(Invited) Ultrafine Porous Polyimide Membrane for Rechargeable Lithium Batteries

Porous membranes have been used in various fields as key materials for separation. Their functionalities strongly depend on their structures such as pore size, pore distribution and porosity. Thus, the structure design of porous membranes has been focused over the years. Particularly, the research and development of ultrafine porous membranes has been intensively engaged. In the research field of battery, porous membranes are used as separators, whose role is to prevent electrical short circuits between anode and cathode but easy permeation of ions. Thus, the safety and power of batteries is strongly dependent on the porous separator, and many advanced porous membranes have been developed for batteries. In this study, we have developed an ultrafine porous polyimide membrane illustrated in Fig. 1, and investigated its performance as a battery separator. This structure is so-called three-dimensionally ordered macroporous (3DOM) structure and has an ideal porosity of 74%. The pores are in a hexagonal close-packed arrangement, so that uniform current distribution is expected during battery charge and discharge.

 3DOM polyimide membrane for a battery separator was prepared by using a colloidal crystal template composed of mono-disperse spherical particles [1]. Fig. 1 shows the characteristics of 3DOM polyimide separator with the pore size of ~300 nm. The uniformly ordered structure with a high porosity of ~ 70% resulted in a very low Gurley value (32 sec), which is about 1/20 of conventional polyolefin separators [2]. The smaller Gurley value leads to the smoother Li⁺-ion transport in the separator. Actually, the 3DOM polyimide separator provided uniform current distribution for rechargeable lithium batteries, and was particularly effective to lithium metal anode which has a serious problem to be used in rechargeable batteries due to dendrite-formed deposition during battery charge process (Fig. 3). This is very advantageous for realizing high capacity rechargeable batteries using lithium metal anode.

References

[1] H. Munakata, D. Yamamoto and K. Kanamura, J. Power Sources, 178(2008) 596-602.

[2] S. S. Zhang, J. Power Sources, 164 (2007) 351-364.