Introduction
Lithium ion battery has been widely used as power sources for electric vehicles. A higher energy density of rechargeable batteries is strongly required. Lithium metal has a large theoretical capacity (= 3861 mA h g-1). However, in case of the using the lithium metal anode, a growth of lithium metal dendrite is a key problem in order to realize lithium metal secondary batteries. Lithium metal dendrite deposits on lithium metal surface during charging process results in a degradation of battery and an explosion of battery in the worst case. One of origins for lithium metal dendrite formation is a non-uniform current distribution. In our group, to realize uniform current distribution, we have focused on a separator. We have reported that the growth of lithium metal dendrite is suppressed by using polyimide separator with 3-Dimensionally Ordered Macroporous (3DOM) structure. Polyimide (PI) haing high thermal stability was chosen for the separator material. In our previous report, spherical particles of silica were used as the template to form pores. However, hydro fluoric acid (HF), which is very toxic, must be used to remove the silica template particles. In order to reduce manufacturing cost and environmental load, a new process without HF acid process is required. In this study, for a new separator preparation process without using HF acid was developed by using polystyrene (PS) particle as template. In addition, the characterization of new separator was performed for Lithium metal secondary battery.
Experimental
PS particle was synthesized by suspension polymerization method in a molar ratio of styrene monomer : 2-hydroxyethyl methacrylate = 1 : 0.023. PI precursor solution / PS slurry was prepared by mixing of PI precursor solution and ethylene glycol and PS particle. The slurry was coated on substrate, and then dried. A heat treatment for this coated layer was conducted to obtain 3DOM PI separator. Li dissolution / Li decomposition test was evaluated by 2032 type coin cell using new 3DOM PI separator. The electrolyte was 1 mol dm-3 LiPF6 in EC. Depth of charge for Li metal anode was 25 %. For comparison to 3DOM PI separator, the same test using a poly propylene (PP) separator was employed.
Results and discussion
Figure 1 (a) shows the SEM image of synthesized PS particles. Uniform spherical particles were confirmed by SEM. Particle size of PS was about 350 nm. Figure 1 (b) and Figure 1 (c) show the SEM image of surface and cross section of 3DOM PI separator, respectively. The uniform spherical pores on PI film were confirmed from Figure 2 (b). In addition, a small pore which appeared in the interior of the macropores was observed, clearly. These small pores are connecting holes between macropores. A presence of the pores inside of PI separator was also confirmed from the Figure 1 (c). This porous structure contributes a uniform current flow to electrode surface.
Figure 2 (a) and (b) show the Li dissolution / Li decomposition test using PP and 3DOM PI separator, respectively. When the short circuit of the cell happens, the overvoltage suddenly decreases due to reduction in an internal resistance. Such behavior was not observed in any cells. In the cell using the PP separator, an increase of overvoltage was observed. On the other hand, in the cell using 3DOM (PS) PI separator, an increase of overvoltage was not observed. The overvoltage of cell using the 3DOM (PS) PI separator was smaller than that of using PP separator. In case of cell using the PP separator, it is considered that lithium dendrite is formed and is easily peeled from electrode surface associated with cycle number, and then this lithium dendrite may be changed to dead lithium leading to high resistance.
Figure 2 (c) and (d) show the SEM image of Li deposited in cell using PP separator or 3DOM PI separator after 100 cycles, respectively. Many lithium dendrites were confirmed on lithium metal surface of cell using the PP separator. On the other hand, lithium granules were confirmed in lithium metal surface of the cell using 3DOM PI separators. It can be confirmed that the 3DOM PI separator suppress the growth of lithium dendrite.
Figure 1
