The separator is a critical component of a battery. It provides a barrier between the anode and the cathode while enabling the exchange of ionic charge carriers from one side to the other.
For over 40 years, the Freudenberg Group has been producing separators for nickel-based batteries, such as the nickel-metal hydride (NiMH) cells widely used in hybrid vehicles since the late 1990s. About two years ago, the company turned its sights to the new lithium-ion market that powers today’s resurgent plug-in vehicles.
The requirements for separators in both Ni-based and Li-based batteries are similar in that both need high homogeneity and very low levels of impurities, much like any high-end material. However, that is where the similarities end. A close look at the specifications of separators will reveal a wide range of different products, even within the same chemistry. Separators vary in terms of fibers, chemistry and surface finishing, resulting in tailor-made mechanical parameters: porosity, wettability, softness and so on. For example, separators used in Ni-based batteries are typically around 100 to 200 microns in thickness, whereas those used for Li-ion batteries are considerably thinner: around 20 to 30 microns.
So, while Freudenberg has had a long history in high-end separator manufacturing, the company had a lot of work to do to design and validate a new line of separators for the Li-ion market.
“Behind our new separator material there is a philosophy,” Dr Christoph Weber, Business Segment Manager for Lithium-ion Battery Separators, told Charged. “We had a look at the main root causes of safety issues with lithium batteries. It starts with mechanical pressure by a particle in the cell. Then this develops into a localized heat-up due to small short circuits. Then a large-scale heat-up. Then an uncontrolled discharge.”
The company decided to design a next-generation separator material that is very robust against mechanical pressure. Also, “a localized heat-up should not allow any shrinkage, and a large-scale heat-up should not allow a complete meltdown,” explained Weber. By integrating these features into a separator material, Freudenberg believes it can protect against the root causes of thermal runaway in Li-ion cells.
Many of the battery separators used in currently available cells are made of polyolefin membranes – either polypropylene or polyethylene. They are manufactured through an extrusion process and then stretched into a film. This process produces separators that have good properties, like low thickness and weight, and have made today’s Li-ion-powered mobile electronics a reality. However, many believe that for widespread use in automotive applications, a safer alternative is needed. The biggest problem is that when the cells get hot, polyolefins begin to shrink, reducing performance and increasing the possibility of short circuits.
Freudenberg decided to use a nonwoven polyester material as the base of its new separators. Nonwoven fabrics are made by bonding together long fibers either by chemical, mechanical, heat or solvent treatments, and Freudenberg is one of the world’s largest manufacturers and suppliers of nonwovens to many different industries. Its polyester nonwoven separator material is made using a wet-laid process, similar to papermaking, in which a slurry of fibers is mixed and then laid out on a sieve and bonded together.
The result is a base material that is fundamentally less prone to shrinkage than extruded and stretched polyolefin material. Also, “We do not have collapsing porosities that you find with softer materials like membranes,” explained Weber. “These collapsing porosities trigger the formation of dendrites, which cause reduced capacity, self-discharge issues and eventually safety issues.”
Impregnated, not coated
Almost all the large-format cells on the market today use ceramics to further reduce shrinkage, increase high-temperature performance and add mechanical ruggedness. Freudenberg does so as well, except that it employs a proprietary ceramic impregnation process.
“With a typical polyolefin membrane, a ceramic layer is placed on top,” explained Fabian Beck, Sales Manager at Freudenberg. “But the properties of the membrane still remain, which includes shrinkage. It’s still a plastic foil. This is like putting plaster on a wound – however, it’s better if you don’t have a wound to begin with. In our case, there is a skeleton of fibers that we stuff with inorganic particles, and then we use a clever way to bond them. So it becomes flexible and you can use that design independent of your preferred cell design.”
The ceramic particles are extremely sturdy mechanical spacers against the roughness of the electrodes, or particles on the electrodes, that could penetrate the separator. This sturdiness translates into enhanced safety, because it maintains a separation between the electrodes, even under various severe abuse conditions. “During safety tests like nail penetration, overcharge and impact, our separator is very thermally and mechanically stable compared to other state-of-the-art separators, including coated membranes,” said Weber. “By the nature of the materials, and the different approach to ceramics, we get to a different level of safety performance. If you coat a material you are adding something on the top of it. The material and the coating are not connected. If you impregnate a material, you bring the ceramics into the other material, so you have a much better connection. Our polyester nonwoven is filled with ceramics, so the whole structure becomes ceramic. We have a true ceramic and flexible separator.”
To verify the strength of its products, Freudenberg offers a demonstration using a soldering iron that is 420 degrees C at its tip. The iron is pressed to a sample of the separator for 10 seconds, and…nothing happens, even under the high pressure and high heat. “In other designs you can see that the separator flows away from the heat source at the tip of the iron,” says Beck. “When you start having a short circuit in a battery, within a second you come to several hundred degrees C localized at the short circuit position. Eventually the short circuit is becoming larger and the cell blows. So this high temperature test gives testimony on the true thermal-mechanical stability of a separator at internal short circuit conditions. The Freudenberg separator assures electrode separation even under such extreme conditions.”
Beck continues, “In, for instance, nail tests of 6.5 Ah NMC pouch cells with typical membrane separators are heating up to around 500 degrees C leading to catastrophic cell failure with fire and explosion. But the cells using our separator stopped at 120 degrees C and then cooled down again. No fire. No explosion.”
Weber explains that for nonwoven materials, production and the following impregnation process need to be controlled very precisely, including the conditions of the production environment. This is where the company’s long history in battery separator manufacturing comes into play. Freudenberg believes its expertise and ability to control all the manufacturing steps in-house gives it a real competitive advantage.
For example, Freudenberg has many years of experience slitting, or cutting, huge rolls of separators into smaller pieces. It’s not an easy process when working with a ceramic material – it’s similar to cutting sandpaper. For Li-ion batteries, quality requirements are extreme, and any rough edges or contamination in the material are very problematic.
“It’s really beneficial to have our history, all the capabilities for the production, from the nonwoven itself to the impregnation with ceramics, and the slitting as well. We can do everything,” said Weber. “That sets us apart from competitors. We have a fast internal feedback loop such that we can adjust to customers’ demands quickly.”
The company says that control over the whole process increases product reliability in many respects, including homogeneity. A homogeneous material, and a homogeneous impregnation process, means uniform porosity and an even distribution of ion flow. If a separator is not homogeneous and uniform, it will generate different heat profiles as ions will flow slowly in some areas and faster in others. The more variation there is in the battery, the quicker it will fail.
Ready for Prime Time
After an intense product development and testing period, Freudenberg now has Li-ion separator products available on the commercial market. The company told us that, at the moment, these high value-added separators have an attractive price/performance ratio, which will become even more compelling with increasing volume demand by customers.
“We are 100 percent sure the use of our separator material will lift lithium batteries, especially the high energy-density nickel-manganese-cobalt cells, to another step regarding safety and reliability,” said Beck. “These are not projections on a Powerpoint presentation, it’s real results in real cells.”
This article originally appeared in Charged Issue 15 – August/September 2014