) –IG composite electrolyte membrane. Lithium bis(fluorosulfonyl)imide and N-metyl-N-Propylpyrrolidinium bis(fluorosulfonyl)imide mixture with a molar ratio 1:1 was used as the IL electrolyte. Polymeric precursor solution for IG composite electrolyte was prepared from methyl methacrylate (MMA), 5 wt% of ethylene glycol dimethacrylate (EDGMA) to MMA, and 0.2 wt.% of 2,2’-azobis(isobutyronitrile) (AIBN) to MMA. The polymeric precursor was casted on the polytetrafluoroethylene (PTFE) sheet, PP separator, or 3DOM PI separator and sandwiched between PTFE sheets and then heated at 75 °C for 12 hours for polymerization in the separator. In the case of pristine IG electrolyte, the thickness of the self-standing membrane was at least 300 μm. The 3DOM PI separator was prepared by using a colloidal template method.

An ionic conductivity measurement was performed to investigate an effect of separators on ionic conductivities of membranes. Figure 1 (a) shows the Arrhenius plots of the ionic conductivity of the membranes. The pristine IG electrolyte membrane exhibited the highest ionic conductivity, followed by 3DOM–IG composite membrane and PP–IG composite membrane. There was no significant difference in the slope of Arrhenius plots, indicating that the ionic conduction mechanisms of these electrolytes were not different each other. The unique pore structure of the 3DOM PI separator provided higher ionic conductivity than the surfactant-coated PP separator. The thickness of the membrane between the anode and cathode is an important parameter for an internal resistance of cell. Therefore, a conductance of real electrolyte membranes should be discussed. Figure 1 (b) shows the Arrhenius plots of the conductance of the electrolytes. The 3DOM PI–IG composite electrolyte exhibited both higher mechanical strength and conductance compared with those of other electrolytes. In conclusion, the composite electrolyte membrane between 3DOM-PI and IG is the most promising separator.

Figure 1