Separators’ heat-resistant coating binder that improves LIB safety

LIBs¹⁾ are used as the power supply for devices ranging from smartphones to electric vehicles, but special care is needed to ensure LIB safety due to their high energy density and organic solvent electrolyte. Among the LIB components, the separator is a crucial part that passes ions to enable an electrochemical reaction, while physically separating the anode and the cathode to prevent short circuits. Separators are comprised of a porous, thin film of polyolefin (crystals with a melting point of around 130-150℃), such as polyethylene and polypropylene. If the interior temperature rises sharply due to overcharge, over-discharge or a battery management anomaly, the crystals of the porous, thin film melt and fill the fine pores, blocking the movement of the lithium ions and thereby curbing temperature increases. This is the separators’ shutdown mechanism. However, if the temperature continues to rise, the polyolefin film shrinks, allowing the electrodes to touch each other and cause a short circuit. This situation can escalate thermal runaway.

Innovative Solution

Higher thermal resistance of the coating layer strengthens the separators and improves LIB safety

To prevent the thermal shrinkage of separators and help them retain their shape when the temperature rises from 130°C to 140°C or 150°C, it is crucial to secure the ceramic coating binder’s thermal resistance, while increasing the physical strength of the separators with ceramic particles firmly adhering.
Resonac’s PNVA is a water-soluble polymer produced by the polymerization of N-vinylacetamide, which Resonac has successfully turned to industrial use. PNVA increases the viscosity in a polar solvent or wide pH range, and demonstrates thermal resistance with Tg at around 190°C and a thermal decomposition temperature at around 370°C. the <GE191 series> offers optimized molecular weight and other properties for use as a ceramic coating binder for LIBs. Its structure with a high number of hydrogen bonds allows the ceramic particles to disperse uniformly and stabilize. After coating, the hydrogen bonds keep the ceramic particles firmly adhered as a coating.
Resonac’s heat-resistant coating binder PNVA <GE191 series> helps improve the safety and durability of LIBs by achieving excellent thermal resistance of the coating layer’s binder and strengthening the separator with the wide dispersion and adhesion of ceramic particles. Heat shrinkage can be restricted even if the coating layer becomes thinner, contributing to LIBs’ higher energy density and reducing the coating process cost.

• Read articles for basic product information on the PNVA <GE191 series> .

 

Appearance of the ceramic coating separator before and after heating

• Heating conditions：150℃, 1 hour　　MD：Mold Direction　Shrinkage rate：1.5%
• ※ The data shown are representative values that represent examples of the results of measurements, calculations, etc., and are not guaranteed values.

Product Features

High thermal resistance

Separators with no coating showed thermal shrinkage rates of over 20% at 130°C, but a conventional ceramic coating using a CMC binder lowered the shrinkage rate to about 3%. However, when the temperature rose further to 140°C and 150°C, the CMC binder allowed the thermal shrinkage rate to increase significantly to around 8% and 20%, respectively. (Resonac’s testing data)
In contrast, a ceramic coating using the binder PNVA <GE191 series> restricted the thermal shrinkagerate to around 5% and 8% when the temperature rose to 140°C and 150°C. When the PNVA <GE191 series> was applied to two types of raw separator films of other makers, the thermal shrinkage rate was also limited to around 5% or less at 150°C in each case.
The acceptance criteria for the thermal shrinkage rate for vehicle-mounted parts is generally 5% or less for one hour at each temperature. Compared with CMC, the PNVA <GE191 series> was confirmed to demonstrate greater thermal resistance even with the temperature increased to 140°C and 150°C.

 

 

Comparison of thermal shrinkage rates at different temperatures

• [Test conditions]　
  Separator：film thickness＝9μm, wet type, polyethylene
  Coating：coating thickness 3μm, Filler: alumina, PNVA binder: <GE191-103>
  MD：Mold Direction, TD：Transvers Direction
• ※ The data shown are representative values that represent examples of the results of measurements, calculations, etc., and are not guaranteed values.

Film thickness reduction

The thermal shrinkage rate of a ceramic coating using the PNVA <GE191 series> at 150℃ was tested with varying coating thicknesses between 2μm and 4μm. For PNVA <GE191 series> with Mw at one million, the thermal shrinkage rate of the 2μm-thick film was 4% or less, indicating its suitability for practical use. It was also found that the effect of improving the thermal resistance compared with CMC became greater as the film was made thinner.
With thermal shrinkage restricted even though reducing the coating layer’s thickness, the material is expected to contribute to increasing LIBs’ energy density, while reducing the coating process cost.

Comparison of thermal shrinkage rates by film thickness

• [Test conditions]　
  Separator：film thickness＝16μm, dry type, polypropylene/polyethylene, three layers
  Filler：alumina
  Binder：PNVA<GE191-103>　Mw=1 million, PNVA<GE191-104>　Mw=300,000
  Thermal shrinkage rate：MD（mold direction）
• ※ The data shown are representative values that represent examples of the results of measurements, calculations, etc., and are not guaranteed values.

• ※ "PNVA" is Resonac Corporation’s registered trademark in Japan, the European Union and China.

Update date: 22 August, 2024