R&D StoryHeat-Resistant Coating Binder PNVATM GE191 Series

Separator in lithium-ion batteries (LIBs) ensures their safety. The more sophisticated these batteries become, the more heat they generate. As a result, there has been increasing demand for higher heat-resistant coatings. Showa Denko used PNVATM, an original water-soluble polymer, to develop a binder for coatings that has excellent coating properties and high heat resistance.

Heat-Resistant Coating Binder PNVA<sup>TM</sup> GE191 Series
What prompted the development?

PNVATM itself has a long history. It was launched in 1997 as a water-soluble polymer produced by polymerization of N-vinylacetamide (NVA). NVA is a functional monomer successfully industrialized by Showa Denko for the first time in the world. At first it was marketed as a water-absorbing material for flocculants and water absorption rubber and so on.

While we no longer sell PNVATM for these uses, its performance remains excellent. For example, 1) it is so heat resistant that it does not deteriorate even at 200 degrees Celsius, 2) it has hydrogen bonds in the polymolecular structure, which exhibit high hydrophilicity and high thickening capacity in a water medium, and 3) it can stably maintain thickening properties in a wide pH range. So, we started searching for new uses of PNVATM with its impressive properties.

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What direction did you set for the development?

Our focus was on high-added-value areas. In the past, we conducted research on PNVATM to use as an additive in cosmetics and pharmaceuticals, and sold it for these purposes. Through this experience, we knew it is excellent in dispersing titanium oxide and zinc oxide and in viscosity properties. Therefore, we decided to focus on the electronic materials field.

PNVATM contains many hydrogen bonds that are effective for metal oxides. It also has high adhesiveness to alumina and boehmite, which are frequently used as dispersing agents for electronic materials and various kinds of additives. The polymer itself has high heat resistance. With these properties taken into consideration, one of the uses we came up with was as an additive for a binder that is coated on the separator of LIBs.

The separator is used to prevent contact of the cathode and the anode. When abnormal heat is generated, the separator can be deformed or deteriorated. If the separator is deformed and the two electrodes contact each other, a short circuit occurs, resulting in ignition.

To prevent an ignition accident and to ensure the safety of the battery, the surface of the base film of the separator is coated with a heat-resistant ceramic layer. The binder serves as glue to harden the ceramic material of the coating. If the binder itself is not heat resistant, it melts at high temperature, and the ceramic particles in the heat-resistant layer are removed. As a result, it losses function as a separator. So, we thought that PNVATM’s high heat resistance could make the material applicable here.

How did you carry out the development?

As I have mentioned, our focus was on the electronic materials field. First of all, we determined where to use PNVATM.

PNVATM is highly adhesive to oxides and exhibits excellent dispersity and coating performance in slurries. These properties allowed us to consider several possible applications of PNVATM. One was for LIBs, where alloy powder is used for the active anode material. Another was for condensers, where the uniform dispersion of silicon dioxide is required. We also looked at the slurries that are used in the manufacturing process of semiconductors.

Our final decision was to use it as the separator of LIBs. I think one of the major factors contributing to this decision was our research and development focused on our expertise in metal oxides, such as alumina, titanium oxide and zinc oxide.

After determining the target use, we adjusted the proportion of components. In verification experiments, we brought samples to clients to see their reactions. We then proceeded with development based on feedback from clients and the results from experiments.

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What is a key to solve the problem?

It took a long time to verify, in particular, that PNVATM is truly suitable for the binder in the heat-resistant layer of the separator.

PNVATM is superior among water-soluble polymers. The most difficult part was identifying how to best make use of some of the features of this product. Through trial and error, we narrowed down to this application. However, the only thing we could do alone was to make a hypothesis from our point of view as chemicals specialists.

Therefore, we brought samples to clients to seek their feedback, based on which we repeated test until the final phase of commercialization. We made many adjustments based on the evaluation results, not only for heat resistance, but also viscosity, ease of coating, and differences in performance by pH values.

The most difficult part was the adjustment of solution concentrations. Viscosity is affected not only by the amount of PNVATM dissolved in the solution but also the molecular weight against the molecular length. Therefore lithium ion conductivity of PNVATM is relatively low. So, if too much PNVATM is added to increase viscosity, the resistance increases. This is a difficult point. We conducted sampling tests to find the optimal concentration through trial and error.

There were also differences, from client to client, in the kinds and amounts of inorganic oxides contained in the coating liquid, as well as in the types of coating equipment and in coating speeds. Even if we used the same amount of PNVATM with the same molecular weight, the results obtained differed depending on the conditions. This made us constantly feel unsure if we were on the right path.

We wouldn’t have been able to develop the product for this application without repeated trial and error in collaboration with our clients. Based on many assessments and a large amount of feedback from clients, we were able to develop several grades of products that can meet various requirements. Commercialization might not have been possible without the cooperation of our clients.

Another factor that helped in the commercialization of the PNVATM GE191 series was advice and support from other departments and internal research laboratories. We applied the expertise of the Advanced Battery Materials Division in LIBs, simulations performed by the Computational Science and Technology Information Center, and analysis results of the molecular structure obtained from the Analysis & Physical Properties Center in improving the manufacturing process in order to enhance the level of perfection of the product itself.

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What are the advantages of the developed PNVATM GE191 series?

The most remarkable feature of the PNVATM GE191 series is its heat resistance, which is better than that of existing products. It is expected that LIBs will become larger in size, more sophisticated in performance and higher in capacity, which will lead to an increased amount of heat generation. A binder containing PNVATM can maintain stability at a high temperature of 200 degrees Celsius. This property makes the binder compatible with the evolution of LIBs, and contributes to improving safety.

Another feature is the high viscosity. It is important that metal oxide particles are uniformly dispersed in heat resistant coating on the separator so as to avoid uneven thickness. This objective has been fulfilled by PNVATM’s high viscosity and its ability to maintain thickening properties in a wide pH range.

Ease of coating is another feature. In general, the higher the viscosity sacrifices coating property. The viscosity of the PNVATM GE191 series decreases with increasing shearing speed, which is one of the series’ remarkable properties. This ensures highly stabile dispersion, enabling quick, uniform coating. Thus, we can expect to shorten the time for commercialization and reduce losses, leading to lower costs.

The PNVATM GE191 series has a good balance of these characteristics, and are very easy-to-use products.

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What will you develop in the future?

As LIBs become more sophisticated, it is expected that demand for improvements to separator properties will increase. We are considering taking two approaches: to respond to the demand for the application as a binder in the heat-resistant layer and to expand the application of the PNVATM GE191 series in other areas. We are currently selling the series mainly in Japan. I hope the product will also be used globally.

I think the series can find more applications with LIBs. We need to make constant efforts to meet the demands of our customers and catch up with the evolution of batteries.

Potential applications include electric double layer capacitors (EDLC), conductive polymer aluminum solid electrolytic condensers, and CMP slurries.

As in EDLC, PNVATM is expected as its binder which has high adhesiveness to aluminum and electrode active material. Applying PNVATM to aluminum solid electrolytic condensers could contribute to stronger adherence of conductive polymers to the aluminum surface, thus improving a condenser’s performance and extending its useful life. CMP slurries are used as abrasives for semiconductors. Fine particles in the slurry need to be dispersed uniformly, for which PNVATM would be effective.

I’m sure many issues that need to be addressed will come up in order to meet these applications. We will continue our efforts to explore the potential of the PNVATM GE191 series so as to widen their applications.

Jun Konishi

Kawasaki Plant
Development Department
Ogimachi Development Group

Manager

Jun Konishi
Atsushi Sugawara

Kawasaki Plant
Development Department
Ogimachi Development Group

Assistant Manager

Atsushi Sugawara