Development of the High Heat Resistance CCS Using Waterborne New Binder
Authors: Fukuchi Iwao , Fukatani Tomoyuki 
Published: February 2020

Introduction:
As the demands for higher power density and higher energy density lithium ion batteries continue to grow, the accompanying safety concerns are issued as a critical challenge. The separators play a significant role in the thermal safety of lithium ion cells, which is critically important in applications such as electric vehicles, ESS (energy storage system), power tool. Commonly lithium ion battery separators are made from polyolefins, either polypropylene or polyethylene. These materials undergo catastrophic shrinkage above 120℃, which leads to shorting in cells that can cause the sparks that will ignite the electrolyte and flammable gasses.
Recently, a variety of approaches for overcoming these technical drawbacks of polyoleﬁn-based separators have been investigated. Among the many alternatives, the ceramic coat separator (CCS) has drawn a great attention because of their superiority in suppressing the heat shrinkage of separators. To improve the thermal stability of CCS, coating layer should be designed to ceramic rich and thermal resistance binder. And also, from the view point of environmental friendliness and cost-effectiveness, adopting the waterborne system is preferable. We will report the high heat resistance CCS using a new waterborne binder which was synthesized based on PAA.
Experimental：
1) materials
Bohemite (APYRAL AOH60(D50=0.9μm) and ACTILOX 200SM(D50=0.35μm), Nabaltech AG), PVP(Poly-vinyl pyrrolidone, K-90L , DKS Co., LTD.), PNVA (Poly-N-vinyl acetoamide, GE191-103, Showa denko K.K.) were used as received. Deionized (DI) water from a PURELITE-PRB-DV25 system (ORGANO Co., >15.0 MΩcm-1) was used. Developed binder (Poly-Acrylic acid based co-polymer, SRJ2 and SRJ3) were synthesized in-house. Micro-porous polyethylene separator (PE, 612SH, SK Innovation Co. Ltd., thickness = 12mm, gurley = 120〜125 sec/100ml) was used.
2) Preparation of ceramic coat separator(CCS)
The coating slurry was prepared by mixing bohemite and binder (Any one of PVP, PNVA, SRJ2 and SRJ3) at a constant ratio in DI water (AOH60/200SM/Binder/water = 83/12/5/200 by weight). After ultrasonically dispersed for 5 min, the slurry was further subjected to vigorous mixing by passing through the shaft bead-mill machine (DYNO-MILL RESEACH LAB, Shinmaru Enterprises Co.) with a speed of 2000 rpm for 4 times. The coating slurry was applied onto one side of the pristine PE separator by automatic gravure roll coating machine (THANK METAL CO., LTD.). After the coating process, the prepared separator was dried under vacuum oven at 80℃ for 12h to remove the residual water in the coating layer. The coating layer thickness of each CCS were controlled in 3 μm respectively.
3) Characterization of the ceramic coating separators
The heat shrinkage of the separators (original size: 5.0×5.0cm) was investigated by measuring their length changes of MD and TD direction using Eq.(1) after heat exposure in an oven at 150℃ for 60 min, where A1 indicate the MD or TD direction length of the samples after oven storage:
Thermal shrinkage ratio (%) = (5.0-A1)/5.0×100　・・・・・(1)
The 180° peel test (as described in standard No. ISO 29862:2007 (JIS Z 0237:2009)) was used to measure the adhesive force between PE separator and coating layer. CCS fixed to a stainless steel plate was adhered the pressure-sensitive adhesive tape of width 2.4 cm (Nichiban Co. SellotapeTM No.405). Then, the 180° peel strength was measured by using peel tester (SHIMAZU EZ-S, Shimadzu Corp.). The air permeability (Gurley number) of CCS was determined by measuring the time for transmission of a 100cc air using a densometer (Inner cylinder weight (pressure) 567gf, transmitting surface hole diameter 28.6mm, permeation area 6.45 cm2, Toyo Seiki Co., Ltd.)
4) Result and discussion：
The heat shrinkage, adhesion and permeability were investigated, and these data are summarized in Table 1. In first, the heat shrinkage of the CCS was evaluated. The developed CCS (SRJ2 and SRJ3 were used) were showed very low length change of MD and TD each directions compare to the reference CCS (PNVA and PVP were used). Next, the adhesion between the PE surface and ceramic coating layer was measured by 180° peel test. The peel strength of the developed CCS were higher than the references. From these results, the heat shrinkage was presumed to be improved when the coating layer was securely binding. The air permeability was also determined. There was no difference in gurley number compared to the reference CCS, so developed CCS were found to have a good air permeability. In poster session, the relationship of adhesion to the heat-shrinkage and the evaluation results of the battery performance of the cells which were used developed CCS will be discussed.

Figure 1