Improvement of Charge/Discharge Performance of Li Metal Battery Using Ionic Liquid Electrolyte with 3DOM PI Separator

Abstract

Ionic liquid electrolytes (ILEs) have attracted attention as promising electrolyte candidates. ILEs can realize non-flammable rechargeable batteries. In addition, other advantages of ILEs are negligible volatility and high thermal stability. These properties can improve the safety of Li metal batteries. However, conventional polyolefin separators cannot be used due to low affinity with highly viscos electrolytes such as highly concentrated electrolytes and ILEs. So far, glass filters with thicknesses of several hundred micrometers have been used as alternatives. However, it is too thick to realize a real cell. Furthermore, the glass filter shows low reversibility of Li deposition/dissolution due to its non-uniform pore structure. It is known that Li deposition/dissolution sometime results in an internal short circuit which is caused by inhomogeneous Li deposition. The separator has a large effect on the uniformity of Li deposition/dissolution behavior. Therefore, new separators with good properties, such as high affinity with electrolyte, hightypemechanical strength, suitable thickness and excellent reversibility of Li deposition/dissolution are strongly required. So far, we have developed a three-dimensionally ordered macroporous polyimide (3DOM PI) memblane with a thickness of 30 micrometers as a separator having a good affinity with ILEs. In this study, we improved the performance of Li metal batteries by using both ILEs and 3DOM PI separator. Three types of separators were used in this study; glass filter (Whatman, GF/A, 260 μm), surfactant coated polypropylene (PP) (25 μm) and 3DOM PI separator (30 μm). 3DOM PI separator was prepared by using a colloidal template method. Lithium bis(fluorosulfonyl)imide and N-metyl-N-Propylpyrrolidinium bis(fluorosulfonyl)imide mixture with a molar ratio 1:1 was used as the ILE. Li deposition/dissolution was carried out by using Li symmetric cell with three types of separators and the ILE, in order to investigate the Li deposition/dissolution behavior. The Li deposition/dissolution capacities for all tests were 1 mA h cm-2. Fig. 1 shows the voltage profiles during the Li deposition/dissolution using different current densities. In the case of the 3DOM PI separator, the stable Li deposition/dissolution behavior was observed up to the current density of 3.0 mA cm-2. In the case of the surfactant coated PP separator, the applicable maximum current density was 1.5 mA cm-2. In the case of the glass filter, an internal short circuit was observed even at lower current density. The Li deposition/dissolution behavior strongly depends on the types of separators, and the 3DOM PI separator provides most stable Li deposition/dissolution, especially at high current density. Fig. 2 shows the voltage profiles during the Li deposition/dissolution cycles at the current density of 1 mA cm-2. In the case of the 3DOM PI separator, the highly stable Li deposition/dissolution was observed over 150 cycles. In contrast, in the case of the surfactant coated PP separator, unstable Li deposition/dissolution was observed after the 50th cycle, probably due to micro-short circuit. In the case of the glass filter, the internal short circuit was more clearly observed at the initial cycles. From the above results, it is confirmed that the 3DOM PI separator can provide stable Li deposition/dissolution behavior. This is due to the uniform structure of the 3DOM PI separator and high stability of ILE. Figure 1